Prestressed_concrete_bridges

Prestressed concrete bridges built using the cantilever method – Design guide

Fig. 6.15 – Form traveler in movement

The form travelers used for the Pays de Tulle viaduct on the A89 highway are good examples of this type of form traveler [LAC 02]. 6.1.2.6 – Other comments regarding form travelers

O t h e r t y p e s o f f o r m t r a v el e r s

As mentioned at the beginning of paragraph 6.1.2.1. of this guide, there are many different types of form travelers and the preceding presentation is far from exhaustive. Special form travelers are commonly built for the construction of bridge decks with struts, lateral shell elements or even hybrid structures (e.g. the bridge over the Bras de la Plaine, the Vecchio bridge, the Corniche bridge in Dole, etc.) [CHU02], [PAU 00]. M o d i f i c a t i o n o f f o r m t r a v el e r s f o r r eu se

Considering the very high manufacturing costs for a pair of form travelers, efforts are often made on new projects to reuse equipment that was designed for a previous site. Unfortunately, as the decks of large bridges have no standardized elements, significant modifications often have to be made to the form travelers in order to reuse them for the construction of a new bridge deck. Considering the risks involved, the engineering design and implementation of these modifications must be handled with the same care as for the original design and manufacturing phases. W e i g h t o f f o r m t r a v el e r s

The weight of form travelers that are custom-made for a specific project is normally quite close to half of the weight of the heaviest standard segment. However, this ratio is sometimes exceeded, especially when a form traveler that was originally designed for a wider bridge deck is reused on a narrower project. 6 . 1 .3 -

Deviator segment s

The vast majority of deviator segments on composite prestressed concrete bridges are constructed in two phases: the standard section is built using the form traveler used for the standard segments; the lower beam and the deviator beams are built using wooden formwork with the concrete introduced through apertures left in the upper slabs directly above the beams (Fig. 6.16). With this method, the reinforcement bars awaiting insertion into the beams must be bent and then unbent - requiring the use of ADX steel - in order to allow the inner core of the form traveler to slide freely. However, it is increasingly common to use mechanical couplers at the junction between the webs and the deviator beams, making these operations unnecessary. It is now also possible to use special high bond strength reinforcing steels which are capable of withstanding a bending-unbending cycle without any loss of strength.

The “Les outils” collection – Sétra

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Fig. 6.16 – Second phase in the construction of a deviator beams

There are several structures in which the deviator beams were built in a single phase. In some of these structures, a special metal core was built specifically for these segments. 6 . 1. 4 -

Segments built by over-cantilevering

In terms of their construction, segments built by over-cantilevering are similar to standard segments. Therefore, they are built using form travelers designed for these segments. 6 . 1 .5 -

Closi ng segment s

Closing segments are normally constructed in a single phase, using one of the form travelers used for the construction of the standard segments. If it is possible to remove the central core once the final segment is in place, the internal parts of the closing segments are encased using the original core, as is the case for the standard segments. If this is not the case, which is by far the most common situation, it is necessary to design and use a special inner core, which can be broken down into small, easily transportable sections. It should be noted that the most common practice is to use standard form travelers for the construction of the closing segments. However, this does not happen systematically, and on certain projects, contractors prefer to build special equipment for these segments, which allows them to remove their form travelers as quickly as possible. During the construction of the closing segment , the formwork equipment is positioned on simple supports at both ends of the cantilevers because it would not be strong enough to withstand the thermal effects that might develop in the continuously-rendered span. If the formwork equipment is specially built, these support conditions pose no problems, but if one of the standard form travelers is reused, special precautions must be taken because the majority of these tools are designed to operate in an overhanging position. On certain sites, it is important for the aforementioned formwork equipment to be equipped with a system designed to prevent certain parasitic movements of the cantilevers - especially their rotation around the axis of the piers due to the effect of wind. Normally, this system consists of longitudinal beams fixed to the overhangs of the last standard segment in each cantilever. The time of the casting of a closing segment must be chosen with care. It is important to prevent the development of significant thermal effects between the setting of the concrete – the moment at which the span becomes continuous – and the stressing of the first continuity tendons. Therefore, casting in the late evening is recommended during very sunny periods. Two to three days are required to complete a closing segment, depending on the project.


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6 . 1 .6 -

End secti ons

In the majority of cases, the longitudinal profile of the natural terrain around the abutments makes it necessary to construct the end sections on horizontal centering resting on the abutment crosshead and on one or more temporary metal pilings (Fig. 6.17). The formwork for the bridge deck is then erected on this centering and the casting is carried out in sections of 3 to 4 m.

Fig. 6.17 – Construction on centering of the end section of an end span

However, it is possible to construct the end sections of the bridge deck on centuring towers, if the natural terrain lends itself to this solution. This is the most economical technique. End zones may also be built directly using the same form travelers used to construct the standard segments. In order to limit the imbalance of the cantilever, a temporary piling is placed under the last standard segment on the abutment side and then caulked after the cantilever tendons associated with this segment have been stressed. This piling also helps to reduce flexion at the end of the bridge deck (Fig. 6.18) [DEM 02].

Fig. 6.18 – Construction using form traveler and temporary piling on the end section of an end span

6 . 1. 7 -

S t a b i l i z at i o n o f c a n t i l e v e r s

The following section contains technical information concerning the stabilization of cantilevers. This supplements Chapter 5 of this guide. 6.1.7.1 – Stitching prestressing

Stitching prestressing is normally carried out by tendons. In the lower section, according to the piers, these tendons may be anchored on the underside of the crosshead, by forming a loop in the shaft or on the foundation bulkheads. In the upper section, they must be anchored to anchor plates housed in the upper slab of the SOP. If this solution is impossible due to the density of reinforcements in these parts, the anchorages are housed in The “Les outils” collection – Sétra

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prefabricated concrete blocks positioned on top of the SOP and removed after the stress has been released from the stitching tendons. A resin or mortar is inserted between these blocks and the SOP in order to guarantee the correct transmission of forces between these elements. Some contractors also use bars to provide the temporary prestressing. However, this solution is not to be recommended because any error in the positioning of the prestressing ducts could result in parasitic flexions in the bars or could even make it impossible to insert the bars. 6.1.7.2 – Concrete blocks

If the pier head unit is big enough to accommodate the bearings, blocks, stitching tendons and jacks simultaneously, the most economical stabilization system consists of temporary prestressing and hooped concrete blocks placed directly on the pier head (Fig. 6.19).

Fig. 6.19 – Concrete stabilizing blocks

During construction, the cantilevers rest on the concrete blocks and the bearings are in place but not in contact with the SOP. After the closing segment is in place, the structure can be placed on the permanent bearings. This consists of: 1. Raising the bridge deck using jacks 2. Caulking the gap above the bearings 3. Bringing the structure down onto the hardened caulking. Once these operations have been carried out, the blocks are no longer in contact with the SOP and may be removed by a crane or other equipment (Fig. 6.21). In this way, the support reaction of the bridge deck is gradually transferred from the stabilizing blocks onto the permanent bearings. 6.1.7.3 – Concrete blocks on sand boxes

If the pier head units are too small to accommodate jacks and blocks side by side, the cantilevers are constructed on concrete blocks placed on sand boxes consisting of a removable metal shell filled with graded sand (Fig. 6.20). Once the closing segment is in place, the gap between the underside of the SOP and the top of the bearings is caulked as before; finally, the sand is blown out of the boxes (Fig. 6.21). Thus, vertical forces are progressively and without jack transferred from stabilizing blocs to permanent bearings.


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Fig. 6.20 – Concrete stabilizing box placed on a sand box

Fig. 6.21 – Detail of an open sand box

A large variety of sand boxes are available. On the simplest projects, they are rectangular and four boxes are used per pier. On more complex projects, the sand boxes are proper cylindrical pot bearings and up to a dozen may be used per pier. Whatever their type, these boxes must be dimensioned and designed to be undeformable. If not, this could lead to an uneven distribution of the support reactions between the sand boxes, thus resulting in structural and/or geometric disorders. It should also be noted that the geometry of the cantilevers has to be perfect with this system, because as a sand box can only be emptied, it cannot be used to raise the bridge in case of problem. 6.1.7.4 - Jacks

Although this technique is rarely used, it is also possible to construct a cantilever entirely on jacks. This technique has the benefit of using the same system to perform both jacking and blocking functions, so that smaller pier heads can be used. For this to be possible, the jacks are immobilized during the construction of the cantilevers. They must therefore be equipped with lock nuts. 6.1.7.5 – Other methods

When the structure is positioned quite low above the natural ground and if the pier head unit is too small to accommodate blocks, e.g. because it is an “exact” match for an older design, it is possible to stabilize the cantilevers using temporary pilings placed either side of the piers (Fig. 6.22). At the bottom, these pilings, which are usually metal tubes filled with concrete, are attached on top of the foundation bulkhead; at the top, they are attached underneath the lower slab of the segments on piers or of the first standard segments.


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Fig. 6.22 – Stabilization of a cantilever by tubular metal pilings

For the second Saint-André de Cubzac viaduct, and still because of excessively small pier head units, composite systems with blocks and cable stays were used tostabilize the cantilevers [JAE00]. In the interests of thoroughness, we should also mention the second Gennevilliers viaduct over the River Seine for the A15 highway, which was equipped with self-leveling pot bearings injected with silicone rubber [CHA 94]. 6 . 1. 8 – P r e f a b r i c a t e d b u t t b l o c k s

In order to allow for the rapid stressing of cantilever tendons, contractors sometimes prefabricate the concrete which surrounds the anchor plates for these tendons. As the cantilever tendons on older structures were often anchored quite low down, as their alignment was designed to reduce shear force, these elements were situated in the webs of the box girder and were rectangular in shape. Today, rectangular blocks are still used when several tendons need to be anchored in close proximity to each other. However, for single tendons, more and more contractors are using prefabricated cylindrical blocks whose only reinforcements are helical hoops with jointed coils which also form the external formwork of the prefabricated block (Fig. 6.23 and 6.24).

Fig. 6.23 – Cylindrical butt blocks


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Fig. 6.24 – Cylindrical butt block in position in the form traveler

6 . 1. 8 -

I n f l u e n c e o f f o r m t r a v el e r s o n t h e d i m e n s i o n i n g o f b r i d g e d e c k s

It is imperative for the weight of the form travelers to be considered in the calculations for longitudinal flexion, as this increases the forces on the structure, particularly during the construction of the cantilevers. The form travelers may also influence the positioning of the cantilever tendons and continuity tendons, as their operation relies on prestressing bars which cross the slabs close to the webs where the internal prestressing tendons are situated. During the design stage for pretressed concrete bridges built by the cantilever method using reused form travelers, it is not possible to choose the position of these bars. This must be accounted for in the design of the cantilever tendons. It is also important to consider the forces caused by the form travelers when analyzing tranverse flexion. These forces usually lead to the localized strengthening of the reinforcements in the upper slab and in the top part of the webs (Fig. 6.25). Réaction de l'appui support de la poutre supérieure

Précontrainte des suspentes des poutres latérales Fig. 6.25 – Example of localized forces exerted by the form traveler on the box girder in the construction phase

In conclusion, it is essential for the organization and methods department and the engineering firm to work very closely together during the construction surveys for a structure built by the cantilever method.

6.2 - Constru cti on by prefabricated segments It is generally considered that it is more economical to prefabricate a bridge deck than build it in-situ when the number of segments to be constructed exceeds 350 to 400 units.


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However, specific difficulties may raise or lower this threshold. Thus, a very short contractual completion time or difficult climatic conditions may increase the appeal of prefabrication. On the other hand, a lack of space close to the site or difficult access conditions may mean that there is no alternative to the in-situ construction of reasonably long structures. French bridges built recently using this technique include the Ile de Ré bridge, the Saint-André viaduct for the A43 highway in Maurienne, the Rogerville viaduct on the A29 [JAC 98], the second A20 viaduct over the River Dordogne at Saint-André de Cubzac [JAE 00], and finally, the Avignon viaducts for the TGV Méditerranée high-speed rail line. 6 . 2. 1 -

The segment prefabrication plant

The prefabrication plant is set up on a site of approximately 2 hectares in area. This is usually located next to the site, but it may also be situated several kilometers away. It can be broken down into two major areas: the first is used for the construction of the segments and the second is reserved for their storage (Fig. 6.26).

Fig. 6.26 – General view of the units of a prefabrication plant

Prefabrication units have been preferred for the construction of segments for the past ten years or so. Thanks to significant improvements in geometrical accuracy, this method has definitively taken over from the costlier long bench method with an ogee mold (Fig. 6.27) which also requires more space.

Fig. 6.27 – Prefabrication using an ogee mold

A prefabrication unit is a construction area the length of two to three standard segments, in which the assembly of formwork and the casting of segments take place (Fig. 6.29 and 6.30). The lateral formwork consists of two metal sides; longitudinally, this function is carried out by a metal plate on the crown side and a by segment n-1 on the pier side, which results in a perfect joint between the different elements (Fig. 6.28). A well designed unit produces one standard segment per day or one segment on pier every two days. In order to maintain such high rates of production, the units are equipped with concrete distribution booms supplied by pipework or conveyors connected to a concrete plant on site. They also take delivery of complete reinforcing cages, which are manufactured on jigs by specialist workshops and are equipped with all inserts (ducts, anchor plates, anchor rails).


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Fig. 6.28 – Breakdown of a prefabrication unit

Fig. 6.29 – Prefabrication units

Fig. 6.30 – Detail of a prefabrication unit (from left to right: the matched mold segment, external formwork, internal formwork)


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Depending on the number of segments to be constructed, there may be between three and several dozen units in total. Some of these units specialize in segments on piers and abutments while the others are reserved for standard segments. Segments on piers are always split into two in order to avoid the need for oversize transportation and hoisting equipment. The half-segments on piers are manufactured side by side in specialized units; after separation, they are transported to the units specializing in standard segments for use as matched molds for the first standard segments of each cantilever. According to the project, the deviator segments are either built in specialized units or in the standard segment units, with the beams being constructed in the second phase outside of the unit in order to avoid having to modify the construction cycle for the standard segments. Specialized units are reserved for the construction of hinged segments, if these are included in the design of the bridge deck. Considering the errors that they have to compensate forr, the closing segments are cast-in-situ rather than prefabricated. They are also reduced to several tens of centimeters in length. The segments are stored on the prefabrication site for a period of one to three months. The concrete is thus extremely strong when the segments are assembled; therefore, the prefabricated anchor blocks described previously (see 6.1.8) can be dispensed with. In general, the segments are stored on just one level. If necessary, and after verification of the segments, they may also be stored on two or even three levels. The segments are first transported by large traveling gantry, initially from the prefabrication units to their storage area and then onto a mode of transport (barge, low-bed semi-trailer, etc.). 6 . 2. 2 -

Transportation of prefabricated segments

6.2.2.1 – Transportation by low-bed semi-trailer

Tyre-mounted low-bed semi-trailers are the most widely used form of transport for segments (Fig. 6.31). Depending on the sites and the chosen assembly mode, the semi-trailer is either driven onto the portion of bridge deck that has already been constructed or onto a track marked out on the ground in line with the structure to be built. Loaded is usually carried out by a gantry crane at the prefabrication site and unloading is performed by the assembly equipment (see 6.2.3 above).

Fig. 6.31 – Semi-trailer used for transporting segments 6.2.2.2 – Transportation by barge

The segments can also be transported by barge if the structure crosses a navigable waterway or one that can be made navigable, e.g. by dredging [JAE 00].


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6 . 2. 3 -

Assembly of prefabricated segments

6.2.3-1 – Assembly using a launch beam

Background inf ormation

Launch beams are the most widely used method for assembling prefabricated beams in successive cantilever segments. This self-propelled handling device is supported by the bridge deck and piers and therefore is free from almost all of the constraints relating to the crossing (Fig. 6.32). The initial cost of a launch beam is very high, but as it is usually designed to be used on several consecutive sites, it can be paid off over a long period. In recent years, numerous bridges have been built using this method. In France, the most notable examples include the Sylvans and Glacières viaducts on the A40 highway [BOU 90], the Ile de Ré Bridge, the Boulonnais and the Rogerville viaducts on the A29 highway [JAC 98] and the Saint-André bridge on the Maurienne highway (A43). Examples in other countries include the access viaducts for the second Severn crossing in England [COM 96], the access viaducts to the Prince Edward Island viaduct in Canada [COM 98] and numerous urban bridges in Thailand and Hong Kong.

Fig. 6.32 – Standard launch beams Structure of standard launch beams

Standard launch beams can be broken down into around ten elements, all of which are made of metal (Fig. 6.33 and 6.34):

•

Two triangular-section lattice girders of between 3 and 5 m tall and 100 to 250 m in length according to the beams, made from sections assembled using prestressing bars

•

Two front and rear lattice towers, acting as main supports

•

Two front and rear legs, acting as secondary supports

•

Two bridge cranes traveling on the lattice girders and used to manoeuvre the segments.


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Fig. 6.33 – Cross-section of a standard launch beam

Fig. 6.34 – Main constituent elements of a launch beam

The lattice towers and legs can be moved longitudinally along the lattice girders using to a system of capstans. The lattice towers can also slide inside their supporting cross beams. This transverse movement is required for the construction of curved sections, for example. The bridge cranes are equipped with spreaders which are used to position the segments as closely as possible to their final position, both longitudinally and transversally and for any type of geometry. A standard launch beam weighs between 300 and 600 tonnes. The completion time for its construction is approximately one year and it costs between 1.5 and 3 million euros.


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S eq u e n c e o f o p er a t i o n s f o r t h e a s se m b l y o f s eg m e n t s

During the assembly of the segments on piers on support Pn, the launch beam rests on its two lattice towers and the front leg. Once both half-SOPs have been assembled, adjusted and stitched, the rear leg is removed and the front lattice tower slides in place on top of the SOP on pile Pn. The beam can then be moved forward so that the rear lattice tower is positioned at the end of the cantilever centred on Pn-1. The front leg is then placed at the end of the lattice tower on the Pn+1 side, so that it does not obstruct the assembly of the standard segments. This operation can now begin (Fig. 6.35).

Fig. 6.35 – Different stages in the sequence of operations for a launch beam

During the construction of a cantilever, the two symmetrical segments S and S' are normally assembled one after the other. On some sites, a “symmetrical” assembly method has been used. This method, which consists of synchronizing the release of the two S and S' segments by both carriages, reduces the stresses imposed on the supports by eliminating non-accidental imbalances. When the bridge deck consists of two parallel bridge decks, it is very common for the adjacent cantilevers of the two decks Fa and Fb to be constructed simultaneously. To this end, the two lattice towers are placed on the transverse rails straddling the central gap, which allows the launch beam to cross over from one bridge deck to the other and thus to assemble cantilever Fa immediately after cantilever Fb. This method is also used when the bridge deck consists of two box girders joined in the middle and when each box girder is built and assembled on-site before the longitudinal grouting of these box girders is carried out. In general, this technique improves the speed of the assembly and prestressing of the segments. In the case of box girders joined side by side, this also significantly reduces the differential creep between the two box girders. S p ec i a l l a u n c h b ea m s

Without going into too much detail, it is worth mentioning certain launch beams that have been specially designed or used by French companies in France or abroad.


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A cable-stayed launch beam was used on the construction of the Ile de Ré Bridge. This solution helped to keep the lattice girders to a reasonable height despite the length of the spans to be crossed (110 m). This also played an important role in helping to reduce wind effects (Fig. 6.36).

Fig. 6.36 – The cable-stayed launch beam used on the Ile de Ré viaduct

The launch beam used for the construction of the H3 highway viaduct on the island of Oahu in the Hawaiian archipelago in the United States, featured two independent upper beams, each of which was positioned on a separate bridge deck. These beams were connected by a bridge crane. This arrangement made it possible to assemble the segments on two parallel bridge decks of different levels and separated by a very wide central gap. 6.2.3.2 – Assembly by crane

If the piers are not too tall and it is possible to use heavy equipment at the foot of the bridge deck, prefabricated segments can be assembled using cranes, which significantly reduces the initial investment costs. If the structure crosses an expanse of water, the assembly is carried out using a 200T to 500T lattice boom crane mounted on a barge (Fig. 6.37). If the structure crosses land, the assembly is carried out using a tracked crane with a lattice boom of the same capacity (Fig. 6.38). Given the order of assembly of the segments, the crane constantly moves from one end of the cantilever to the other.

Fig. 6.37 –Assembly using a river crane


Fig. 6.38 – Assembly using a land-based crane

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Whatever type of crane is used, a spreader is positioned between the hook of the crane and the segments. If the transverse slope and longitudinal profile are minimal, the segment can be guided by joining frames and the spreader can therefore be very simple. If the transverse slope or longitudinal profile is very pronounced, it is better to offer up the segments in a position close to their final position, which requires much more complex spreaders with transverse and/or longitudinal adjustments. Several structures have been built using this technique in recent years. The most notable examples are the Arcins viaduct over the River Garonne in Bordeaux [ABE 94], the second viaduct for the A10 highway over the River Dordogne at Saint-André de Cubzac [JAE 00] and the "central" and “Expo" viaducts of the bridge over the River Tage in Lisbon. 6.2.3.3 – Other assembly methods

In addition to the launch beam and the crane, other methods exist for the assembly of prefabricated segments. When each segment can be positioned on the ground in line with its final position, it is possible to “winch” segments or parts of segments up to the bridge deck using lightweight metal girders. This method has been used on at least four sites in France: on the Falaises viaduct on the A20 highway, on the Ottmarsheim bridge, where it was used to raise whole segments and on the Sermenaz and Arrêt-Darré viaducts ([SER 90], Fig. 6.39), where it was used to raise sections of segments.

Fig. 6.39 – Winch assembly principle (on the site of the Arrêt-Darré viaduct)

As the structure for the West Kowloon Expressway project, in Hong Kong, occupied a totally virgin site, the segments were assembled using a gantry crane operating on both sides of the bridge deck (Fig. 6.40). The same system was used for the Khurays Road viaducts in Riyad and, closer to home, on the A10 highway bridge over the River Loire at Tours.


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Fig. 6.40 – Assembly using a gantry crane

6 . 2 .4 -

Tempor ary lashi ng

In order to release the assembly equipment as quickly as possible, the segments being assembled can be lashed in place before the cantilever prestressing is brought into operation. This lashing generally consists of Ø36 mm bars anchored in blocks at the top and bottom of the webs, in the web uprights or in the slabs (Fig 6.41). It is dismantled after the stressing of the permanent tendons and the hardening of the epoxy adhesive. A normal stress of approximately 0.2 MPa must be exerted by these bars, which guarantees the correct distribution of the adhesive and helps it to set, while preventing any decompression at the end of the segments.

Fig. 6.41 – Anchor block for a lashing bar

6 . 2 .5 -

Fig. 6.42 – Construction of a closing segment with strip formwork

Mid-sp an clo sur es

The mid-span closures between cantilevers are constructed in the traditional manner, by casting an in-situ joint of approximately 20 centimeters in length. For such short joints, contractors often use strip formwork (Fig. 6.42). 6 . 2. 6 -

S ec t i o n s o n t h e a b u t m e n t s i d e o f e n d s p a n s

As for in-situ casting, the extremities of end spans on the abutment side are usually built on falsework. In the case of prefabricated segments, the corresponding segments are positioned and assembled on falsework. In order to allow the movement necessary for perfect joints at the crowns, blocks or jacks are placed between the bottom of the segments and the top of the falsework.


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6 . 2. 7 -

Bondi ng of the segments

Before assembly, an epoxy adhesive is spread on the pier-side face of the segment being assembled. This adhesive is spread by hand just before assembly to a thickness of approximately 1 mm. 6 . 2. 8 -

S t a b i l i z at i o n o f c a n t i l e v e r s

As mentioned in the part of this chapter devoted to cast-in-situ segments, the choice of a stabilization system for cantilevers is highly dependent on the size of the pier head units. Under these conditions, the stabilization techniques for bridges made from prefabricated systems are the same as those described in section 6.1.6 above. However, as it is difficult to correct the geometry of the cantilevers once the segments are joined, the cantilevers are often built on supports featuring jacks equipped with lock nuts. If conventional blocks are used, caulking must be applied between the top of the blocks and the bottom of the segments on piers in order to correct any construction defects or errors in the assembly of these parts. To be exhaustive, it is worth mentioning that cantilevers are sometimes stabilized by the launch beam itself. This technique was used for the construction of the Baldwin viaduct in the USA [FUZ 94], and for the B3 South viaduct to the north-east of Paris. 6 . 2. 9 -

S p e ed o f c o n s t r u c t i o n

The instantaneous construction rate on a site using prefabricated segments varies significantly. At the start of the project, no more than 3 m per day are completed. However, under normal operating conditions the speed of construction reaches 12 m or 4 segments per day. The average speed is therefore 6 m per day. 6 . 2. 10 -

Influence of methods on the dimensio ning of struc tures

Assembly using a crane or gantry crane creates very few longitudinal forces on the bridge deck, as the loads applied to the cantilevers are limited to the weight of any work platforms that might be used. If the segments are assembled using a launch beam or winch, greater forces develop in the bridge deck due to the weight of this equipment and the segments being handled and must therefore be allowed for in the design. For localized flexion, it is important to verify that the reinforcing of the segments is capable of taking up any transverse moments that develop during the storage and handling of the segments, regardless of the assembly method used. If the assembly uses a launch beam and/or requires the use of a low-bed semi-trailer, it is also necessary to consider the localized forces created by this equipment when dimensioning the non-prestressed reinforcements of the segments. As for construction using cast-in-situ segments, it is important to perform a thorough analysis of the positioning of the handling hangers and the internal cables in order to prevent any interference between these elements. In conclusion, as for the construction of bridge decks using cast-in-situ segments, the design and construction of bridge decks made from prefabricated segments using the cantilever method requires a very close collaboration between the engineers of the engineering and design department and those of the organization and methods department.


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7 - On site Monitoring This chapter covers the monitoring operations and particular precautions that need to be implemented during the project. It places particular emphasis on topics relating to the construction of bridges by the cantilever method. In the process, it extends its scope to the monitoring that must be performed and the particular precautions that must be taken during the construction of all types of large engineering structures.

7.1 - Backgr ound i nfor mation On-site monitoring mainly relates to quality, quantities and the monitoring of deadlines. This chapter only covers qualitative monitoring. Monitoring prior to construction is primarily designed to evaluate the methods which are likely to be used by the contractor and which will go some way towards defining some of the specifications of the contract. Some of these monitoring operations will not be covered in this chapter, including the certification of companies, the monitoring of plans and design calculations, the inspection of the SOPAQ (Schéma organisationnel du plan d’assurance qualité - Quality Assurance Plan Organizational Scheme), and the Quality Assurance Plan itself. However, other monitoring operations performed prior to construction will be developed in the following subchapters, including the inspection and approval of materials, the monitoring of the materials used and process controls. Inspections carried out during construction are designed to verify the correct application of the technical specifications and compliance with the rules of good practice. Different aspects of these inspections will be described in the following sub-chapters. Post-construction inspections are designed to ascertain whether the initial objectives have been achieved. If both of the previous types of inspections have been carried out, post-construction inspections will be a formality. Therefore, they will receive only minimal coverage in this chapter.

7.2 - Inspecti on o f g eometry The quality and accuracy of the topographic surveying that is carried out before, during and after construction, depends to a considerable extent on the care that is taken over the development of the geometric framework. Technical Document 4 entitled “Instruction technique sur la surveillance et l’entretien des ouvrages d’art : topométrie" (Technical instructions for the monitoring and maintenance of civil engineering structures: topographic surveying) is currently being prepared by Sétra. In Appendix 3, it provides a number of definitions which help to specify the vocabulary to be used for clarifying the exchanges in this field. The concepts of measuring points, reference points, frames of reference and networks are thus defined and are clearly explained in the following section. M ea su r i n g p o i n t

A measuring point is a point to which coordinates are attributed in order to monitor relative or absolute movements. Three categories of measuring points can be defined: The “Les outils” collection – Sétra

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A visible point on the structure: rivet head, hole, paint mark, punch mark

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A point given by a part that has been fixed or welded to the structure: target, leveling stud

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A specific point in relation to a part that has been fixed or welded to the structure and designed to accommodate accessories positioned by forced centring.

Reference poin t

A reference point is the association of a system of orthonormal axes and a point representing the origin of the system of axes and whose movements are to be measured. As a general rule, the orthonormal reference point is defined by:

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The Ox axis following the tangent to the longitudinal axis of the bridge deck.

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The Oy axis following the normal to the longitudinal axis of the bridge deck,

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The Oz ascending vertical axis.

F r a m e o f r ef e r e n c e

A frame of reference is a system of orthonormal axes (Ox, Oy, Oz) associated with a point of origin O, which is used to identify the position from any point in space. This conventional system, which is the working reference point established by the surveyor at the start of the topographical operations, is used to calculate the position of the measuring points. Network

A network is an arrangement of points determined by a set of planimetric and/or altimetric measurements. These points are usually marked out on the site. Two types of networks may be used in the topographical monitoring of civil engineering structures: absolute reference networks or relative measurement networks. Topometric operations consist of determining the positions of certain points whose movements will be monitored, in relation to a previously defined network. 7 . 2. 1 -

Inspection of pier geometry

Obtaining the correct bridge deck geometry involves considering any data that might have a bearing on the geometry of the piers and using reverse deflections to negate any drift that these data are likely to produce. 7.2.1.1 – Accounting for vertical settlement

The following effects must be carefully evaluated:

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The settlement of foundations under the action of the selfweight of the pier and the cantilever

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The elastic shortening of the pier due to the same actions


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•

The effect of shrinkage and creep due to the weight of the cantilever.

The bridge deck is normally supported by bridge bearings which operate between concrete support blocks. An inaccurate estimation of the vertical settlement of the pier may be compensated for by adjusting the thickness of these elements. However, this is a difficult operation requiring careful preparation. In the case of relatively small structures, it might be possible to consider the settlement of the foundations only. This approach guarantees a successful closure of the span. While the aforementioned effects may be small for small bridges, they increase in proportion to the height of the piers and the length of the spans. Piers of a hundred or so meters tall, span lengths of approximately 180 m and a bridge deck of 20 m wide may result in downward loads of approximately 20,000 tonnes and a vertical shortening of around 4 centimeters including creep. 7.2.1.2 – Accounting for other phenomena

In the case of a rectilinear bridge deck, the construction tolerances inherent to any structure may lead to shifting in the horizontal plane between cantilevers. However, it is important to remember that a thermal gradient may act upon a pier shaft and cause a horizontal shift of the pier head unit capable of affecting the measurements of movement during the construction of the cantilevers. In the case of curved bridge decks, it is necessary to examine the effect of the torsional moment applied to the segment on pier by the cantilever under construction. This effect increases in proportion to the height and/or flexibility of the pier. Indeed, when a curved cantilever is built, the torsional moment applied to the segment on pier causes flexion in the pier shaft which can alter the position of the pier head unit. Therefore, rather than building a perfectly straight pier, it is necessary to anticipate this movement by building in a pre-deformation in the opposite direction. The construction of the cantilever will have the following effects:

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The pre-deformation will be accentuated for the first segments, because the weight of these segments remains offset in relation to the plane containing the pile axis and tangent to the curve of the horizontal alignment of the bridge deck

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The construction of the following segments will gradually straighten out the pier until it becomes vertical, provided that the estimate was correct.

The accuracy of the result is dependent on the awareness of the rheology of the concrete used. The concrete must have consistent characteristics and the conditions of application must be the same throughout all stages in the construction of the pier shaft. Creep tests should therefore be performed on the chosen formula of concrete in order to obtain an accurate estimate of the deformations in the structure. 7.2.1.3 – Measuring points to monitor geometry

Several types of measuring points can be positioned on the structure in order to help the surveyor monitor the geometry. Each point has a specific function


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Rivets

A rivet is a measuring device with a rounded head. It is used to monitor the leveling of a horizontal plane. This inexpensive tool must be made from a hard-wearing, undeformable material designed to resist the effects of weather and impacts on the site. A sufficient number of rivets must be used in order to monitor the elastic curve accurately. Medallions

A medallion is a piece of rustproof metal which is fixed to the structure. It features a rounded upper support surface. The horizontal plane tangent to the top of the sphere supports the sight. Medallions are used to monitor the levelling of a vertical plane and cost approximately the same as the rivets. As for the rivets, exploitable data are only obtained if a sufficient number of medallions is used. This is particularly important if a bi-directional phenomenon is being monitored. Targets

Self-adhesive targets are discs marked with concentric circles which are attached to bonded metal elements or directly onto the concrete itself, depending on the type of model used. They are used for monitoring verticality in particular. Targets can be used to measure both angles of orientation of a viewing axis from a known reference marker. If repeated from three markers, this operation provides a good estimate of the position of the target point. A retroflective target gives a third component in space: namely the distance from the observation point to the measuring point. In certain cases, this component can increase the accuracy of the calculations, provided that this distance is no more than approximately 100 meters. Therefore, it is important to make sure that the type of theodolite used is compatible with the constant of the target in question, as this may vary from one model to another. The limitations of targets concern the mediocre accuracy of the measurements and the fact that their bonding fails after a few years. Furthermore, on uneven sites, it can be difficult to position three markers with clear visibility between each one. Prisms

Prisms are a little more expensive and are positioned on a bonded base in the form of a corner plate that allows the angle of the prism to be adjusted. A prism gives a direct measurement by defining the sighting angle and the distance to the measuring point from a single known reference marker to which it is permanently directed. The prism must be sited and angled in such a way as to reduce the damaging effects of dust, bad weather and birds. This type of measuring point has numerous advantages:

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Greater accuracy for a measuring point which is difficult to access

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Saves time for the surveyor

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Single measurement reduces the risk of error.


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As with targets, it is important to check that the modele of theodolite used is compatible with the constant of the prism in question, as this may vary from model to model. 7 . 2. 2 -

Monitori ng the geometry of the brid ge deck

A cantilever consists of several segments, which are manufactured, assembled and stressed at different ages, using a material whose characteristics and composition may vary over time. It is important to predict the exact extent of the deformation of the cantilever element in order to determine an adequate reverse deflection which will be implemented in the prefabrication unit if the segments are prefabricated or in the form travelers if the segments are cast in-situ. When the structure is isostatic, the deformation of the cantilever is due to:

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The weight of the concrete beam

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The weight of the form traveler or the assembly equipment

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The cantilever prestressing (Fig. 7.1).

The effects of concrete creep and the delayed prestressing losses are added to the instantaneous deformation. After the closure of the different cantilevers, when the structure becomes continuous and hyperstatic, the bridge deck continues to deform due to the following effects:

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The interior and/or exterior continuity prestressing

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The removal of the form traveler or the assembly equipment

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The removal of the temporary piers and the temporary cantilever stabilizing systems

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The erection of superstructures.


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Fig. 7.1 – Concreting curve for a single cantilever

Fig. 7.2 – Concreting curve for a standard bridge built by the cantilever method

Following this, the deformations due to concrete creep and delayed prestressing losses continue to develop. It is therefore necessary to build in a reverse deflection in order to compensate for these different types of deformations (Fig. 7.2). The calculation of this reverse deflection must account for the probable values of the different applied loads:

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The density of the concrete must be realistic for the calculation of selfweight

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The coefficients of friction for the cantilever tendons in a straight line and on curves will only increase very slightly

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The weight of the form traveler and the site equipment situated at the end of the cantilever will be evaluated as accurately as possible.

The modulus of elasticity for concrete varies according to the age of the load and the duration of this load. Therefore, it is always difficult to predict and control the deformations resulting from the construction of a cantilever with accuracy. This difficult problem obviously supposes that the actual position of the segment on pier in space is accurately known and is as close as possible to the theoretical position. For a more detailed consideration of geometric monitoring, it is important to distinguish between cast-in-situ segments and prefabricated segments. Indeed, a different frame of reference applies to each of these methods of construction. For segments which are cast in-situ using a form traveler, the frame of reference is absolute and relates to the pier, which is also globally referenced. The construction of a new segment makes it necessary to adjust the form traveler considering its position in space in relation to the overall geometry of the cantilever.


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For segments which are cast in a prefabrication unit, the frame of reference relates exclusively to the preceding segment. When a new segment is constructed, it is necessary to adjust the prefabrication unit in relation to the end of the segment that is postioned as a matched mold. 7.2.2.1 – Bridge decks cast in-situ u sing form travelers

M o n i t o r i n g of l e vel l i n g

The monitoring of the levelling of segments under construction is based on a document drawn up by the engineering firm called the "construction pyramid". This indicates the theoretical dimensions to be obtained for each stage in the progression of the cantilever and for each segment joint. These dimensions incorporate all of the forces that apply to the cantilever under construction. This pyramid is used to determine the dimensions for the adjustment of four key points on each segment joint: both cantilevered ends and both ends of the lower slab, i.e. points P1to P4 shown on Figure 7.3.

P6

P5

P1

P4 P2

P3

Fig. 7.3 – Key points for levelling adjustment

Two additional points will be chosen on site in areas which seem to be the least affected by secondary deformation, i.e. close to the webs, namely points P5 and P6 in Figure 7.3, which will be the first measurement reference to be determined on site. The other points will be verified in accordance with this reference. The only effect not to be considered in the engineering firm’s calculation is the deformation of the form travelers. This phenomenon must include the form traveler’s suspension bars. The construction of the first segment will incorporate a deformation assumption which will then be adjusted in accordance with the measurements taken after casting. This deformation assumption may include the deflection of the form traveler’s main girders if they were load tested during the acceptance inspection. A d j u s t m en t o f t h e f o r m t r a v e l er

The adjustment of the form traveler is carried out by localization using localized reference points in relation to the segment on pier, which is repositioned globally. When the form traveler has been moved forward in order to build segment Vn, and before the final tightening of its lashing onto segment Vn-1, the alignment of the formwork equipment and horizontal adjustment of the form traveler are verified by theodolite. The form traveler is now considered to be properly adjusted in relation to segment Vn-1 and the following operations are carried out:

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Inspection and adjustment of the leveling and horizontal alignment of points P5 and P6

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Inspection of the levelling of the cantilevers and adjustment of points P1 and P4 if required


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•

Adjustment and monitoring of the leveling of points P2 and P3 defining the height of segment V

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Inspection of the spacing of points P2 and P3, i.e.of the width of the bottom slab.

Once the form traveler has been adjusted, all of the reinforcements for the segments can be installed. Two corner irons are bolted onto the mask. These are marked with a punch and are integral with the concrete of the upper slab close to points P5 and P6. The upper face of these corner irons is horizontal and is situated 20 mm underneath the extrados for protection. This upper face is kept clear of the concrete during casting. Finally, an additional levelling inspection is carried out at the level of the mask. The internal formwork of the form traveler is adjusted:

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At the back, by placing it against the concrete of the previous segment

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At the front, by altimetric adjustment in relation to the external formwork at the level of the supporting beams (points P7 and P8).

M o n i t o r i n g t h e g eo m et r y

Once the casting of the segment Vn has been carried out, it is necessary to carry out a general inspection of the levelling of the cantilever. This operation begins as early as possible the following morning. This is the best time of day to avoid the manifestation of parasitic forces due to temperature. A cantilever that is subjected to a temperature gradient behaves like a highly sensitive bimetallic strip. The operation must be completed as quickly as possible. The corner iron is unbolted so that the mask can be removed. The corner iron becomes the measuring point for monitoring the deformations of the cantilever. The levelling measurement is carried out from segment Vn, which has just been concreted to the SOP, and is repeated at each joint of the segment. The values measured are recorded on the monitoring documents and are compared with the values given by the construction pyramid. By analyzing the deviations that are observed (which is always a difficult operation), corrections can be made to the adjustment of the form traveler for the construction of segment Vn+1. The levelling measurements are completed before the form traveler is moved forward and before the prestressing is tightened. Particular attention must be paid if heavy equipment such as a mobile crane is present on the cantilever that has already been constructed, in order to ensure that the measurements are taken under identical load conditions. Additional measurements can be performed in other phases if necessary. It is important to pay great attention to the consistency of the measurements taken on site and the information given by the construction pyramid. Any correction of the measurements must be performed after examining the assumptions and the data used to create the construction pyramid. If differences go uncorrected on the first cantilever, the risk of error becomes greater the closer we get to the closing segment with the cantilever already constructed The simultaneous monitoring of the geometry of the support is essential to ensure that the surveyor is always capable of relating the pyramid to the absolute dimensions of the objective to be achieved.


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Finally, it should be mentioned that for the construction of cast-in-situ segments using form travelers, the sequence of operations always starts on the same side of the cantilever in general. There is no point in accounting for this dissymmetry in the calculations if the form travelers are always adjusted on the basis of the measurements made in the morning. On site, however, this sequence of operations could be modified by reversing the order of casting if the levelling measurements reveal a systematic discrepancy between the two sides of the cantilever. 7.2.2.2 – Bridge decks consisting of p refabricated segments

During the setting up and commissioning of the prefabrication units in the prefabrication workshop or plant, a number of precautions must be taken to facilitate the work of the surveyor and increase its reliability. This means verifying:

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That the longitudinal axes of the units follow the same alignment, which is called the prefabrication axis

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That the points which are used to measure the alignment and to mark out the prefabrication axis are adequately spaced and perfectly visible from the units

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That the theodolite support frame is not exposed to hot sunshine, which reduces the size of the alignment corrections

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That the support for the theodolite and the level are correctly in line with the prefabrication axis and that these appliances are placed slightly above the upper level of the concrete of the segments

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That the reference point used for measuring the levelling is separate from the mask. This point could consist of a measuring point on the structural steelwork of the unit or a graduated rule permanently attached to a concrete post.

A d j u s t m e n t o f t h e m a t c h - c a st se g m en t

This is a complex topographical problem, as it involves constructing the independent segments on the ground in such a way that they can be assembled in space in accordance with the established geometric data (Fig 7.4).


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Fig. 7.4 – Adjustment of the match-cast segment

A prefabrication unit is described in Paragraph 6.2.1 and Figure 6.28 of this guide. Three parts of this unit can be considered from the perspective of adjustments and monitoring:

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An initial mobile part: the core or internal formwork of the segment, which leans against the mask on the one hand and the matched mold on the other; it requires no adjustment

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A fixed part, the mold or the unit itself, consiting of the mold base, the sides, wings and the mask; in theory, this is fixed and its assembly requires great precision

•

A second mobile part: the matched mold, consisting of the previous cast segment, placed on jacks used to adjust the angle and position according to the geometric data.

The choice of adjustment points is thus of paramount importance. Their positions are identified by corner plates bearing a punch mark or, better still, with a hemispherical indentation into which a ball bearing can roll. The recommended configuration for these corner plates is shown in Figure 7.5:

•

The adjustment points for levelling are situated in the areas least affected by deformations

•

The adjustment points for alignment follow the segment’s true axis.

The measuring points may also consist of small plates capable of housing a mini-prism.


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Fig. 7.5 – Adjustment points for a prefabricated segment

It is essential to position the points in such a way as to avoid any possibility of them being moved at elevation when the casting is completed and to set them back slightly from the joint in order to protect them. An additional precaution is taken for structures with a curved longitudinal profile or a variable transverse slope: the distance from the measuring points to the joint and the axis of the segment shall be identical from one segment to another. T o p o g r a p h i c a l i n s pec t i o n i n s t a n d a r d c o n st r u c t i o n c y c l e s

The construction of a standard segment Vn requires three topographical inspections to be carried out on segment Vn-1:

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Mapping of the levelling points and the prefabrication axis on Vn-1 in the prefabrication unit

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Adjustment of Vn-1 in the match cast position

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Mapping of Vn-1 in the match cast position after casting of Vn.

Two precautions must be taken during these operations:

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The surveyor must take the measurements before the removal of any of the formwork, as any impacts could compromise the accuracy of the measurements taken

•

It is advisable to check both alignment measuring points systematically before and after each operation, as slight deformations may affect the base of the theodolite (due to strong sunshine or instability).

The final inspection concerns the recording of measurements of length. A d j u s t m en t o f t h e seg m en t o n p i er

The segment on pier is particularly difficult to adjust. Accuracy is of the utmost importance because the accuracy of the geometry of the cantilever to be built depends upon this adjustment. In order to obtain a precise adjustment in space, it is possible to use angled blocks, wedges, bolts, etc. to make fine adjustments to the position of the segment. During the manufacture of the segment in the prefabrication unit, it is also important to provide secondary alignment measuring points which mark out the entire width of the transverse axis of the segment (Fig. 7.6).


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Fig. 7.7 – Adjustment of the segment on pier at ground level

Fig. 7.6 – Alignment markers on the segment on pier

These measuring points are used to make adjustments at ground level through the observation of both axes of the segment (Fig. 7.7). M et h o d f o r m o n i t o r i n g c h a n g es i n g e o m et r y

Base curves, namely the alignment curve and the two levelling curves i.e the longitudinal profiles given by the position of the rivets, are used to monitor and control changes in geometry. The effects of the weight and bevahior of the concrete, prestressing and superstructures are anticipated and compensated for via reverse deflections calculated by the engineering office. Although it is essential to incorporate these reverse deflections, their calculation is only approximate and uncertainties remain with regard to the accuracy of the geometry. In order to track the changes in the actual geometry of the cantilever, simultaneous digital and visual inspections are carried out in the form of graphs, base curves and the monitoring of lengths. These inspections obviously incorporate the adjustment dimensions that result from the measurement of the existing segments and are applied to the next segments to be manufactured. The monitoring may reveal a systematic lateral drift if the stressing of the cantilever tendons always starts from the same web. To counteract this, the order in which the cantilever tendons are stressed can be alternated. E x c ep t i o n a l c o r r ec t i v e a ct i o n s

It is possible that a mistake in the prefabrication might be overlooked on the prefabrication site, thus leading to a fault in the geometry of the cantilever during the assembly process. In the event of a major fault, the solution might be to disconnect one or more segments and carefully place wedges in the joints with the segments securely lashed together. The remaining spaces between the joints must be caulked before the segments are prestressed, taking care not to create obstructions in the sheaths that pass through the modified joints. This illustrates the importance of ensuring the accuracy of the geometry for prefabricated segments.


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7.2.2.3 – Comments concerning both techniques

C o m p a r i s o n o f d ef o r m a t i o n s

For the construction of both cast-in-situ and prefabricated segments, it is necessary to analyze each phase in the construction of the cantilever and to determine the deformation curve for each cantilever element, phase-by phase. An example of a five-segment cantilever is shown in Figure 7.8 hereafter. The line 1-2-3-4-5 represents the envelope of the different deformations, or the trajectory followed in space by the end of the cantilever at each phase of construction. By modifying the angular positions of the segments in small angles of α1, α2, etc., the cantilever may be constructed in such a way that once completed, it has a satisfactory longitudinal profile, as shown in Figure 7.9 for the example in question. In each section, the modified profile will effectively compensate for the future deformation.

Fig. 7.8 – Deformations phase-by-phase

Fig. 7.9 – Correction of the profile to compensate deformations

It is interesting to compare the relative importance of the deformations and reverse deflections affecting prefabricated and cast-in-situ segments. Figure 7.10 below shows these relative values for a structure designed according to these two methods.


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Fig. 7.10 – Comparison of deformations for prefabrication and casting in-situ

The design assumptions given in this figure show that in the majority of cases, the difference between the methods will be greater if a cycle of less than one week is used for the in-situ casting of the segment and if the prefabricated segments are stored for more than two weeks. Whatever the circumstances, the deformations affecting a cast-in-situ cantilever will normally be two to three times greater than for an equivalent prefabricated cantilever. I n s p ec t i n g t h e g eo m e t r y o f t h e c o m p et e d c a n t i l e v er

Once completed, the geometry of the cantilever must be inspected; this inspection must be carried out as early as possible in the morning. This involves plotting curves corresponding to all of the measuring points in order to obtain a geometrical representation of the area obtained. The advantage for a cast-in situ cantilever is that corrections are made after each casting. Corrections are therefore less likely to be needed when the cantilever is completed. If the entire cantilever is incorrectly aligned, it is possible to improve its geometry to a certain extent by using hydraulic jacks to alter the position of the segment on pier after the stitching tendons have been slackened off, if the cantilever is simply supported on the pier. This may require the modification of the adjustment pyramid for the adjacent cantilever if it has not yet been constructed. If the errors are significant, the general longitudinal profile will have to be rectified. If the cantilever is embedded on its pier, the misalignment can only be compensated for over the length of the closing segment. The lashing of the last cantilever segment to the opposite cantilever, or the loading of the higher cantilever may partly rectify this misalignment. However, such a manoeuvre is ill-advised, as it has the disadvantage of modifying the stress state that was designed for the structure. If the curves corresponding to all of the measuring points reveal localized irregularities, the following actions may be considered once the whole structure is completed and continuous:

•

It may be possible to plane off the humps, provided that a sufficient thickness of concrete remains over the reinforcements


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•

If hollows exist, they must first be outlined by a trench of at least 10 mm; they can then be filled with resin. After the application of the waterproofing, a first layer of asphalt will be applied and then planed in order to modify the profile, supposing that the profile has been accurately determined in advance and allows for thermal effects, for example. A second layer will then be applied. The disadvantage of such a process is that it limits the thickness of any new layers of asphalt that will be added to the pavement at a later date

•

An extra layer of asphalt may be sufficient to level out the irregularities, but this technique also limits the thickness of any new layers of asphalt that will be added to the pavement at a later date.

7 .2 . 3 – M o n i t o r i n g t h e g e o m e t r y o f t h e f i n i s h e d s t r u c t u r e 7.2.3.1 – The compromise between commissioning and infinite time

For 15 to 25 years after the bridge is commissioned, creep continues to affect the structure. After this time, the evolution of delayed deformation is normally no longer significant. Therefore, in the first part of the life span of the structure, deformations continue to affect the bridge deck. For the majority of structures, this is illustrated by a lowering of the reference axis, the extent of which varies according to the sections in question. The theoretical reference axis during construction must therefore be specified. If the theoretical longitudinal profile were to be obtained at the start of the bridge’s service life, this would lead to a gradual decrease in user comfort over the years, corresponding to the development of creep within the structure. On the other hand, a policy of aiming to achieve the theoretical profile in the long term would result in a structure that might be uncomfortable at the start of its service life, but this would improve with age. Therefore, a compromise can be sought: the target profile is considered to be midway between the profile when the bridge is commissioned and the long-term profile. The comfort of the structure can be optimized by adjusting the thickness of the surfacing. This may be adapted throughout the decades in which the structure is in use. It is essential that the chosen profile and the profile that is actually obtained are clearly defined in an appropriately referenced document. Both of these elements must form part of the as-built file and shall be included in the zero-point file for the structure. 7.2.3.2 – Monitoring the deformation of structures subjected to loading tests

For non-standard structures, these tests shall involve the systematic testing of all of the characteristic sections of the structure, i.e. at least the sections on supports and the sections of greatest flexion on the span. If the structure consists of a large number of almost identical spans, the testing of bearing sections can be limited to just a few spans. However, it is important to carry out a load test on the middle of each span. Although load tests involve no more than a simple observation, a number of special precautions must be taken. C a r r y i n g o u t t h e m e asu r em e n t s

When test trucks are used, it is important to start by loading the supports. The corresponding topographical measurements are recorded for information purposes. A zero-point measurement is then taken which is used to accurately determine the height differences between the supports and the middle of spans. The “Les outils” collection – Sétra

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For box girder bridge decks, it may be beneficial to perform precise levelling measurements from inside the box girder as this provides good measuring conditions and there is no interference from the trucks. Before performing the measurements under load, it is necessary to wait for the structure to stabilize. This period can be estimated at 10 minutes after the trucks have been positioned. In any event, the measurements cannot be taken until the trucks’ engines have stopped. The measurements are taken from points marked by studs or rivets with rounded heads which have been carefully positioned and provide a single point of contact with the base of the levelling rod. Laser sightings may also be performed. A n a l y zi n g t h e r esu l t s

The precise analysis of the results requires an adequate knowledge of the unladen thermal behavior of the structure is established, which also allows the thermal effects to be dissociated from the combined effects of the test loads. The deflection values obtained during the tests are considered to be satisfactory when the measurements taken coincide with the authorized range for the values calculated (1.1 times the probable values / 0.8 times the probable values), allowing for the uncertainty of the measurement. This range, which conforms to the value recommended by the "Guide des épreuves des ouvrages routiers" (Guide to Testing for Road Structures) currently being prepared by Sétra, corresponds to a design-based approach that is more accurate than the current methods. The design assumptions must not be too conservative. Instead, they should be as realistic as possible as physical quantities will be measured. In particular, it is important to account for:

•

Deformation due to shear force in the beams

•

The contribution of the superstructures to the rigidity of the structure

•

The law of the actual behavior of the materials derived from the testing of test samples

•

The rigidity of the supports, mainly with regard to rotation

•

The skew of the structures

•

The curvature of structures

•

The cracking of parts operating as reinforced concrete and the reduction of torsional inertia due to the cracking

•

The effective width of the slab on supports

•

The probable prestressing value (using the results of the measurements of transmission factors) for partially prestressed structures

•

The actual geometry of the structure, if on-site incidents have led to modifications.

If the deflection values are unsatisfactory, the results must be examined with a critical eye in order to find an explanation for the anomalies: non-linearity of the behavior, abnormal changes in the deflections measured for a The “Les outils” collection – Sétra

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non-standard section, etc. A new calculation must establish a range of theoretical values to be monitored on the structure, based on realistic high and low assumptions made for the significant physical values. In all cases, after performing a global analysis, the engineering office draws up a summary report. The Owner decides whether or not the deflections are satisfactory according to the Construction Manager’s recommendations.

7.3 - Inspection of temporary st ruct ures It is increasingly common for temporary structures to be listed in the first category under the terms of Article 41 of fascicule 65A of french CCTG. This does not include scaffolding, working platforms and protective structures. Article 42 of this fascicule defines the role of the Chargé des Ouvrages Provisoires (Head of Temporary Structures – COP). It is important to emphasize that the COP cannot, under any circumstances, replace an external inspection agency approved by the French Ministère du Travail (Ministry of Labour), either at the design stage or during construction on site. This important point should be clearly explained in the STC (Special Technical Clauses). 7 . 3. 1 -

Inspection of form travelers

The different phases in the inspection of a form traveler, from its design through to its use, are sequenced in the following way:

•

External inspection of the design calculations for the dimensioning of the form traveler focusing particularly on the load-bearing elements.

•

External inspection of the supporting beams using load testing to examine their elastic behavior and evaluate their remanent deformations, to examine the welds after loading and to measure and monitor the deflections.

These inspections are carried out by an organization approved by the Ministry of Labor such as french APAVE, VERITAS, SOCOTEC, etc.

•

External inspection by the Construction Manager who verifies that all documents guaranteeing the traceability of the different inspections are available in accordance with the company’s Quality system

•

Manufacturer’s inspection of the equipment in order to verify the conformity of the different manufacturing stages

•

External inspection of the conformity of the assembly of the form traveler on the segment on pier, carried out by an approved organization upon completion of the assembly and before casting.

It should be emphasized that an inspection by an approved organization involves much more than a simple visual inspection of the apparent condition of the equipment. This inspection must produce a general report, confirming that all of the inspections required for the approval of the equipment have been performed and guaranteeing that the construction is consistent with the design. It must be rounded off by a definitive acceptance report attesting to the conformity of the equipment i.e. its fitness for use.


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•

Inspection by the COP of the conformity of the assembly of the equipment on the segment on pier, performed upon completion of the assembly and before casting

•

Inspection by the COP of the validity of the procedure used to move the form traveler forward. This procedure will also be internally monitored on a systematic basis every time the equipment is moved forward.

Before the first time the form traveler is properly used, it is essential to define the maximum number of times the high-tensile steel bars forming the hangers can be used. This number shall be determined prior to the start of work on site (see paragraph 7.5). Finally, although this concerns the use to which the equipment is put rather than the form traveler itself, it is important to mention that the contractor shall perform an internal inspection of the adjustment of the equipment after it has been moved forward and before the reinforcements are installed. 7 . 3. 2 -

Trial assembly of a form traveler

This does not refer to an assembly carried out by the manufacturer of the equipment in the factory, but to a “test” assembly that is normally carried out on site, in order to test the supporting beams, which are the main structural components. These beams are assembled on a flat surface, in opposition with the insertion of jacks. The resulting deflections in these elements are verified under loads which increase progressively until the nominal load corresponding to the forces encountered when the equipment is in service is reached. 7 . 3 .3 -

Centerin g

Chapter IV of fascicule 65A of the french CCTG covers temporary structures, with centering specifically described in Article 45. 7.3.3.1 – Testing of bearing capacity of the soil

The testing of the bearing capacity of the soil for centering must be covered in the company’s QAP. It is essential to verify the soil that will support the centering. To this end, it is possible to perform a static plate load test defined by the standard NF P 94-117-1 of April 2000. This involves determining a value for the modulus of the soil by subjecting it to the action of a plate whose diameter and stiffness are normalized according to a standardized procedure, and then by measuring how far it sinks down. Plate load testing provides a model, but the design verifications for centering are expressed in the form of pressure exerted on the soil. For typical cases concerning 1 m x 1 m sole plates, with compaction limited to 3 mm and for soils with no underlying soft layers, the inspection of bearing capacity may be performed while assuming that the value of the modulus at the plate Ev2 is 350 times greater than the permissible soil pressure. In the case of vertical types of centering, it is important to be particularly attentive to the risk of differential compaction due to the presence of a large number of supports and to the effects of successive compactions on the geometry and stresses. 7.3.3.2 – Inspection of equipment

The inspection of the equipment used in the centering must be covered in the company’s QAP.


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It is important to highlight the dangers that could be caused by any laxity with regard to the inspection of the equipment in use. Any makeshift repairs could soon lead to a catastrophe. Therefore, the COP must certify that all of the elements used in the centering are fully functional. When inspecting the centering, two factors must be considered: the wear of the equipment and the extent of any corrosion. The wear of the equipment relates to its use on other sites. Fascicule 65A (Article 4 of Appendix B, in addition to Article 44) uses a coefficient α in order to reduce the bearing capacity of the equipment. The value of this coefficient can be as low as 0.75 for equipment with more than 10 consecutive uses. In the same way, the corrosion of the equipment due to its different uses and periods of storage is determined in the calculations using a reduction factor β equal to:

Corrosion condition

Absence of corrosion 1 Slight corrosion (presence of oxide modifying 0.95 the color of the element) Notable corrosion (small, thin particles of oxide) 0.85 Major corrosion: the element cannot be used The two coefficients α and β are accumulated. 7.3.3.3 – Inspection of the erection

The inspection of the centering erection must be covered in the coontractor’s QAP. The inspections must target the areas of greatest risk in the centering structures. Therefore, the following areas and inspections must be covered: At the base of the centering:

•

Verify that the manufacturer’s recommended runout ranges for the jacks are not exceeded

•

Check that drainage has been installed on the platform in order to channel the runoff

•

Make sure that improvised extensions and wedges are not used.

In the intermediate section:

•

Inspect the bracing on towers, posts and girders

•

Verify the convergence of the joints in the centering structure

•

Makes sure that the horizontal forces are correctly taken up.

At the top of the centering:

•

Follow the same precautions as at the base


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•

Verify that the structural beams or girders are correctly centred in the forks

•

Make sure that any risk of accidental relative movement is avoided above the metal-to-metal contacts.

At the level of the planking:

•

Check that the load transmission areas have been stiffened

•

Verify that there is no irregular and/or complex stacking

•

Check the reliability of the support conditions

•

Verify the existence of lateral limit stops if there is a danger of slippage

•

Make sure that bracing is fitted in all directions.

7.3.3.4 – Inspection of deformations

It is important to inspect any deformations of the centering throughout the entire casting phase and to monitor the compaction of the supporting soils. A simple way to monitor this compaction consists of attaching vertical bars under the centering which meet a fixed independent marker at ground level. The monitoring of deflections is designed to:

•

Verify whether the final target profile will be obtained

•

Detect any anomalies that might be indicative of an imminent accident.

7.4 - Inspecti on o f c oncr ete Article 76 in fascicule 65A of the french CCTG defines all of the elements involved in the inspection of the manufacture and application of concrete. 7 . 4. 1 -

Inspection of the compo nents

Article 23 of fascicule 65A describes the inspection procedures for the constituents. The following additional points should also be noted. 7.4.1.1 – Inspection of the mixing water

The mixing water must conform to the standard P 18-303 of August 1999 which prescribes appearance tests, chemical tests and defines the testing methods, the frequency of these tests and how to evaluate the conformity of the water being tested. Drinking water usually conforms to the standard. However, under specific climatic conditions, particularly in tropical regions, drinking water may not conform to the requirements of the standard. The use of seawater is strictly prohibited.


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7.4.1.2 – Monitoring of cement

The acceptance inspection involving the rapid identification test defined by the standard P 15-466 of August 1983 is used by the contractor to verify the conformity of the cement at every delivery. Precautionary samples are thus taken. External monitoring tests in the framework of the standard P 15-300 of December 1981 enable to contractor to confirm this conformity. For example, these tests may involve the following operations:

•

Measurement of specific surface

•

Determination of strength after 2 days

•

Measurement of anhydride sulfurique SO3 content, or Na20 equivalent, or shrinkage test for cements exposed to the action of deicing salts

•

Measurement of hydration heat, tricalcium aluminate C3A content, or shrinkage test if these characteristics are significant, etc.

External inspection tests are carried out by the Construction Checker and its laboratory. 7.4.1.3 – Inspection of aggregates and additives

Internal inspections must be carried out at two levels: by the producer and by the contractor. Their main elements are: • For sands: Sand equivalent, particle size, fineness, hygrometry • For chippings and pebbles: particle size and cleanliness • For aggregates: sulfate, sulfide and chloride content • For additives: expiry date, characteristics specified by the standards. The contractor’s external inspection consists of an acceptance inspection and possibly spot checks corresponding to tests laid down in the standard P 18-540 of October 1997. Inspections of storage conditions for the aggregates and additives may be added to these. 7.4.1.4 – Monitoring the alkali reactivity potential of the aggregates

Monitoring the alkali reactivity potential of aggregates is a complex task which is covered in detail in the “Recommandations pour la prévention des désordres dus à l’alcali-réaction" (Recommendations for the Prevention of Problems Relating to Alkali Reactivity” published by the LCPC (Central Public Works Research Laboratory) and in the appended document "Guide pour l’élaboration du dossier carrière" (Guide to the Creation of the Quarry File). Furthermore, Sétra’s "Guide pour la rédaction des pièces écrites des marchés" (Guide to the Drafting of Written Documents for Contracts) concerning the "Prevention of Problems Relating to Alkali Reactivity" lists the clauses to be inserted into the written documents for the DCE (Document de consultation des entreprises) [Contactor Tender Document] and the contracts.


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Considering these two documents, major structures are usually placed in category C. As a consequence, only NR (non-reactive) or PRP (potentially reactive to pessium effect) aggregates shall be used in their construction. 7.4.1.5 – Ettringite formation in concrete

Presentation

The deterioration of concrete due to sulfates has been known about since the beginning of the 20th century. It may have internal or external origins. If it has external origins, it is caused by industrial emissions, the presence of sea water or the action of constituents of the soil in contact with the concrete (this applies to gypsum, for example). This is a known effect and the recommendations of the standard NFP 18-011 of June 1992 help to control it via the choice of cements and the concrete formulas. If the problem has internal origins, it bring sulfates in the cement into play. These consituents are required for slowing down the setting of the concrete. As the problem appeared relatively recently, less is known about it. Indeed, its existence was only reported in 1986, concerning problems in prefabricated parts which look similar to those caused by alkali reactivity, but which are, in fact, a different type of pathology. This sulfate reaction of endogenous origin, which modifies the hydration reactions of concrete, has been observed in many countries, although France has only recently started to pay serious attention to it. It is true that it remains a limited phenomenon, as it only concerns around 10 structures in France at present. However, as certain application conditions and certain current trends appear to be favorable to its appearance, it wouild appear to be important to summarize the current levels of knowledge about this problem ([DIV 98] and [DIV 00]). Parameters

While the alkali reactivity of concrete is governed by the three main parameters of moisture, alkali content and reactive silica content, ettringite formation in concrete seems to be a more complex phenomenon. Moisture is essential to the process. Parts of the structure which are subjected to alternate wetting and drying, such as those exposed to differences in water level, are all the more vulnerable. Temperature increases are equally important. Different authors suggest different upper limits, and their recommendations relate more to a precautionary approach than to objective studies. However, this temperasture range does depend on the type of cement used. A low sulfate and alkali content allows for a higher temperature limit. The composition of the cement is important in terms of the contents of three of its constituents: tricalcium aluminate, sulfates and alkalis. The type of aggregates must finally be mentioned. Indeed, with a limestone aggregate, the cement mixture has better adhesive properties, which seems to prevent interstitial dissolution. It is noticed that there is a greater number of parameters than for alkali reactivity. This indicates that it is a more complex, though less frequent phenomenon. P r o b l e m s , a n a l y se s a n d p r e ca u t i o n s

The problems observed have three underlying causes:


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•

Construction systematically carried out during the summer, a period in which the temperature of the concrete is higher

•

Use of cements that are too rich in high-risk components

•

Use of highly exothermic cements.

This demonstrates the importance of choosing the correct type of cement. Indeed, the clinkers in modern cements are ground increasingly finely, rendering the mixture particularly volatile and subject to large increases in temperature. This risk seems to increase for the construction of bigger parts; in this case, particular care should be taken to choose less exothermic cements. There are certain statutory texts which cover this problem at least partially:

•

Standard ENV 13 670-1 of September 1999 (construction of concrete structures). Paragraphe 8.5 "Cure et protection des bétons coulés" (Curing and Protection of Cast Concrete) limits the temperature in large parts to 65°C

•

The European pre-standard Pr EN 13 369 of 1999, which requires testing and recommends very low temperature limits.

In this respect, France is far behind many other industrialized nations. Based on the current state of knowledge, it would seem to be advisable to check the moisture levels of the environment; in a dry environment, the risks are obviously very small, but in a wet environment every precaution should be taken to ensure that the temperature of the concrete does not exceed 65 to 70° C and to opt for cement with a low alkali content. Indeed it would not be reasonable to allow concrete temperatures to reach 80 or 90° C as this could risk reducing the life span of our structures. 7.4.1.6 – Suitability test and control test

The suitability test is used to verify the likelihood that the nominal formula of the concrete and its implementation conditons satisfy the requirements of the contract in terms of its strength and the application conditions. It is particularly useful for verifying the concrete resistance to frost and the effects of deicing salts, where necessary. Article 77.1 of fascicule 65A provides information about the nature of this test. The control test is used to verify the conformity of a batch of concrete. The control procedure is detailed in Article 77.2 of fascicule 65A. 7 . 4. 2 -

Informatio n tests

7.4.2.1 – The main information test

The aim of the information test is to verify that, under actual hardening conditions, especially with regard to ambient temperatures, the strength achieved at an early age j, corresponding to a very precise phase of construction, is above a predetermined value f cj . This value f cj is establishjed after analysis in order to make sure that the strength of the concrete us compatible with the construction procedures set out in the QAP, particularly in the casting program or prestressing program.


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For example, these tests shall concern:

•

Removal of formwork from pier shafts

•

Removal of formwork from pier crossheads

•

Removal of formwork from lateral overhangs on the bridge deck cross section

•

Removal of formwork equipment from inside the box girder

•

Removal of falsework from parts of the structure built on centering

•

Stressing of certain tendons (generally cantilever tendons): in this case, the tensile strength value f tj is just as important as f cj

•

A temporary or permanent loading operation.

The information test may also have other aims, e.g. to verify that the compactness of the concrete, a genuine guarantee of durability, conforms to the expected values. 7.4.2.2 – Additional information tests

Additional tests are normally carried out as of the 7th day in order to make sure that the required level of strength at is likely to be obtained at 28 days. A third type of information test may be performed in order to obtain information about characteristics that are not specified in the contract such as the modulus of elasticity, tensile strength (if this was not evaluated in the main information test), long-term strength, creep effects, etc. In this case, these tests could be considered to be control tests. 7.4.2.3 – Conditions of execution

The test samples used fore the information tests shall be obtained and conserved in accordance with the provisions of the standard P18-405, and of articles 1.5 and 6.3 of the P18-504 documentation booklet. The concrete shall be cured in accordance with the recommendations of Article 74.6 of fascicule 65A. 7 . 4 .3 -

Maturo metr y

7.4.3.1 – The principle of maturometry

Maturometry is based on the existence of a relationship between the quantity of heat released by concrete when setting and its mechanical properties. If the same type of concrete is subjected to different thermal conditions but releases the same amount of heat since the start of the manufacturing process, it will have the same mechanical strength. In physical terms, this is shown by the fact that the hydration levels of the concrete will be identical.


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Arrhenius’ Law is used to express the changes in the strength of concrete according to a temporal parameter: the concept of equivalent age, which is independent of temperature. The concrete is characterized in an instrinsic manner using a “reference curve”. 7.4.3.2 – Practical evaluation of strengths

The reference curve is established by measuring the strength of standardized test samples stored at 20°C on precise dates. The thermal monitoring of the concrete in these test samples is used to calculate an equivalent age of this concrete at the desired reference temperature on each of the test dates at which the strength is tested until breaking. The apparent activation energy of the concrete must be known for this last operation. This depends on the type of cement used and is adjusted according to the temperature. In addition, thermistors linked to a central readout station are implanted in the concrete of the most recently cast part of the structure and the temperature is recorded at regular intervals. To exploit the data from these measurements, it is necessary to: • Transform the actual age of the concrete into an equivalent age (time required for the concrete to develop the same degree of hydration under isothermic conditions at 20° C) • Read off the strength value corresponding to this equivalent age on the reference curve for the concrete. This evaluation can be performed directly by the maturometer, which shows the level of strength obtained in the concrete at the different positions where the thermistors have been implanted. 7.4.3.3 – Advantages of maturometry

In practice, the temperature histories of the test samples used for the information tests are very different to those of the concrete that forms part of the structure. Therefore, there is a risk that the strength measurements obtained for these samples may not be representative of the actual situation. Furthermore, the validity of the laboratory compression test is affected by the method used to remove the samples from their container. This illustrates the advantages of maturometry, because this process is not affected by different temperature histories and only uses the total quantity of heat. It also shows the advantage to measure the maximum temperature obtained in the middle of the concrete. The significant reduction in the number of test samples required is also an advantage of this process, although it is essential to use a sufficient number of samples to guarantee the validity of the measurements. For every five findings by maturometry, one test sample shall be verified using a conventional information test. 7.4.3.4 – Requirements for the use of maturometry

The method requires the use of an activation energy value that corresponds to the type of cement, to any additives used (e.g. extra fillers), and to the external ambient temperature. This activation energy will then be confirmed or adjusted by referring to the strength/equivalent age curves obtained from the analysis of test samples.


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These tests are performed on samples stored at different temperatures, e.g. externally, in the laboratory, in an isothermic box kept in the laboratory, etc. Calibration may be quite a lengthy process; it will therefore be necessary to allow a sufficient period of time for this process starting from when the concrete formula has been specified (3 to 4 months may be required). It should also be noted that the maturometer must be recalibrated at regular intervals (see manufacturer’s instructions). As far as the measurements themselves are concerned, the first choice to be made is to determine the critical areas of the structure in which the evolution of strength is representative of the operation to be tested. If the aim is to determine the age at which the concrete is strong enough to resist the removal of formwork, instruments will be placed in the area in which the last mix of concrete was added. It is also necessary to position thermocouples so that information can be obtained for parts of the bridge deck box girder, for example. Remote temperature sensing elements are used to evaluate the mass effect in the webs, which helps the maturing process, and measure heat losses in the upper slab close to the edges. These can be positioned according to the plan shown in Figure 7.11 below, for example.

Fig. 7.11– Maturometry: an example of a layout for temperature sensing elements

7 . 4. 4 -

Inspecting the application of concr ete

7.4.4.1 - Application

For the construction of cast-in-situ segments, the planned sequence of operations generally starts at the same side of the cantilever. This sequence of operations can be modified by reversing the order of casting if the levelling measurements reveal a systematic discrepancy between the two sides of the balanced cantilever. The inspection of the application of concrete must focus on the following points:

•

Cleanliness of the bottom of the formwork

•

Availability and condition of the equipment required

•

The release of pressure from the jacks used to adjust the matched mold for segments prefabricated in the prefabrication unit

•

The conformity of the composition of the concrete shown by the information given on the delivery slip. It is essential that the delivery slip shows the following information: the differences between the theoretical weight and the actual weight of the constituents expressed as a percentage, and the water content of the aggregates so that the total quantity of water can be recalculated


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•

The slump test and the estimate of entrained air. The slump test is difficult to perform when the concrete is in liquid form: the wattmetric readings of the output of the mixer motor are therefore an important source of information and these readings must be attached to the delivery slips. The presence of this information will be verified during the acceptance inspections

•

Compliance with the casting plan for the application of the concrete (fascicule 65A, Article 75.3): maximum time from the end of the manufacturing process to the completion of casting, casting phases, height of drop, vibration, floating of the upper face of the lower slab for which formwork was not used

•

At the end of casting: surface condition, possible use of thermal protection devices, cleaning of starter bars, and treatment of construction joint surfaces

•

Check that any thermal devices are operating correctly.

Fig. 7.12 – Precautions to take for the casting of segments

It is particularly important to comply with the casting plan. Figure 7.12 above shows the main precautions to be taken. An addtional precaution to be taken for the casting of very tall segments is to use tubes which are designed to reduce the height from which the concrete is supplied, therefore minimizing the risk of segregation. These tubes can be shortened as the operation progresses. For high-quality casting, the vibration of the concrete must be carefully controlled in order to eliminate segregation and voids. The main precautions to be taken are summarized in Figure 7.13 below. It is particularly important to make sure that vibration is not carried out in the immediate vicinity of prestressing ducts. As a precaution, guide channels can be used in conjunction with the vibrating needle. These consist of expanded metal mesh cylinders which are securely attached to the reinforcements.


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Fig. 7.13 – Precautions for the vibration of concrete

The final inspections for the application of concrete consist of:

•

Verifying that the required levels of strength have been obtained before authorizing the removal of formwork

•

Performing a general inspection of the segment, either at the extremity of the cantilever or in the reception area for a prefabricated segment.

After the inspection of a prefabricated segment in the reception area, a specific identification number is painted onto the concrete of the web inside the box girder on the side of the mask. Special conditions apply to the manufacture and application of concrete if the temperature recorded on the site falls below 5°C or rises above 25°C. In this case, special provisions must be made to produce concrete at a temperature of approximately 15 to 20°C. Upper and lower limits of 5°C and 40°C must not be exceeded during the manufacturing process. These provisions are described in detail in "Procédés généraux de construction - tome 1" (General Construction Procedures-Information Document 1) by J. Mathivat and C. Boiteau, and "Le béton hydraulique - mise en œuvre" (Hydraulic Concrete – Application) by J.M. Geoffray [GEO 96]. The recommendations of Article 74.3 of fascicule 65A must be followed for the treatment of construction joints. 7.4.4.2 – The test segment

It is always essential to construct a test segment. This may be reduced to a half-test segment for economic reasons. This element must be manufactured under actual site conditions in order to give an accurate picture of the problems encountered in the casting of segments. This element must be truly representative of the difficulties that are likely to be encountered on site. Therefore the test piece will be artificially subjected to the full range of potential problems. Therefore, this segment will normally be at least two meters long in order to include a prestress anchor block. The “Les outils” collection – Sétra

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The following points will be verified in particular:

•

Conformity of the facings (color, appearance, etc.)

•

Casting methodology (application, vibration, drying)

•

The setting behavior of the fresh concrete in lower slabs without upper formwork

•

The practicality of the implementation of internal ducts, transverse single-strand tendons and reinforcing bars, particularly in problematical areas

•

It will also be verified that the physical references for the adjustment of the alignment of prestress tendons are accurate, unambiguous and fully understood by everyone working on site.

Differences and changes in temperature over time may also be recorded. Samples may also be extracted for subsequent testing. A specific procedure must be developed for the construction of the test segment. 7 . 4. 5 -

The application of high-perfor mance concr ete

A certain number of specific precautions must be taken with regard to the manufacture and application of high performance concrete. The concrete plant must be capable of handling silica fume, and staff must be properly trained. When transported in the truck mixer, the concrete behaves like a fluid, with the associated risks of spillage. Vigilance is therefore required. In view of the fluidity of HPC, the consistency of the fresh concrete must be inspected on site by measuring its spread on a flow table in accordance with the P 18-432 and NF EN 12350-5 standards. Slump test measurements are neither appropriate nor accurate for fluid concrete. The application of the concrete requires completely leak-tight formwork, as the slightest loss of laitance could result in segregation. The floating and levelling of the surfaces must be carried out by teams of workers aware of the viscous and self-adhering behavior of this type of concrete. In the case of a structure with variable inertia, the angle of the lower slab on segments close to the piers could be steep enough to cause the concrete to slide down the slope. Formwork must then be erected on the extrados of the slab. It may also be beneficial to use concrete of different consistencies for the webs and slabs. Full-scale testing is required to verify this. The curing of HPC must also be more meticulous and more intense than for ordinary concrete. HPC is more sensitive to the effects of drying, because as the water contained within the concrete has been used up in the hydration process, there is practically no more free water. While this constraint improves the durability of the concrete, it also increases the risk of cracking in areas where shrinkage could be obstructed. Therefore, it is important pay attention to the proportion and design of the non-prestressed reinforcements, using small diameters and reduced center distances between the bars. This is particularly important for the construction of cast-in-situ segments using the cantilever method.


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7.5 - Inspection of form wor k The inspection of the formwork on the form traveler or in the prefabrication unit covers the following points:

• Numbers and indices of the plans used (which must be approved plans) •

Cleanliness of the work area and flushing out of the formwork bases before installation of reinforcements

•

Systematic verification that any laitance deposits on construction joint surfaces have been purged and cleaned prior to the closure of the formwork

•

Stability of centering, stem clamps and of the rigidity of the equipment

•

Condition of the equipment: structural condition, flatness of panels, absence of distortion at the edges which form joints, leak-tightness of different seams and foam strips, cleanliness, elimination of traces of laitance and oxidation, and oiling of outer skin

•

Tolerances of panels for unevenness and the conformity of the dimensions in relation to plans

•

Hydraulic jacks positioned under the segment acting as a matched mold - straightness, squareness of application of load and performance of lock-nuts

•

Conformity of inserts and fixings in relation to the plans: positioning, dimensions, type and number.

For the construction of cast-in-situ segments, it is very important to scrupulously observe the procedures governing the movement of formwork tools and systematically inspect the supporting and attaching elements. The rigorousness of the inspection procedures reduces the likelihood of accidents. In particular, systematic visual inspections of the hangers are carried out in order to check for straightness and the absence of impact marks or welding spots. Checks shall also be performed to ensure that bolts are easy to tighten and that they act squarely on the plates. If there are any doubts, the hanger is be scrapped – or better still, cut into one metre long pieces and replaced. Furthermore, the hangers will be reused no more than 30 to 40 times and they will be automatically replaced when this limit is reached. The number of uses could be painted on the hanger after each operation in order to monitor their use. Needless to say, the hangers must be new at the start of construction on site.

7.6 - Inspecti ng t he reinforc ements The following list describes the main inspections to be performed concerning the reinforcements for a segment prior to casting:

•

Inspection of the numbers and indices of the plans used

•

Acceptance inspection of the reinforcement bars: existence of the approval certificate, condition of the bars (straightness, rust, dirt,), compliance with reinforcement bar list, welds

•

Verification of conformity to the reinforcement plan: diameter, length and bending


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•

Inspection of implementation: position, coating and setting, solidity of binding of blocks which must be clean and made from concrete, absence of contact between these bindings and the near wall, length of overlapping between the reinforcement bars and the starter bars

•

Inspection of stability, rigidity and strength of the entire reinforcement cage with a view to casting. The bar chairs should not be used to improve the rigidity of excessively flexible cages. It is better to use frames to make stiffening trusses.

It is important to pay particular attention to the hooped reinforcements required due to the application of concentrated forces. The accuracy of the positioning of the hoops on the prestress anchorages must be carefully inspected. The centering on the trumplate must conform to its theoretical positioning, with the minimum deviation allowed from this position. The scale of the plans used for these areas must be large enough to allow for the precise management of the positions and sizes of the reinforcements. Three-dimensional modeling may be used for certain critical areas. The test segment (see 7.4.4.2) will provide important information about the feasibility of these arrangements. For parts whose formwork has one or two lines of symmetry but whose reinforcements are asymmetrical, it is important to verify that the reinforcement cage is not inverted. The inserts designed for the structural element in question will also be inspected.

7.7 - Inspection o f prestr essing Prestressing is carried out under the supervision of a specially qualified manager called the CMP (Chargé de la Mise en Précontrainte - Prestressing Manager), whose expertise is recognized by the Construction Manager and approved by the distributor of the prestressing process used. Article 95.1 of fascicule 65A describes the CMP’s role. 7 . 7. 1 -

Inspection of materials

7.7.1.1 - Procurement

The delivery conditions for the prestress reinforcements conform to Article 5 of fascicule 4, Titre II. The renewal of the protection is carried out in compliance with the stipulations of Article 66.2 of the supplement to fascicule 65A. The delivery of all of the components of the anchorages must be protected according to the provisions of Article 92.1.2 of fascicule 65A and Article 71.2 of its supplement. These conditions must also comply with the provisions of Article 3-5 of Circular no. 86-64 of September 4 1986 concerning the delivery conditions for parts and the checks to be performed. 7.7.1.2 - Storage

Storage procedures are defined in Article 92.1.3 of fascicule 65A and Article 76.2 of its supplement, and also in Articles 4.1 to 4.3 of Circular 94-34 of April 19 1994). The storage conditions for prestressing elements must be rigorously inspected and particular attention should be paid to the following points:

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The hard steel, factory-oiled prestressing cable reels must be stored in enclosed and well-ventilated premises. They shall be placed on battens or pallets in order to keep the rims off the floor


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•

Prior to use, the parts can be stored temporarily under a shelter with a removable roof or under ventilated tarpaulins, making sure there is no contact between the tarpaulin and metal. Wood with a high tannin content must not be interposed between the reels, as this could affect the oiling

•

Anchorages must be stored under a ventilated shelter and their crates or packaging are placed on battens or platted to avoid contact with the ground. Key bolts are kept in their original packaging until use

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Ducts are stored in bundles under ventilated tarpaulins. They are also placed on battens or pallets to avoid contact with the ground. 7 . 7. 2 -

Inspection of impl ementation

The accuracy of the installation of the prestress components is dependent on the methods used on site. It is unrealistic to demand accuracy greater than 5 mm. The calculations and construction provisions must take account of this. 7.7.2.1 – Geometry of ducts

The inspection of the installation of the ducts is a complex matter. Indeed, problems regularly arise on site due to inadequate positioning. This applies to the internal continuity tendons which may move during casting if they are incorrectly attached. It may also result in the spalling of the lower slab due to delamination under the parasitic radial tendon forces that are thus generated (Fig. 7.14 and 7.15).

Fig. 7.14 – Risks related to inaccurate duct geometry

Fig. 7.15 – Spalling of the lower slab/Angled view from below

These problems must be borne in mind when designing the reinforcement plans, with regard to the systems used to fix the internal continuity tendons in place and during the construction and implementation of these systems. A spacing of approximately 0.75 m must be allowed between the ducts supports. These systems are designed to align the ducts perfectly in accordance with the prestressing plans. Rigid sections of ducts can be specified around joints with a view to maintaining the continuity of this alignment. The same precautions must be taken for the cantilever tendons close to the facing on the upper slab.


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Problems due to the incorrect geometrical positioning of the ducts are also observed on the external tendons. These problems normally concern two phenomena: the localized spalling of the concrete due to parasitic forces resulting from badly positioned deviators, or an excessively high number of breakages of single-strand tendons when the structure is stressed and if parasitic angular cracks have developed close to the anchorages.

Fig. 7.16 – Spalling of a deviator when the structure is stressed

Fig. 7.17 – The deviator after cleaning

To prevent these problems, it is first of all important to make sure that an efficient procedure exists for adjusting the angle of the deviator tubes. As many basic errors are still committed on sites – e.g. inverting the deviators – it is important to carry out general inspections and to take a regular overview. It is also important to make sure that the beams situated at the base of the deviator beams are correctly reinforced, including their edges, because the large-diameter bars which are commonly used require large bending radii. Figures 7.16 and 7.17 show the problems caused by an incorrect reinforcement of one of these edges on a deviator beam. These problems could have been minimized by the addition of small-diameter non prestressed reinforcement bars positioned close to the facings. Finally, deviator tubes should be designed in such a way that the pressure exerted by the tendon on the concrete is directed into the body of the structure and stops before the facings in the coating areas. Accordingly, the concrete formwork tubes must be bell-mouthed so that the curvature of the tendon effectively starts around 10 cm from the facings (Fig. 7.18). The reader is advised to consult the Sétra document entitled "Précontrainte extérieure" (External Prestressing), published in February 1990. The geometrical accuracy of the ducts is very important. Therefore, in the contract documents, it is necessary to designate a person in charge of monitoring the geometry of the prestress ducts before and after casting.

Fig. 7.18 – Cross-section of a deviator tube 7.7.2.2 – Installation of the prestressing components

The following inspections must be performed during the installation of the prestressing elements:

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Verification that the sleeves of the thin metal ducts are correctly tightened: the prefabricated lengths of ducts with sleeve extensions are designed to provide a satisfactory fit at the level of the facing


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Verification that the alignment of the ducts and their attachments conforms to the plan (Fig. 7.19)

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Verification that the ducts are in good condition and that there are no significant distorsions or out-ofroundness

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Adjustment of the duct supports in relation to formwork if necessary

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Inspection of the attachment of all ducts to the non-prestressed reinforcements

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Accurate inspections of the alignment of the ducts, particularly in the areas of deviation close to the anchor block attachments (Fig. 7.20).

In general, this inspection must be carried out according to unambiguous physical references which are fully understood by everyone working on the site. It is assumed that the plans show dimensions relating to existing physical objects or lines marked out on site. In areas of large deviations (emergence of tendon from anchor block, deviation area close to an anchorage, etc.), in which it is better to use rigid tubes rather than thin steel ducts, it is possible to mark out the outline of the curvature of the duct by making a light saw mark on the end sections of tube. This tip can also be applied to the deviator tubes for the external prestressing ducts.

Fig. 7.19 – Conflict between non-prestressed reinforcements and ducts

Fig. 7.20 – Verification of a duct alignment in space


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When completed, the installation of the ducts and the anchorages must be verified by a competent surveyor who must check the following points:

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That stiffening devices are used to ensure a continuous alignment of the ducts around the joints, without sudden changes of angle. These stiffening solutions may be used for the ducts throughout the entire length of the segment to be cast; the angles at which the ducts emerge from the joint sections are inspected for compliance with the values shown on the plans

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Use of coupling sleeves and the adhesive sealing of the sleeving joints

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Fixing of the trumplate to the formwork: verification of orientation, rigidity of the fixing and angle of the injection hole

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Fitting of the duct in the end of the trumplate via a coupling sleeve

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Absence of sudden changes of angle, i.e. verification of the coaxiality of the different elements and of the adhesive sealing

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Distribution of vents and the positioning of the vent outlets in the facings.

For the external prestressing, it is necessary to use temporary supports for the ducts in order to make sure that the tendon is in the correct position before it is stressed (Fig. 7.21). The number and spacing of the supports will depend on the number and strength of the prestressing units. A spacing of 4 to 5 m is typically used. Should transverse prestressing be required, it is particularly important, given the thinness of the parts, to verify the accuracy of the duct alignment and the presence of a support system designed to maintain this alignment (Fig. 7.22). Furthermore, as tranverse reinforcement made from small-diameter bars is likely to be significantly t distorted during casting, it is advisable to stiffen the reinforcement cages in order to make sure that the ducts remain in the correct position.

Fig. 7.21 – Support system for HDPE ducts for external tendons

If the prestressing passes through the webs, the type and frequency of the tendon supports should be carefully inspected in view of the forces exerted during casting; this also applies to the ducts left empty in reserve (Fig. 7.23).


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Fig. 7.22 – Inspection of supports for transverse prestressing

Fig. 7.23 – Supports for prestress ducts in the webs

It is also important to make sure that “removable” elements can actually be removed, especially when double ducting is used on the deviators.


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Finally, before casting, it is highly advisable to verify that the ductsducts have not been blocked by any foreign objects: bottles, cans, tools, etc. This inspection can be carried out by flushing a ball through the sheath with compressed air. 7.7.2.3 – Visual inspection of strands

It is essential to reject strands showing signs of pitting due to oxidation, scratches, nicks or other defects likely to affect their mechanical strength. A corrosion table specifying up to five levels of corrosion, of which only the fifth level was unacceptable, was once used by certain contractors. We consider this table to be far too lax and believe that corroded reinforcements should never be accepted. 7.7.2.4 - Threading

Strand-by-strand threading is the most commonly-used threading method for bridges constructed by the cantilever method. During this operation, it is particularly important to make sure that:

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The strands have been sufficiently well oiled in order to reduce the friction forces in the later phases

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The distance between the pay-off reel and the cable pusher is kept to a minimum and that, over this distance, the strand is well protected by a tube or duct

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The cable reel has been placed in the static pay-off reel in such a way that the lay of the coil being unwound lies in the same direction as lay of the stranding, so that the strand tends to pull itself tight while is unwinding

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Each tendon is made up of strands from the same supplier

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The additional length required for the stressing has been allowed for before the strand is cut to length

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The strand has not been marked by the rollers on the cable pusher; this particularly applies to the last strands to be threaded, when the pressure of the rollers tends to increase

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A thick wooden shield is securely fitted over the strand outlet in order to protect staff

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Particular attention is paid to the threading of the last two strands of the tendon, as these are always the most difficult to thread through the duct (for long tendons, i.e. over 50 m in length, speed of threading is preferred to force in order to prevent the jamming of the last strands on the approach to the outlet)

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After the threading of every strand of an external prestressing tendon and the positioning of the anchorages, it is important to verify that the entire tendon can move inside the duct. This helps to position one strand in relation to another and also helps to check that there are no blockages. 7 . 7. 3 -

Inspection of stressing

7.7.3.1 – Inspections prior to stressing

Lubrication, oxidation and cleanliness play a vital role in the correct installation of a traditional strand-jaws anchorage.


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The anchorage must be properly lubricated if it is to work correctly and the lubrication conditions must be specified in the internal procedures recommended by specialist distribution companies. The exact type of lubricant to be used must also be specified. Oxidation must not be considered to be acceptable in the mechanical assembly of the anchorage. Finally, cleanliness is essential and a there must be no laitance whatsoever. These three parameters are vital for the correct operation of the conical wedging of the strand-jaws in the anchorage. In addition to the fact that successful stressing depends on the quality of casting in the anchorage area, it is also important to verify the following points when fitting the anchor head on the tendon:

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That the strands are clean

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That they are disentangled if necessary, so that there are no brakages due to crossed strands near the anchorage

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That the conical holes in the anchor head are clean and have no rust that cannot be wiped off.

It is also important to check that the jack is aligned concentrically on the anchor head and coaxially in relation to the tendon. 7.7.3.2 – Inspection plan

An inspection plan must be drawn up prior to the start of the stressing operations. It specifies the types of inspections, the frequency of the inspections and identifies the tendons to be inspected. These inspections may be systematic in terms of:

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Measuring the extension and pressure in stages

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Monitoring the position of tendons during threading in order to detect any possibility of crossed ducts.

They may also take the form of spot checks, e.g. when measuring transmission factors, which at first are systematically verified for each family of tendons at the start of stressing operations, and then are checked at random. Before the jacks are threaded, it is advisable to cut all of the strands to the same length; this shows up any problems with the jaws and reveals any broken strands. The marking of the strands at the back of the jack is another, albeit less effective, solution. Finally, before the start of operations, it is necessary to verify that engineering office has allowed for the additional lengths of cable which are gripped by the jacks in their calculations of the extension of the tendons. 7.7.3.3 – Precautions during stressing

During stressing, the following points require special attention:


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Verify that the internal friction on the jacks and their anchorages has been correctly accounted for in the interpretation of the pressure readings on the manometer

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Do not forget to correct the pressure reading in accordance with the calibration chart for the pump manometer: the pressure is used as the reference measurement and not the extension, which is indicative

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Check that the correction for the compaction of the jaws has been properly allowed for in the measurement of the extension on the piston of the jack.

7.7.3.4 – Coefficients of friction

Coefficients of friction are monitored by measuring the transmission factor. The principle behind this measurement is described in detail in Article 95.4.2 of fascicule 65A and Appendix I of its supplement. Before carrying out this measurement, it is essential to make sure that the jacks have been properly tared (using a calibration chart). It is also important to verify the calibration chart for the manometers on the pumps (test date). 7.7.3.5 – Precautions to be taken with regard to concrete

During the construction of a cantilever with cast-in-situ segments, it is essential to verify that the value f cmin which is mentioned on the french CIP (Commission Interministérielle de la Précontrainte-Interministry Prestressing Committee] accreditation and in the construction design calculations has been achieved by the concrete before stressing is carried out. The use of a sclerometer to perform this verification is strictly prohibited. The cantilever tendons are sometimes only stressed after the form traveler has been moved forward. This operation, which is designed to shorten the segment construction cycle, means that the last segment to be cast behaves as reinforced concrete with regard to the loads exerted by the form traveler. Normally, this poses no problems in terms of general flexion, but particular vigilance is required with regard to the localized stresses applied to the new concrete. This operation must be described in a detailed construction procedure and the engineering office must apply specific design calculations to it. 7 . 7. 4 -

Inspection of injection

7.7.4.1 - Injection

The injection of prestressing ducts and anchorages is intended to:

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Completely fill the void left in a duct

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Passivate of the steels due to the action of the products used

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Protect steels against corrosive external agents.

The combination of these three fundamental actions produces a permanent barrier against corrosion and guarantees the longevity of the prestressing. 7.7.4.2 – Cement grout

Two types of grouts are identified:


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Traditional grout, which is fluid after manufacture

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Thixotropic grout: a traditional grout to which a thixotropifying agent is added at the end of the manufacturing process.

The main characteristic of this second type of grout is its capacity to change from a gel when stationary to a liquid when shaken. This has two advantages: the injection conditions remain the same as for a traditional grout, and the front wall of the grout remains almost perpendicular to the duct and does not collapse as it passes through high points. However, this property depends on the shearing threshold of the grout for a given slope and on the steepness of the slope for a given grout. Numerous tests must be performed on the grout. These are specified by European standards and are referenced in Article 92.3 of fascicule 65A. They involve:

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Measuring fluidity, which determines the flow

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Measuring exudation, which defines the stability of the grout

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Measuring the variation in volume, which characterizes the filling volume

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Measuring compressive strength.

Testing using a transparent inclined tube, using the method described in Appendix 3.1 of Circular no. 99-54 of August 20 1999, is essential in view of the representativeness of this test and the unsuitability of the other tests with regard to the characterization of the in-situ stability of grout. However, grouts may be used without preliminary testing if they have been given a favorable technical evaluation report or a favorable provisional report by the CIP, except when testing is specifically requested by the Construction Checker, provided that the components of the grout, the equipment used for their injection and the temperature conditions for their application comply with the the conditions described in the technical evaluation report. If just one of the conditions is not respected, it is essential to perform a new inclined tube test on site. 7.7.4.3 – Specific inspections and precautions during the injection of cement grout

P r e c au t i o n t o b e t a k e n d u r i n g t h e i n j ec t i o n s t u d y

Injection tests have shown that the appearance of pockets of air due to the collapse of the front wall of the grout as it passes through high points can be prevented by optimizing the speed of advancement of the grout in accordance with the alignments and the prestressing units used. Therefore, it is important to use an appropriate speed of injection, generally: Horizontal or undulating tendons with ducts of 8 to 14 m per min Φ < 100 mm Horizontal or undulating tendons with ducts of 16 to 20 m per min Φ > 100 mm Vertical or steeply angled tendons 3 to 8 m per min

Furthermore, the phasing of the injection must be adapted to suit the alignment of the tendon in question. This analysis must be carried out upstream of the process in order to make sure that the position of the vents is consistent with this phasing.


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P r e c au t i o n s t o b e t a k e n d u r i n g c o n s t r u c t i o n

Bleeder valves and vents are identified by marking or labeling according to the vent plan which must be supplied by the contractor. During the construction of the bridge, these vents must be closed in order to prevent water from seeping into the ducts. The bleeder valves are kept open. P r e c au t i o n s t a k e n w h en p r e p a r i n g f o r t h e i n j ec t i o n

Mixers and storage tanks must be protected from water penetration and prevented from drying out due to direct sunshine. An air compressor and a pressurized water inlet must be operational on the structure and kept ready in case of problems during the injection process. P r e - i n j e c t i o n i n s p ec t i o n s

Before any injection takes place, it is compulsory to test the leak-tightness of the ducts in order to identify any potential anomalies. This test can be carried out using compressed air or by creating a partial vacuum. This also helps to check that the numbering of the ducts matches that of the vents. If the test shows the existence of any connections between the ducts, the tendons concerned will be injected simultaneously. Preparations for the injection must also take account of the weather forecast. The rheology of the grout must be adapted to the ambient temperature and humidity. Finally, it is important to remember to check the equipment and make sure that the necessary materials are supplied in sufficient quantities. I n sp ec t i o n s du r i n g i n j ec t i o n

During pressurized injection, the monitoring of the injection pressure is of paramount importance to the success of the operation. The fundamental value to be considered is that the pressure in the duct must not exceed 1.5 MPa. It is also necessary to perform systematic tests of the bleeding of the anchor caps and vents at the high points. Fluidity and exudation tests are performed according to the recommendations of Article 95.5.l.B of fascicule 65A. P o st - i n j e c t i o n i n sp ec t i o n s

After the injection, the different levels will be inspected in order to make sure that the ducts are properly filled. This involves:

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Visual inspections of the ducts

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A statistical inspection by removing the caps

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An acoustic sounding inspection

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A gammagraphic inspection, if allowed by the thickness of the concrete (e < 0.50 m).


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7.7.4.4 – Injection of external prestressing

Circular no. 2001-16 of February 28 2001 relative to the “Conception de la précontrainte extérieure au béton” (The design of prestressing external concrete) describes the possible solutions used for the design of external prestressing and mentions the risks involved in its removal. In practice, this leads to the use of protected and sheathed single strands in a general duct injected with cement grout or ungalvanized strands in a general duct injected with a flexible product, usually petroleum wax. 7.7.4.5 – Flexible products

Wax

Wax is a malleable, crystallized solid material which is preferred to grease due to the absence of syneresis, which improves its stability over time. As wax must be injected at temperatures higher than its melting point, between 90 and 120° C, certain important precautions must be taken. Firstly, it is of paramount importance to consider the thermal exchanges with the adjacent materials and the ambient temperature during the injection process. During cooling, the wax contracts significantly, to the order of 5 to 10 %, and this can lead to the creation of voids. Therefore, it is important to take special precautions such as re-injecting the high points. However, in the case of the external prestressing, a large proportion of the shrinkage of the wax is compensated for by the cooling of the HDPE duct, which was heated by the passage of the hot wax. It is absolutely essential to prevent any leakage of wax during the injections. If this happens, staff could be burned and the surrounding concrete could be impregnated with wax, with disastrous consequences. To reduce this risk to a minimum, it is important to only use materials which are resistant to high temperatures and to make sure that the ducts and anchorages are perfectly leakproof by carrying out rigorous inpections and tests ( As the risks can never be entirely eliminated, it is important to ensure that everyone present is protected against the risk of burning by the wax). Grease

Because of its syneresis, it is prohibited to use grease for the protection of prestressing units that are not selfshielded. For self-shielded units such as those which are often used in transverse prestressing, the protection of anchorages using grease must be carried out on site. The greatest care must be taken to ensure there are no leaks.

7.8 - Other import ant point s 7 . 8. 1 -

Adhesive and its application (prefabricated segments)

7.8.1.1 - Characteristics

The adhesive used for prefabricated segments consists of epoxy resins mixed with a hardener. Its main properties are:

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Density of approximately 1.50

•

Compressive strength of between 15 and 25 MPa


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•

A low modulus of elasticity of between 1,500 and 2,500 MPa.

Adhesives bearing the "NF-Produits spéciaux destinés aux constructions en béton hydraulique" (French Standard – Special products intended for hydraulic concrete structures) label in the structural adhesives category, or products bearing an equivalent European label are highly recommended. 7.8.1.2 – Adhesives and their role

The term “adhesive” is not strictly accurate. In fact, results from the laboratory testing of concrete-to-concrete bonding, structural demolitions and analyses of structural behavior show that under normal conditons on construction sites, the joints are not actually bonded because tensile strength is not created in the joint. Nevertheless, adhesive plays an important role by:

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Filling the openings between joints caused by the differential shrinkage of different parts of the section of the box-girder; these openings may reach 0.3 mm in size

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Partially correcting imperfections in joints because the adhesive has a similar compressive strength to the concrete

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Lubricating the surfaces when the parts are brought together, minimizing the damage caused by the inevitable minor impacts during handling operations

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Helping to center the match casted surfaces

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Sealing prestress ducts.

7.8.1.3 – Testing of setting time

Before the segments are assembled and bonded, it is necessary to test the time taken for the adhesive to set, i.e. the time between the start of the mixing of the components and the moment when the mixture no longer adheres on contact. The following precautions must be taken when performing this test:

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The components of the test sample must be similar in quantity to the mixture that will be used on site in practice

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Care must be taken to avoid trapping air when mixing the components of the glue

•

The temperature and humidity conditions for the test must be representative of the actual conditions and conform to the specifications.

7.8.1.4 – Application precautions

The following precautions should be taken when applying the adhesive:

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Ensure that if it rains, protective measures such as the sheeting of the bonding area and the creation of an run-off barrier near the joint are implemented in order to prevent contact with rainwater run-off before and after the bonding of the segments and until they are securely lashed together

•

Verify that a sufficient amount of adhesive is applied to the joints of both segments to form a seam of excess adhesive on the edges of the joints


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•

Verify that this seam has not been ground down after polymerization, except on the bridge deck extrados, in order to facilitate the surfacing required for the application of the waterproofing layer.

It should be noted that the formation of a seam of adhesive as mentioned below simplifies inspections in the short and long term by guaranteeing that the joint has not been been altered after the clamping of the parts. 7.8.1.5 – Repair of bonding defects

A major bonding defect can only be repaired by an injection of resin according to a specific procedure adapted to each case. Resurfacing with cement mortar may make it harder to evaluate the type and extent of any defects. This evaluation must imperatively be carried out before any repairs are made. Minor defects such as impact damage to the concrete on the edge of a joint are better left exposed. If resurfacing is necessary, a recess should first be created around the damaged area and an epoxy resin-based mortar should be used for the repair. 7 . 8 .2 -

Segment facin g

The inspection of the general appearance of a segment concerns three criteria: shape, corresponding to the different geometrical dimensions of the segment - texture, which is given by the formwork skin - and color, which is governed by the concrete. These three criteria are interrelated. For example, color is quite difficult evaluate in isolation because it depends on how the surface reacts to the light. In fact, the type of surface finish can attenuate differences in the color of the concrete. Color therefore relates to texture. The color obtained also depends on the number of times the formwork is reused – which turn depends on the resources allocated to the project. Three factors play an important role in determining the color of the segment facings on structures built by the cantilever method: the quality of the concrete, the formwork and vibration. 7.8.2.1 – Concrete quality

The final appearance of the segment facings should be considered when choosing the concrete formula because the color depends on the rheology of the concrete. The amount of water used, the proportions of additives and the temperature are the three key, interdependent factors which must be controlled. The additives and their effects on the water and temperature are particularly important. It is now known that changing the formula of the concrete to suit the season plays a key role in producing a high-quality facing. Concrete for the construction of bridges built by the cantilever method is now manufactured during several seasons. A reference formula for spring and autumn has to be changed to allow for higher temperatures and lower rainfall in summer and is changed again in winter to account for the reversals of these same parameters. It is particularly important to account for the variation in the water content of the components. Only a detailed rheological study will satisfy this requirement. Maturometry, which generalizes the evolution of the hydration parameters for concrete, helps to improve the management of the quality of concrete. It is also a valuable indicator for the removal of formwork, as it can be


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used to determine how long the formwork needs to remain on the part and this is one of the key factors in determining the homogeneity of the facing. This factor is particularly important for the prefabrication of segments in prefabrication units. The daily cycle which leads to the removal of formwork in the morning, is actually quite hard to reconcile with the stoppage of work at the weekend. 7.8.2.2 – Formwork and formwork skins

Today, the most widely used formwork is made of steel or bakelite plywood (film-faced plywood, which is less expensive, may cause problems depending on the type of aggregates used). These materials are used primarily for economic reasons and their characteristics are not ideal for the creation of high-quality facings. A double formwork skin is better suited to this task. This consists of a thickness of no more than 10 mm of ordinary plywood fixed to the standard formwork. In this case, the formation of bug holes can be prevented by making sure that the wood fibres are aligned in the same direction as the pouring of the concrete. The drawback to this technique relates to the time required for the assembly and the dismantling of the formwork skin each time it is replaced, rather than the costs of the materials. Indeed, beveled vertical slats of unfinished wood have already been used for this purpose. Formwork skins, made from composite materials which can be reused at least three hundred times, are a new arrival on the market. These materials are very expensive, however, and are not suitable for all shapes. Another important contribution of the formwork skin to the creation of a high-quality facing is to dampen the vibrations transmitted to the formwork during the vibration of the concrete, thanks to the column of air between the formwork skin and the formwork itself. 7.8.2.3 - Vibration

A large number of defects are caused by incorrect vibration. Two pitfalls must be avoided:

•

Over-vibration - meaning that part of a component has been vibrated for too long. On a constant cycle, which is the most common situation, this means that another part of the component has not been vibrated for long enough

•

Post-vibration - involving the creation of secondary vibrations caused by the hard spots of the reinforcements or formwork structures.

The importance of the type of formwork skin is illustrated by the fact different materials have different capacities to absorb vibrations. Flat vibration, e.g. of a slab, often creates intermittent vibrations in the reinforcements which act as secondary vibrators when the concrete is tightly compacted. To make sure that the upper section of a layer of freshly-poured concrete is sufficiently vibrated, it is necessary to vibrate it for one to two minutes after air bubbles have ceased to emerge from the concrete. Finally, it is important to follow a vibration plan corresponding to the shape of the component. This plan is drawn up by a specialist or with specialist help (see paragraph 7.4.4 concerning the inspection of application).


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Differences in the appearance of the facing can be reduced by the use of white cement or light-colored cement. Clinker has a distinctive color and additives can be used to attenuate its impact. Therefore CHF-CEM III/A and B, and CPJ-CEM Il/A and B cements produce light-colored facings. The test segment is used to experiment with all of the aforementioned factors and eventually arrive at a comprehensive solution for the production of high-quality facing. However, it is also necessary to anticipate changes in the quality of the formwork skin. For example, if the skin is made from painted sheet metal, this paint will gradually disappear, thus changing the final appearance. As fascicule 65A does not address all of the issues relating to facings, the Construction Manager may also refer to the Projet National Qualibé (National Concrete Quality Project) report for additional information.


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8 - Pathologies and repairs This chapter begins with a quick reminder of the changes in the regulations applicable to structures built by the cantilever method. It then goes on to describe the main pathologies affecting these structures and the corresponding repair techniques.

8.1 - Histor y of t he regulations This paragraph mentions certain key events in the evolution of French regulations and the technical texts relating to prestressed concrete bridges built by the cantilever method. 8 . 1 .1 -

1 9 46 - 19 5 2: t h e f i r s t p r e s t r e s s e d b r i d g e s

In this era, there were no French regulations concerning prestressed concrete. 8 . 1. 2 -

1 95 3- 19 64 : t h e f i r s t r eg u l a t i o n s f o r p r e s t r e s s e d c o n c r e t e

The first French regulations applicable to prestressed concrete appeared on October 1 1953, (Circular no. 141 issued by the Ministère des Travaux Publics, des Transports et du Tourisme [Minsistry of Public Works, Transport and Tourism] - a Provisional Instruction concerning the use of prestressed concrete). Essentially, it recommended the use of fully compressed concrete (with minimum compression on one axis equal to at least 8% of the maximum compression), but it hinted at the use of partially prestressed structures. This instruction recommended the inclusion of certain clauses in the Cahiers des prescriptions spéciales (Special Conditions) [CPS]) relating to the composition of concrete (minimum cement batching of 400 kg/m3) and the quality of prestressing reinforcements. It also made the use of strong sealed metal ducts compulsory for prestressing by post-tensioning. 8 . 1 .3 -

1 9 65 - 19 7 5: c o n s t a n t l y c h a n g i n g r e g u l a t i o n s

This was a time of enormous changes and constantly changing regulations. In 1965, Circular no.44 of August 12 1965, relating to the instruction provisoire (Provisional Instruction) for the use of prestressed concrete (called “IPl”), replaced the 1953 Provisional Instruction. Complete prestressing remained, but there was no lower compression limit (σmin ≥ 0). Cylinder strength at 28 days replaced cube strength at 90 days. It should be noted that this circular was written primarly for bridges with prefabricated girders. It incorporated the advances made since 1953, especially regarding the calculation of losses of tension (e.g. formulae for relaxation losses). 1966 saw the publication of the Provisional Directive for the construction of prestressed concrete bridges. This was intended to prevent accidents during the construction of bridges or on structures in use. It made changes to the “construction” section of the IPl and insisted on:

•

The need for a waterproofing layer

•

The use of APC (Artificial Portland Cement) 325 or CFA (Cement – Fly Ash) 400 cement, batched at 400 kg/m2 for the manufacture of concrete, in order to reduce the risks of corrosion (the use of PCS [Portland Slag Cement] cement had to be approved by the LCPC)


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•

The almost total prohibition of admixtures after the collapse of a beam on the Guerville viaduct. This accident was caused when an incorrect dose of an admixture prevented the concrete from hardening properly; as a result, the bottom flange of the girder was crushed during prestressing

•

Precautions for the storage of prestressing reinforcements on site, in order to reduce the risks of stress corrosion

•

Inspections during stressing operations in order to reduce prestress losses

•

The care to be taken over the injection of ducts and the need to allow a maximum period of 8 days between the stressing and injection operations, again with a view to reducing the risks of stress corrosion (only APC 325 cement was allowed for the injection grout and sealing mortar).

1971 saw the introduction of the Directives Communes relatives au Calcul des Constructions (DC71) [Common Guidelines relating to Structural Design], which was the first document to cover the concept of limit states. These guidelines were intended to be used as the basis for the creation of new design rules for metal, reinforced concrete and prestressed concrete structures. In the same year, the 1960 load regulation was replaced by Titre II of fascicule 61 of the Cahier des Prescriptions Communes (CPC) [Common Conditions]. This regulation significantly reduced the intensity of the distributed loads A (l) for bridges of moderate length (between 15 and 80 m) and introduced degressivity factors according to the number of loaded lanes and the category of structure. The Provisional Directive for the injection of ducts on prestressed concrete structures appeared on March 28 1973. This highly informative document covered the requirements for grouts (traditional and special), the characteristics of study and suitability tests, the manufacturing conditions for grouts, the execution and inspection of injected materials, accidents, etc. This directive led to real improvements in the quality of the protection applied to prestress tendons. In December 1972, Sétra published the first version of its Technical Bulletin on bridges built by the cantilever method, called “BT7”. This document covered the history, engineering, design and construction of these structures. Appendix I of this document featured a design example incorporating changes due to creep and thermal gradients. Appendix II included a collection of 55 monographs on structures built between 1960 and 1972 in France and abroad. In 1973, Circular no. 73-153 of August 13 1973 introduced Provisional Instruction no. 2 (called “IP2”) relating to the use of prestressed concrete. This consisted of a design rule based on “limit states” methods. The IP2 introduced the use of:

•

The characteristic strength of concrete instead of the nominal strength

•

Types or classes of prestressing (from type I: no decompression of concrete, to type III: limitation of the opening of cracks)

•

Characteristic values of actions due to prestressing (P1 and P2)

•

Specific rules relating to end or support areas and to concentrated forces during post and pre-tensioning

•

Rules to account for delayed deformations in concrete due to shrinkage, creep, etc.


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During this period, private and public engineering firms were not ready to face up to the practical difficulties relating to the application of this new regulation (complete overhaul of design programs, difficulty of applying the range of characteristic values of the prestressing when there was a redistribution of efforts due to delayed obstructed deformations, etc.). Therefore, the IPl, which remained valid, continued to be used except for the pretensioning, the distribution of concentrated forces and for the precise calculation of delayed deformation effects. Circular no. 74-60 of April 23 1974 made changes to the following articles of the IP1 in accordance with the new Titre II in fascicule 4 of the CPC of March 5 1971, and March 26 1973:

•

Article 10, relating to the calculation of relaxation losses in the prestressing reinforcements, established relaxation at 3,000 hours

•

Article 12 reduced the initial tension of the tendons

•

The values of the coefficients of friction f and φ specified in the IP1 were used in the certificates of approval, except for structures with numerous joints for which they were determined by the CPS (Special Conditions).

The Direction des Routes et de la Circulation Routière (Department of Roads and Traffic) published a circular on April 2 1975, after analyzing and determining the causes of cracking observed on a number of bridges built by the cantilever method. This document was designed to supplement the IPl with regard to the following points:

•

Redistribution of forces due to delayed deformation

•

Distribution of prestressing loads (concentrated forces) and the accumulation of shearing stresses with or without transverse or vertical prestressing (stressed stirrups)

•

Traction forces exerted by tendons anchored in the slab of a box girder

•

Thermal gradients (5°C under rare combinations and 10°C under practically permanent combinations in the sense of the BPEL 91, while taking account of the instantaneous deformation modulus of the concrete)

•

radial tendon forces in curved slabs

•

The continuity of reinforcements close to joints, etc.

By introducing rules to allow for creep and the thermal gradient, this circular brought an end to the era of second-generation bridges (see Chapter 1) and paved the way for a new generation of much better dimensioned bridges. 8.1.4 -

1975-1982

This period saw the publication of new regulations which, on the one hand, incorporated the concept of quality and on the other, abandoned the "allowable stresses" calculation in favor of "limit-states" calculations. 1979 saw the publication of four important circulars:

•

Circular no. 79-23 of March 9 1979, which included the instruction of January 15 1979 relating to the inspection of concrete quality (conditions for the execution and interpretation of study, suitability and inspection tests which were not included in fascicule 65 of August 13 1969, and described in Circular no. 69-92, or in a usable form within Appendix B of the IP2)


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•

Circular no. 79-78 of August 16 1979 relating to the implementation of prestressing units, which established: The values of the coefficients of friction f and φ (according to the radius of curvature, the number of joints crossed and the type of tendons: smooth wires or strands) – The moduli of elasticity of the wires (200,000 MPa) and strands (190,000 MPa) – The measuring conditions for the transmission factors – The choice of ducts (ducts and tubes), in addition to the diameters, radii of curvature and the continuity of the ducts – The limit of the initial stress to be 0.7 Frg for reinforcements with a small radius of curvature (stressed stirrups) – The strength of the concrete close to the anchorages and the minimum distances to the facings of the anchor plates – The conditions of execution for the tensioning – The need and the implementation conditions for temporary protection and sealing Circular no. 79-121 of December 14 1979 concerning the reprint of the IPl, which incorporated the modifications made by Circulars 74-60 of April 23 1974 and 77-67 of April 25 1977 –

• •

Circular no. 79-25 of March 13 1979 relating to the new Directives Communes pour le Calcul des Constructions (D.C.C.C.79) [Common Guidelines for Structural Design]. These guidelines served as the basis for the creation of BAEL and BPEL rules and the design rules for foundations.

A supplement to Technical Bulletin No.7 also appeared in April 1979, introducing the requirements of the Circular of April 2 1975 and its implications concerning the quantities of materials, new design programs, the stability of cantilevers under construction, design examples, construction advice (prefabricated segments, measurement of transmission factors, form travelers, stressed stirrups, segregation of concrete, workability, protection of anchorages, etc.). 8 . 1 .5 -

1 9 83 t o t h e p r e s e n t d a y

This period was marked by the generalization of "limit states" design rules. The introduction of Quality Assurance and the development of French and European standardization led to significant changes in the rules relating to the construction of structures. The development of “external prestressing” had a major impact on the development of prestressing procedures, and much less of an influence on the development of prestressing reinforcements. Significant changes were made to the design of large prestressed concrete structures, but construction techniques remained very similar. It should also be noted that the effects of Circular 82-50 of May 24 1982 began to felt. This involved the notions of quality of use, the organization of quality, internal and external inspections and independent inspections, etc. All of these notions concerning quality were applied to each of the texts relating to the construction of structures. In 1983, the regulations relating to the design of prestressed concrete bridges were revolutionized by the appearance of fascicule 62 Titre l Section 11 of the french CCTG (General Technical Clauses, technical design and calculation rules for prestressed concrete structures according to the limit states method, called the “BPEL


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83”). It should be noted that the BPEL 83, the IPl and the IP2 were all applicable until December 31 1985 according to certain procedures established by decree. The BPEL 83 rules introduced:

•

External prestressing

•

The notion of verification categories for structures (Class I authorizes no traction and is mainly used for prefabricated bridges; Class II, in which the tensile stresses are limited, is used for major bridges and large standard bridges or those situated in harsh environments; Class III, covering “partial prestressing”, was applied to prestressed concrete bridges built by the cantilever method, certain types of standard bridges in reasonably benign environments (2nd and 3rd category road bridges) and for bridge slabs with transverse prestressing

•

A characteristic thermal gradient value of 12° C for verifications at service limit state

•

Characteristic strength as the specified strength for concrete

•

The possibility of performing verifications with the probable prestressing Pm, instead of range calculations using P1 and P2 (characteristic values).

In 1984, Sétra published two information notes:

•

The first covered the anchoring strength of external prestress tendons in the event of overtension in order to prevent the blockage of wedges by the cement grout

•

The second dealt with the use of single and multi-strand couplers, the precautions to be taken with keyed anchoring and problems relating to the use of “super” strands.

In February 1990, the CTOA (Centre des techniques d’ouvrages d’art - Center for Engineering Structure Techniques), a division of Sétra, published a guide entitled "Précontrainte extérieure" (External Prestressing). It gave additional information on subjects covered by the BPEL 83, with a particular emphasis on external prestressing technology. The BPEL 83 rules were replaced by the BPEL 91 rules at the start of 1992. The major modifications introduced by this new version included:

•

Extending the rules to concrete with characteristic strength of between 40 and 60 MPa

•

Improving SLS verifications for the different classes I, II and III

•

Introducing a "θ" coefficient into the formula giving the value of the compression limit stress of concrete at ULS, as a function of the duration of applied of loads

•

Reduction of the 1983 shear limit stress which was considered to be too forgiving

•

Reduction of the working stress ratio for stressed stirrups

•

For coastal structures: increasing the depth of coating on non-prestressed reinforcements without individual protection from 4 to 5 cm


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•

Appendix 3, which assigned numerical values to the coefficients of friction f and φ for tendons inside the concrete, sheathed and protected strands and tendons outside the concrete.

It should be noted that the BAEL 83 rules were simultaneously replaced by the BAEL 91 rules. At the end of the 1990s, the BPEL 83 was modified again, becoming the BPEL 91, revised in 1999. An additional appendix (Appendix 14) was included in this new version, relating to concrete with a characteristic strength of between 40 and 60 MPa. The end of the 1990s was also marked by the appearance of two circulars relating to prestressing using posttensioning techniques:

•

The first, dated August 20 1999, prohibited the washing of sheathed tendons with water, established a number of new requirements for HDPE tubes and promoted the injection of external tendons with wax

•

The second was published on February 28 2001 and imposed new requirements concerning the injection of external prestress tendons (see paragraph 7.7.4.4).

8.2 - Pathologi es specific to t he cantil ever con str ucti on techniq ue As for most other types of bridges, specific types of problems have affected the first prestressed concrete structures built by the cantilever method. The purpose of this section is to briefly outline the principal specific disorders that have been identified for these structures and to describe the lessons that have been learnt with regard to the design of modern structures built by the cantilever method. In the interests of relevance, we shall only cover problems with structural origins, as problems caused by other factors (materials, use, maintenance, etc.) are not exclusive to the cantilever construction method. For more detailed information about problems affecting prestressed concrete structures, the reader is invited to refer to Sub-Section 32.2 of the second part of the Instruction technique pour la surveillance et l’entretien des ouvrages d’art (Technical Instruction for the Monitoring and Maintenance of Civil Engineering Structures), published in 1979 and modified in 1995. 8 . 2. 1 -

R em i n d e r c o n c e r n i n g c r a c k i n g o n p r e s t r e s s e d s t r u c t u r e s

The appearance of significant deformations and/or cracks on prestressed concrete structures may be evidence of structural disorders. Not all of the cracks found on prestressed concrete structures are signs of defective behavior. When analyzing the problems on a structure, it is important to consider all of the characteristics of the cracks observed. 8.2.1.1 – Opening and widening of cracks over time

The opening and widening of cracks if they are active are obviously the first elements to consider. However, this information is insufficient in itself to evaluate the condition of the structure.


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Prestressed concrete structures built by the cantilever method are often only prestressed longitudinally. They therefore behave like reinforced concrete in the transverse direction. As a result, fine longitudinal cracks may develop and in most cases, these are evidence of normal transverse flexion behavior in reinforced concrete. This applies to the fine cracks that can be observed in the upper slabs. It also applies to the fine cracks found in the deviators on structures with external prestressing. As a guide, it shall be considered that the opening of the cracks on elements behaving like reinforced concrete must not exceed: – 0.1 mm in the case of systematic cracking (multiple, evenly-distributed cracks), for the average width of all

of these cracks – 0.2 mm for the average width of an individual crack – 0.3 mm in localized cases for the width of an individual crack.

These openings are values under dead loads and were established in the framework of studies relating to the penetration of corrosive agents into the concrete. For structures or parts of structures situated in relatively noncorrosive environments, these values could therefore be increased. 8.2.1.2 – Design assumptions and the influence of regulations

Structures built according to the Instruction Provisoire n° 1 (Provisional Instruction no. 1) or the BPEL in Class I were designed to be totally prestressed, as, in theory, no traction is allowed with regard to flexion when the bridge is in service. As a result, minimal longitudinal reinforcement is used and this would be incapable of compensating for any unforeseen tensile forces. On this type of structure, transverse cracks are usually evidence of abnormal longitudinal flexion behavior. In particular, cracks crossed by prestress tendons are problematic because of the risk of fatigue in the prestressing reinforcements. On the other hand, tensile forces are allowed at the design level for structures dimensioned according to Classes II or III of the BPEL. Efforts are made to keep the opening and widening of any cracks to acceptable levels in order to: – Prevent the risk of fatigue in the tendons crossing areas likely to be tensioned – Limit the penetration of corrosive agents through the gaps caused by excessively wide cracks.

With this in mind, stresses or stress variations within the concrete and reinforcements are minimized and non prestressed longitudinal reinforcement bars are added to control the cracking. These structures were built more recently and benefit from the lessons learnt on older structures. Fine transverse cracks, which are closed when the structure is not loaded, are usually evidence of normal behaviour in prestressed concrete, in accordance with the regulations. Excessive cracking (open cracks when the structure is not loaded, etc.) is obviously a sign of abnormal behavior. 8 . 2. 2 -

Stability of cantilevers

8.2.2.1 – Problems observed and their causes

Problems arose during the construction the cantilevers on some of the first structures built using the cantilever method and a spectacular accident occurred on the site of the Viosne viaduct on November 13 1970, when the first cantilever tipped over, fortunately without any serious consequences (Fig. 8.1).


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Fig. 8.1 – Tipping of the first cantilever of the viaduct over the Viosne

Accidents have also been caused by the collapse of prefabricated segments and form travelers, e.g. on the bridges of Calix, in 1975 and Bellegarde in 1982. 8.2.2.2 – Impact on design

Design rules governing the stability of cantilevers under construction were established and published in the supplement to Technical Bulletin (BT) no. 7, published by Sétra in 1979. These rules, which are included in this guide along with several additions, have now been successfully used for over twenty years. No cantilevers have collapsed since their publication, despite the collapse of several form travelers. 8 . 2. 3 -

Cracks due to insuf fici ent longi tudin al strength


The majority of structures built using the cantilever method prior to 1975 have suffered from transverse cracking or the opening of joints in the lower slab, towards the middle of the central spans or in so-called “low bending moment” zones (Fig. 8.2).

Fig. 8.2 – Flexion cracks

These cracks are evidence of insufficient resistance to longitudinal flexion. This type of cracking is particularly serious given that the cracks or open joints are often crossed by continuity tendons inside the concrete, thus exposing these reinforcements to the risk of fatigue.


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These cracks have several causes which are often cumulative and include:

•

The failure to account for the redistribution of forces due to the delayed obstructed deformation of materials in the calculations. For simplicity’s sake, these forces are often put down to "redistribution by creep" (in general, this error leads to the normal stress in the lower axis at the crown being overestimated by 2 to 3 MPa)

•

The failure to account for the thermal gradient in the calculations (in general, this omission also leads to an overestimation of the normal stress in the lower horizontal plane at the crown of approximately 2 MPa for a thermal gradient of 6° C)

•

The overestimation of the prestress effect, due to over-optimistic coefficients of friction or underestimations of relaxation losses

•

Parasitic forces locked into the structure when the closing segment is assembled due to adjustments made on site (realignment of cantilevers, correction of reverse deflections, unforeseen site loads, incorrectly controlled weight of the form traveler etc.).

8.2.3.2 – Impacts on design

The Circular of April 2 1975 and the supplement to the BT7 of April 1979 defined the design rules to be adopted in response to redistributed forces due to the delayed obstructed deformation of materials and in response to the thermal gradient. For structures dimensioned according to the IPl, this resulted in a significant reduction in the recommended slenderness ratio at the crown, from l/50th to 1/40th, i.e. the bridge deck became considerable thicker in this area. With regard to the coefficients of losses by friction, the values adopted by the BPEL were more conservative than those in the previous regulations. The BPEL paid particular attention to the case of tendons crossing numerous joints, as happens on bridges built by the cantilever method. In addition, empty ducts allow for additional prestressing to be used if the actual friction is greater than the friction values allowed for in the design. Finally, the design allows for the use of additional prestressing (anchorages, deviator tubes, etc.) in order to facilitate any future structural repairs or reinforcements (see the recommendations in Chapter 3 of this guide). 8 . 2. 4 -

Cracks and probl ems due to excessive radial tendon forces


Two types of disorders may point to the existence of excessive radial tendon forces: longitudinal cracking in the lower slab in the crown area and localized cracking, delamination or spalling of the concrete. Longitudinal cracking in the lower slab is caused by downward thrusts in continuity tendons inside the concrete in structures of variable depth and most commonly affects the central span (Fig. 8.3 and 8.4).


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Fig. 8.3 – Cracking due to downward thrust at the web-slab junction

Fig. 8.4 – Cracks caused by downward thrust

These problems affected first-generation structures because a large number of low-strength continuity tendons were spread across the entire width of the lower slab, mainly in the central span. Also, many of these early structures were built without gussets at the junction between the web and lower slab. Localized cracks, delamination and the spalling of concrete are caused by the errors in the positioning of the ducts such as the festooning of ducts between two joints, badly-positioned tendons, sudden changes in angle close to the joints etc. These problems create radial tendon forces. 8.2.4.2 – Impacts on the design

Gussets are now systematically used at the junction of the webs and the lower slab of box girders. Furthermore, the practice of spreading the tendons across the full width of the lower slab has now been abandoned, especially for bridge decks of variable depth. Tendons are now usually placed in the gussets or very close to them, which is made easier by the fact that fewer internal continuity tendons are needed for mixed prestressing. To combat problems caused by incorrectly positioned ducts, the Instruction Technique pour la Surveillance et l’Entretien des Ouvrages d’Art (Technical Instruction for the Monitoring and Maintenance of Civil Engineering Structures) recommends using a reinforcement plan that is designed to take up any parasitic radial tendon forces due to angular fractures in proximity to the joints.


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Fig. 8.5 – Suggested reinforcement plan designed to compensate for accidental radial tendon forces close to joints

8 . 2 .5 -

Other patho log ies

For information, we shall also mention two other important pathologies found on prestressed concrete bridges built by the cantilever method:

•

Badly fitting joints in the case of prefabricated segments

•

Deflections of cantilever arms close to joints.

Bridges built by the cantilever method are, of course, not immune to the more general problems affecting prestressed concrete structures (distribution of concentrated prestressing forces, injection defects, corrosion of tendons, etc.). It should be noted that the positioning of cantilever tendons inside poorly protected channels on the extrados - a technique specific to bridges built by the cantilever method - proved to be particularly damaging in terms of the corrosion of tendons and was quickly abandoned. To round off this section, it should be mentioned that external prestress tendons have failed on several occasions in recent years. This problem has affected tendons whose ducts were injected with a cement grout which had failed to set properly in certain areas due to the phenomenon of settling (fresh grout).

8.3 - Main r epair tech niq ues In France, the first significant repairs to prestressed concrete bridges built by the cantilever method were carried out at the beginning of the 1970s (Bussang bridge etc.). Since then, major structural repairs have been carried out on around sixty structures using a variety of techniques described below. 8 . 3. 1 -

Addit ion or replacement of prestressing

The most commonly used repair technique consists of adding extra prestressing or replacing the original prestressing. In the latter case, the original prestressing must be dismantled, which is a difficult process, especially for tendons situated inside the concrete. It should be noted that these techniques are also used for reinforcing operations. L o n g i t u d i n a l p r est r e ssi n g

The tendon layout may be rectilinear or polygonal. In the latter case, it is angled by deviators added to the initial structure (Fig. 8.6).


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Fig. 8.6 – Additional longitudinal prestressing

Fig. 8.7 – Anchor block for additional longitudinal prestressing

The additional anchor blocks may be situated beyond the abutment, as on the Corbeil bridge, in Essonne, or more conventionally, inside the box girder (Fig. 8.7 above). T r a n s v er se p r e st r e ssi n g

Transverse prestressing may be carried out on the lower slab (Fig. 8.8), the webs, or both (Fig. 8.9). Further information on this subject is given by the following articles [RIC 93], [DEL 94], [BAR 94], [PER 94], [JEH 94], [DEL 98], [BAR 98], [POI 99], [TAV 00], [BOUT 1] and [GIA 01] listed in the bibliography in the appendix to this guide. 8 . 3 .2 -

Other techn iqu es

Other techniques are also used. For information, these include adjusting the height of the supports and adding extra material, glued steel plate sheet or FRP (cf. article [POI 92]). 8 . 3 .3 -

Repair desig n

There is little information concerning repair design for prestressed concrete civil engineering structures. However, the following documents provide useful information for anyone interested in the repair of prestressed concrete bridges:

• Maintenance et réparation des ponts (Bridge Maintenance and Repair) [Presses de l’ENPC – 1997] •

French standard NF P 95-104 - Réparation et renforcement des ouvrages en béton et en maçonnerie Spécifications relatives à la technique de précontrainte additionnelle (Standard NFP 95-104 – Repair and Reinforcement of Concrete and Masonry Structures – Specifications Relating to the Additional Prestressing Technique)

• Ponts en béton précontraint par post-tension (Post-tensioned Prestressed Concrete Bridges) [HA, Sétra, TRL, LCPC] • Annales de l’ITBTP n° 501 de février 1992 : Journées réparation et renforcement des structures de bâtiments et d’ouvrages d’art (Annals of the ITBTP no. 501 of February 1992: Information Days for the Repair and Reinforcement of Buildings and Civil Engineering Structures).


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Fig. 8.8 – Additional transverse prestressing of the lower slab

Fig. 8.9 – Additional vertical prestressing of a web and transverse prestressing of the lower slab


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9 - Provisions to facilitate maintenance This chapter describes the provisions to be made at the design stage with a view to facilitating the maintenance of bridges incorporating box-girder bridge decks. These provisions are intended for prestressed concrete bridges built by the cantilever method, but they are also applicable to concrete structures with box-girder bridge decks built by other methods (on falsework, by incremental launching and by temporary cable-staying).

9.1 - General pri nci pl es In general, the bridge’s design and construction should incorporate measures to facilitate the inspection and maintenance of all parts of the structure, including the interior of hollow sections, without requiring heavy equipment. With these provisions in place, it must be possible to perform bridge management and maintenance operations in accordance with labour legislation and particularly with regard to french Law no. 93-14-18 of December 31 1993, which introduced the Dossier d’Intervention Ultérieur sur Ouvrage (DIUO) [Post-Construction Works File]. Finally, these access arrangements must not make the structure vulnerable to acts of vandalism.

9.2 - Bri dge deck 9 . 2 .1 -

Prestr essin g

9.2.1.1 – Replaceability of external prestressing

Mixed prestressing has been used on the vast majority of large bridges built in recent years. This means that a certain number of tendons are situated outside the concrete. To benefit fully from this technique, it must be possible to replace these tendons without having to demolish any part of the structure. The requirements concerning the tendons and their layout are set out in the Sétra guide entitled "Précontrainte extérieure" (External Prestressing), published in 1979. It should be noted that a large number of the provisions specified in this guide are included in Chapter 7 of the supplement to fascicule 65A of the french CCTG and that, notwithstanding any contrary provisions in the STC, these provisions are compulsory. The provisions concerning the injection of these tendons are defined by the french circular of February 2001 relating to the injection of prestress tendons situated outside the concrete. It should be remembered that this circular only authorizes the use of non-adhesive products (factory-applied grease and wax), unless special equipment capable of absorbing the energy of the tendon during its removal is used. 9.2.1.2 – Provisions for the implementation of additional pres tressing

At a given moment in the life of a bridge, it may be decided to strengthen the prestressing. This may be due to a pathology affecting the structure or to a change in its functional design. For a box-girder structure, it is important to allow this possibility by building in the means to add additional external tendons if they are required. This includes leaving room to pass formwork tubes housing an extra pair of tendons through cross beams and deviators, and placing extra trumplates in the cross beams. It is also necessary to allow the forces added by this extra pair of tendons when dimensioning the reinforcements for the deviators and cross beams. The original type of tendons is used for the extra tendons and they are aligned according to the same rules. They


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are then stressed according to the same procedures used for replacement external tendons. (Further information on this point can be found in Article 64 of the supplement to fascicule 65A of the CCTG and its comments). 9.2.2 -

A c c es s i n s i d e t h e b o x g i r d e r

9.2.2.1 – Minimal depth of bridge deck

To allow for easy movement inside the box girder (Fig. 91), it is important to design bridge decks of sufficient depth. For bridge decks of a constant depth, the problems caused by an insufficient depth could affect the entire structure; therefore, it is recommended to fix the minimum total external depth at 2.20 m. For bridge decks of variable depth, the access problems are occasional and only concern areas in the middle of the spans or close to the abutments, so a minimum depth of 2 m can be accepted. It is important to note that these depths are given for bridge decks without transverse ribs. For bridge decks with transverse ribs, it is necessary to add the internal height of the ribs (usually 0.75 m to 1 meter) to the minimum depth.

Fig. 9.1 – Example of a very small box girder 9.2.2.2 – Normal movement

In the majority of structures, there is sufficient room – at least 0.75 m – between the external tendons nearest to the middle of the box girder to walk comfortably on the concrete surface of the lower slab. When this is not the case, either because the lower slab is very narrow or because there are too many external tendons, it is possible to install a gridded steel walkway over the tendons. This passageway may be cramped in the middle of the spans due to tendons rising up close to the piers. (N.B. If the box girder is too narrow for normal maintenance, it is possible to use completely internal prestressing, which is not as bulky as mixed prestressing).

Fig. 9.2 – Passageway clearances


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9.2.2.3 – Crossing of cross beams and deviators

Cross beams on modern box girder bridges always have access shafts to allow access for construction workers and, when the bridge is in service, maintenance staff. For easy and, above all, risk-free movement through this passageway, it is important to ensure that the height clearances in the shafts correspond to the values shown in Figure 9.2. Furthermore, if these deviators and crossbeams have low beams which hinder the movement of staff and equipment – a common situation – it is advisable to install metal or concrete stairways or concrete ramps on either side of these elements (Fig. 9.3). Important: if the vertical height exceeds 1.50 m, these elements must be equipped with guard rails. 9.2.2.4 – Access to pier head units

An access shaft must be made in line with each pier in order to allow access from the bridge deck to the piers and vice-versa. For obvious reasons, this shaft must be positioned in line with the inspection pits in the pier head units (see 9.3.2.1 below). In order to prevent this shaft from adversely affecting the mechanical behavior of the slab or lower part of the cross beam, it should be circular in shape and 80 cm to 1 m in diameter. For safety reasons, it is essential to keep this shaft closed under normal conditions, using a metal grid which cannot be removed and closes automatically. An effective system consists of placing a square metal grid inside a square rebate built into the top of round access hole.

Fig. 9.3 – Ramp for crossing the lower beam of a deviator 9.2.2.5 – Electrical system

All large prestressed concrete bridges must have an electrical system that is used to light the inside of the bridge deck and to supply power for any tools required during maintenance operations (flood lights, power drills, etc.). The lighting in the box girder must be powerful enough to allow staff to move around in complete safety (Fig. 9.4). For this purpose, bracket lights with spherical light bulbs or, more commonly, neon tubes or fluorescent strip lights are used. These lights are positioned centrally underneath the upper slab with a longitudinal centerto-center spacing of no more than fifteen meters and the high-risk areas around the cross beams and deviators must be adequately lit. If the total length of the structure does not exceed 400 to 500 m, all of the bracket lights can be operated simultaneously using 2 two-way switches situated at both ends of the bridge deck. For bridges of over 500 m in length, it is recommended to organize the lighting system into sections of 300 to 400 m. In this case, in addition to the two switches on the abutments, two-way switches must also be fitted at the ends of each section N, in order to control the lighting for sections N and N+1. In the interests of economy, it is also possible


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to fit a timer to the lighting system, but this must be set to stay on for a long time (at least 10 hours) in order to avoid plunging workers into darkness when they are still inside the box girder.

Fig. 9.4 – Internal lighting in a box girder

Electricity is supplied by power sockets, which are usually positioned one meter above the lower slab, directly below the light fittings. To allow for the use of power tools inside the box girder, a 220- volt system with a frequency of 50 Hertz must be installed. In certain systems, these sockets are accompanied by safety sockets supplying 24-volt current. In all cases, the power socket circuit in the box girder must be totally independent from the lighting circuit, so that a problem with a tool does not suddenly extinguish the internal lighting. The electrical system must comply with the general standards in force for low-voltage electrical systems (i.e. the NF C 15-100 standard on the publication date of this guide). After installation, it must be approved by an accredited organization. There must be no risk of damaging the non-prestressed reinforcements or the prestress tendons during the installation of the system.

Fig. 9.5 – Access hatch for cabling

9 . 2 .3 -

Miscell aneous

9.2.3.1 – Access hatches for equipment

As explained in the Sétra “Précontrainte Extérieure” (External Prestressing) guide, it is necessary to examine different ways of routing extra or replacement tendons through the bridge deck, and for transporting the equipment needed for their installation. If it turns out to be impossible to route the tendons via the abutments, another solution would involve making a 1 m x 0.80 m hole in the lower slab, directly in line with a road lane if possible. Under normal conditions, this hole would be closed using a galvanized steel hatch (Fig. 9.5 and 9-6). For further information about this point, the reader is recommended to refer to section 5.45 in the guide mentioned above. These hatches may also be used to supply any other equipment, of course.


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In the past, certain box-girder structures featured access hatches in the central part of the upper slab. As these hatches cause maintenance and durability problems, this technique must be completely prohibited. 9.2.3.2 - Drainage of the lower slab

Water frequently penetrates into the box girder due to leaks in the structure or during maintenance operations. In order to allow this water to drain away quickly, it is essential to leave holes of 10 to 15 cm in diameter in the lower slab, equipped with a beveled tube extending through the intrados to form a drip groove. The longitudinal positioning of theses holes must take account of the longitudinal profile of the top of the lower slab, which is very different to the longitudinal profile of the road for a bridge deck of variable depth. It is also important to consider the position of the cross beams and deviators, which form a barrier preventing the drainage of water. Care must betaken to ensure that this configuration does cause water to run down the facing of the piers. Transversally, it is also important to account for the transverse slope of the top of the lower slab and of the tendons which might be installed there.

Fig. 9.6 – Piercing of a lower slab required by the lack of an access hatch 9.2.3.3 – Fixing rails for future networks

In order to facilitate the installation of networks inside the box-girder at some future date, it is common practice to install anchor rails or couplings on the underside of the upper slab. Usually made from galvanized or stainless steel, these fittings form the upper anchorages for the bars to which pipework, or a metal frame supporting the networks, can be attached. Fittings situated close to the tendon anchorages on piers may also be used for jack hoists, subject to their nominal load being greater than 20 kN. The principal characteristics of these rails and bushes (position, orientation, length, nominal load, cross section, etc.) must be determined according to a precise set of specifications for the networks they are designed to support. This guarantees their suitability for their future tasks. 9.2.3.4 – Identification and orientation

It is advisable to number the segments using indelible paint and a stencil according to the construction plans. This type of identification prevents the use of different numbering systems in inspection reports carried out over the years. On certain very large structures, it might also be useful to label cross beams on piers with the number of their corresponding pier as this could facilitate the identification of certain works.

9.3 - Piers


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9 . 3. 1 -

Space between the undersid e of the bridg e deck and the top of piers

In order to facilitate monitoring and maintenance operations, it is recommended to leave a minimum space of 0.50 m between the top of the piers and the underside of the bridge deck. 9 . 3. 2 -

D es i g n o f t h e p i e r h e a d u n i t s

9.3.2.1 – Access and inspection pits

If the height of the piers exceeds 8 to 10 m, it is recommended to build a pit into the pier head units. This pit, resembling a bathtub in shape, allows work to be carried out "comfortably" in the pier head area, in spite of the limited space between the top of the pier and the underside of the bridge deck (Fig. 9.7 and 9.8). These pits are usually 0.80 to 1 m deep and one meter wide; their length depends on the center distance between the bearings. A ∅ 100 mm drain hole is built into the pit for the drainage of any water that might enter, especially during construction. 9.3.2.2 – Position of the jacking points for the bridge deck

A bridge deck often has to be jacked up after commissioning in order to change the bearings, reset the slide plates or make adjustments due to the subsidence of a pier or a geometrical defect. To prevent problems or damage to the structure during jacking, it is necessary to determine the position of the jacking points at the design stage. The size and location of these jacking points depends on the amount of space available on the pier head unit. It is also important to position the jacking point as close to the axis of the nearest web as possible. In general, two jacking points are provided for each bearing. However, if the pier head unit is too short to accommodate two jacking points per bearing, it is possible to allow two points between both bearings, i.e. one point per bearing. These points will also be be situated as close to the webs as possible. Once these jacking points have been determined, the next stage is to design and install the reinforcements or prestressing required to take up the forces generated in the piers and bridge deck under these support conditions. It is also necessary to mark the jacking points on the plans and identify their positions on the top of the pier. Support blocks (Fig. 9.9) are usually used for this operation, although durable markers (studs, etc.) may be used if the geometry of the structure prevents the use of blocks. 9 . 3. 3 -

Inspection of hollow piers

9.3.3.1 - Inspection

Hollow piers are usually equipped with a system of safety ladders and landings throughout their full height (Fig. 9.10) for use during internal monitoring and inspection operations. This system is allows for the regular examination of the inside of piers with a level of accuracy that satisfies the requirements of scheduled inspections. For more detailed inspections, it is possible to use binoculars or additional ladders if stairs are fitted.


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Fig. 9.7 – Access to bridge deck from piers and vice-versa Fig. 9.8 – Inspection pit on pier head unit

Fig. 9.9 – Block marking the location of a jacking point

The dimensions of the system (height and depth of steps, frequency of landings) are determined according to the standards in force relating to stairs in buildings. NF E 85-010 "Échelles métalliques fixes avec ou sans crinolines" (Fixed Metal Stairs With or Without Safety Bows) was the applicable standard on the publication date of this guide. Considering the financial investment required for this type of equipment, precautions must be taken to guarantee its longevity. Stairs or ladders must be made from hot galvanized steel thick enough to ensure that minor corrosion does not affect the strength of these elements. Landings can be made from galvanized steel or the same concrete that was used to construct the pier shafts. In recent years, some Owners have decided to restrict the fittings inside hollow piers on large bridges to a single metal gridded platform situated just underneath the crosshead and couplings to which harnesses can be attached (Fig. 9.12). This creates a working platform from which inspectors equipped with climbing gear can operate. This system reduces the initial investment costs and guarantees that inspections will be carried using the latest totally compliant equipment. However, an inspection of the piers under these conditions requires more equipment and more time, which also increases the cost of the operation, of course.


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Fig. 9.10 – Safety ladders and landings inside a pier

Fig. 9.11 – Access door to a pier or an abutment with five-point safety lock

Fig. 9.12 – Minimal fittings inside a pier 9.3.3.2 - Access

Whatever the context, it is necessary to allow access to hollow piers from the bridge deck. For this, a shaft is usually created at the bottom of the inspection pit described in Paragraph 9.3.2.1. As before, this hole must normally be closed using a metal grid or plate. It must also be offet in relation to the shaft built into the cross beam on pier, to make sure that if someone falls from the bridge deck, they will not fall further than the bottom of this pit. In the majority of cases, a second access point is also provided via a metal door at the base of the pier. Considering the risks of vandalism, it is necessary to use the same type of doors as those recommended for the abutments (see 9.4.2 and Fig. 9.11). It is also possible to situate these doors three or four meters above ground level, requiring a ladder for access. These doors are rarely aesthetically pleasing and it is important to make sure that the architecture of the pier is as complimentary as possible. If it is impossible to provide doors at the base of the piers (e.g. piers on the sea bed, risk of vandalism too great, insufficient strength, etc.), it is necessary to create a reasonably large shaft leading from piers to the bridge deck. Indeed, this passageway must be big enough to be used for transporting and removing maintenance equipment. In the event of an accident during a maintenance operation, any victims will also have to be evacuated via this single passageway. 9.3.3.3 - Lighting

Hollow piers must be equipped with a lighting system similar in design to the system used for the bridge deck.


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9.4 - Abu tment s 9 . 4. 1 -

Access to abutments

Access points to the abutments must be designed and built with care. In general, access is gained from the supported road, via the return walls of the abutments. However, it is also possible to enter abutments from another road below the structure if parking spaces are provided. These access points must be created with care (concrete surfacing, concrete or wooden stairs, guard rails if necessary, etc.). However, they must also be as unobtrusive as possible in order to avoid attracting unwanted attention. 9 . 4. 2 -

R es t r i c t i n g a c c e s s i n t h e a b u t m e n t s

On large bridges built in recent years, the front face of an abutment is usually closed off by a concealing wall, which is entered through a metal door. This system is highly recommended, as it denies access to unauthorized persons and therefore prevents malicious damage to the bridge bearings, external continuity prestressing and the guttering under the joints. This solution is also aesthetically pleasing, given that these walls hide the inside of the abutments which are never very attractive. It is also important to fit high-quality locks on the doors. Five point safety locks bearing the "A2P trois étoiles" label awarded by the french APSAD ( Assemblée Plénière des Sociétés d’Assurances Dommage - Plenary Assembly of Damage Insurance Companies) are particularly suitable for this task (see Fig. 9.11 on previous page). In addition to this important provision, as little room as possible should be left between the concealing wall and the outer edges of the box girder. No more than 15 cm should be allowed for this gap, including between the underside of the bridge deck and the top of the central part of the concealing wall. 9 . 4 .3 -

Electr ical sys tem

Although covering a much smaller area than a bridge deck, an abutment must have the same type of electrical system (lighting and power sockets). It is important to fit a light switch next to the door mentioned in 9.4.2 above. 9 . 4 .4 -

Depth of abutm ents

T y p i c a l e x a m p l e o f a b u t m e n t s wi t h c a b l e p u l l i n g c h a m b er

One of the essential requirements for changing the external prestressing is the ability to gain access to the anchorages to stress the new tendons. As mentioned in section 5.42 of the "Précontrainte extérieure" (External Prestressing) guide published by Sétra, the design of an abutment must incorporate a cable pulling chamber if the external tendons are attached to stressed anchorages on the cross beam directly above the abutment. In order to determine the effective length L of this chamber, i.e. the distance between the internal face of the abutment wall and the end of the bridge deck, not including the corbel, it is necessary to consider the phase of the replacement process which requires the most room. For external tendons injected with a flexible product (wax or grease), this key phase in the design of this chamber is when the new tendons are in their ducts and the tensioning jack is positioned at the end of the strands prior to “swallowing” them up (Fig. 9.13). In this phase, the space required corresponds to the length of the jack Lv plus the length of the strands connected to the jack prior to stressing Lt.


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Fig. 9.13 – Key design phase for determining the minimum length of the cable pulling chamber

The following table gives the lengths Lv, Lt and (Lv+Lt) for the 19T15S, 27T15S and 37T15S types of tendons used in certain French prestressing systems. Unit

19T15S

27T15S

37T15S

Supplier

System

Lv

Lt

Lv + Lt

Lv

Lt

Lv + Lt

Lv

Lt

Lv + Lt

Freyssinet

C with jack Cl 000F FUT SB

1.28

1.11

2.39

1.29

1.12

2.41

1.25

1.08

2.33

1.07 1.14

1.25 1.20

2.33 2.34

1.18 1.16

1.40 1.25

2.58 2.41

1.20 1.18

1.40 1.38

2.60 2.56

SEEE Spie Pré contrai nte

At the design stage, it can therefore be considered that the effective length L of the chambers must be at least 2.40 m for 19T15S cabling and 2.60 m for 27T15S or 37T15S cabling. Sp ec i f i c e x a m p l e s o f a b u t m e n t s w i t h o u t c a b l e p u l l i n g c h a m b er s

In certain very specific cases, cable pulling chambers may not be built into abutments. In this case, the additional length described above is no longer required. However, the distance between the end of the bridge deck and the abutment wall must not be reduced too much. A minimum distance of 0.80 m to 1 m is essential for inspecting and correctly maintaining these parts of the structure. 9 . 4. 5 -

Collection of water under the expansion joint

Expansiont joints are fitted at the ends of almost all large modern bridges, and are therefore situated directly above the abutments. Despite the care and attention devoted to their design and fitting, these joints are never totally watertight. Therefore, it is necessary to collect any water that percolates through them in order to protect the abutment crossheads, bridge bearings and the external prestressing anchorages situated at the ends of the structure. The best systems are shown in Figures 9.14 to 9.l6, consisting of a metal gutter positioned under the joint. Supported by galvanized steel brackets, this gutter must be positioned centrally under the expansion joint, which requires a corbel of 30 to 40 cm on the side of the bridge deck. In order to prevent splashes, it is necessary to channel the water using vertical neoprene splash guards enclosing the space between the joint and the gutter. These splash guards, which are usually different to those supplied with the joint, must be weighted for maximum resistance to the draft of air created by passing trucks. To facilitate the cleaning of the gutter, it is recommended to fit a water tap on the abutment, provided that the structure can be connected to the drinking water network at reasonable cost. The pipes must then be protected from freezing.


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Fig. 9.14 – Collection of water under pavement joints

Fig. 9.15 – Gutter under joint (the distance to the slab allows for easy cleaning)

9.4.6 -

Fig. 9.16 – Gutter attachment

Space between the underside of the bridge deck and the top of the crossheads

As for the piers, it is advisable to leave a minimum gap of 0.50 m between the top of the abutment crossheads and the underside of the bridge deck in order to facilitate maintenance operations. 9 . 4. 7 -

Bridg e deck jacking point s

Like piers, abutments must have jacking points for the bridge deck. These are determined according to the procedures described in paragraph 9.3.2.2 above.


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10 - Recommendations for the creation of a Contractor Tender Document This section contains a number of recommendations and instructions for the creation of a Contractor Tender Document for a prestressed concrete bridge built by the cantilever method, and particularly with regard to the drafting of the written documents required. Given that it is impossible cover all of these aspects, these recommendations focus above all on items which are strongly influenced by this particular construction method and on the associated bridge deck structures.

10.1 - Nature of the tender enq ui ry In France, the vast majority of bridges built by the cantilever method are constructed in the framework of a restricted invitation to tender, i.e. with a preliminary shortlisting of applicants. In most cases, the contract is not divided into tranches or packages. Very large projects, however, may sometimes be broken down into tranches in order to stagger the financing of the works, although this system is not be recommended because the tranches can rarely be considered in isolation.

10.2 - Creation of a Contr actor Tender Doc ument Apart from in very specific situations, Contractor Tender Documents consist of three sub-groups or schedules. Sub-group 0 is reserved for the tender regulations (règlement de la consultation [RC]). Sub-group I contains the documents that make up the contract. It contains the frameworks for the tender document, the price schedule, cost estimate, draft the Special Administrative Clauses (Cahier des Clauses Administratives et Particulières [CCAP]), the Special Technical Clauses (STC), and occasionally, price breakdowns and breakdown frameworks. It also includes a series of documents appended to the STC, including:

•

Site plan

•

Plan view

•

Longitudinal cross-section

•

Transverse cross-sections of bridge deck

•

Detailed plan of cross beams and deviators

•

Detailed plan of bridge deck superstructures

•

Pier formwork plans

•

Abutment formwork plans

•

Contractual part of the geotechnical survey, i.e. usually consisting of the borehole logs.


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In certain cases, sub-group I also contains specific studies which have a direct influence on the design of the structure: hydraulic studies, wind-effect study, etc. In other cases, certain architectural studies may also be included in sub-group I in order to make the information relating exclusively to these studies contractually binding (reference to a matrix for the bottom shuttering matrix, aggregates, etc.). Finally, if the structure has to be built over or close to a traffic-bearing road, railway or waterway, sub-group I must also contain a document listing the road, rail or water-related constraints facing the contractor during the works. Sub-group II is made up of documents intended for information purposes only. For structures built by the cantilever method, this sub-group usually includes the following items:

•

Plan of the areas capable of accommodating the site installations

•

Plan of possible access routes to the site

•

Longitudinal cabling plan

•

Transverse cabling plan (for structures featuring transverse prestressing)

•

Plan of the bridge deck showing a breakdown of segments

•

Scheme of construction operations

•

Plan of the cantilever stabilization system

•

Preliminary quantity survey

•

Architectural study

•

The non-contractual part of the geotechnical survey: usually the pre-design for the foundations produced by the laboratory in charge of this survey.

In addition, sub-group II sometimes contains initial plans for the bridge deck reinforcements, along with design calculations or miscellaneous studies containing information for contractors. In recent years, sub-group II of certain Contractor Tender Documents has also included reports concerning the formulation of concrete, including the results of specific studies carried out upstream by the Construction Manager (HPC, alkali reactivity, frost and salt-resistant formulas, etc.)

10.3 - Tender regu lati ons 1 0 .3 .1 -

S u p p l e m e n t s t o t h e S p e c i a l T e c h n i c a l Cl a u s e s / T ec h n i c a l p r o p o s a l s

Technical proposals are detailed precisions that contractors are obliged to include in their bid in addition to their proposed basic solution. Article 2.3 b of the Tender regulations specifies the structural elements for which technical proposals must be submitted. With regard to the bridge deck of a structure built using the cantilever method, the following points must be covered in these proposals:

•

Origin of the components, the composition and the application of the concrete

•

Bridge bearings


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•

Prestressing processes

•

Procedure for the waterproofing layer

•

Expansion joints.

10.3.2 – Technical variants

The Owner may authorize contractors to offer variants with a view to encouraging competition. Contrary to the technical proposals, these variants, if they are accepted, may require alterations to be made to the price schedule, cost estimate and of course the Special Technical Clauses and the attached plans. The permissible technical variants are listed in Article 2.3 of the Tender Regulations. The most common variations include:

•

The breakdown into segments

•

Types of prestressing units used

•

Internal form of bridge deck gussets

•

Scheme of construction operations

•

Cantilever stabilization system.

More flexible variants are sometimes permitted. These may include:

•

The replacement of concrete webs with lightweight metal webs (for long span bridges)

•

Substitution of the cantilever construction technique by the incremental launching method, with or without pilings depending on the situation (for moderately long bridges of a constant depth)

•

A change of construction technique for the foundations of one or more supports.

10.4 - Tender doc ument 1 0 .4 .1 -

Period of validit y for bids

The evaluation phase for the bids for a structure to be built by the cantilever method is often longer than for a basic structure. Therefore, the first article of the tender document should specify a reasonably long period of validity for the bids (a minimum of 180 days), or even longer if contractors are likely to propose major variants. 1 0 . 4. 2 -

Preparatio n perio d

As we have already mentioned, construction surveys for structures built by the cantilever method are long and complex. Sufficient time must also be allowed for Construction Managers to evaluate these surveys. For certain structures, work on site may sometimes advance at a faster rate than the construction surveys can be carried out. In order to prevent the interruption of work due to delays in the availability of construction plans that have been approved by the Contruction Manager, it is essential to allow a sufficiently long preparation period in which the engineering firms can reach a stage that is well ahead of the work carried out on site. The period required varies The “Les outils” collection – Sétra

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from structure to structure, but the amount of time required is inversely proportional to the time allowed for the construction of the piers.

10.5 - Special A dmin istr ative Clauses 1 0. 5 .1 -

G e n er a l c o n t r a c t d o c u m e n t s

For a structure built by the cantilever method, the list of general documents to be included in the contract includes at least the following documents:

•

Fascicule 61 Titre II of the french Cahier des Prescriptions Communes (Common Conditions) [CPC]) : "Programme de charges et épreuves des ponts-routes" (Program of Loads and Tests for Road Bridges)

•

Fascicule 61 Titre IV Section II of the french General Technical Clauses (GTC): “Actions de la neige sur les constructions” (Effects of Snow on Structures) (DTU P 06-006 of September 1996)

•

Fascicule 62 Titre I - Section I of the french GTC: "Règles techniques de conception et de calcul des ouvrages et constructions en béton armé suivant la méthode des états limites" (Technical Rules for the Engineering and Design of Reinforced Concrete Structures According to the Limit State Method) [BAEL 91 revised in 1999]

•

Fascicule 62 Titre I - Section II of the french GTC: "Règles techniques de conception et de calcul des ouvrages et constructions en béton précontraint suivant la méthode des états limites" (Technical Rules for the Engineering and Design of Reinforced Concrete Structures According to the Limit State Method) [BPEL 91 revised in 1999]

•

Fascicule 65-A of the french GTC and its supplement: "Exécution des ouvrages de génie civil en béton armé ou précontraint" (Construction of Reinforced and Prestressed Concrete Civil Engineering Structures) in their updates published in August 2000

•

Fascicule 62 Titre V of the french GTC: "Règles techniques de conception et de calcul des fondations des ouvrages de génie civil" (Technical Rules for the Engineering and Design of Civil Engineering Structures)

•

Fascicule 68 of the french GTC: "Exécution des travaux de fondation des ouvrages de génie civil” (Construction of the Foundations for Civil Engineering Structures).

For structures supporting oversize loads, it is also necessary to mention:

•

French Circular no. R/EG3 of July 20 1983 entitled "Transports exceptionnels, définition des convois types et règles pour la vérification des ouvrages d’art" (Exceptionally Large Trucks: Definition of Standard Loads and Rules for the Verification of Civil Engineering Structures), published by the Direction des Routes (Highways Department), for structures supporting these types of vehicles.

In earthquake zones, it is also necessary to add the following specific documents:

•

The AFPS 92 guide for the seismic protection of bridges, edited by the Association Française du Génie Parasismique (AFPS) [French Seismic Engineering Association], published by the École Nationale des Ponts et Chaussées (National School of Civil Engineering)


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•

The officially approved standard NF P 06-013, more commonly known as the "Règles de construction parasismique - règles applicables aux bâtiments - PS92" (Antiseismic Construction Rules – Rules Applicable to Buildings – PS92), with regard to the foundations

•

Order no. 91-461 of May 14 1991 relating to the prevention of seismic risks and to the Decree of September 15 1995 relating to the classification and rules for antiseismic constructions applicable to bridges in the “normal risk” category. 1 0 . 5. 2 -

Preparatio n perio d

Article 8.1 of the Special Administrative Clauses (Cahier des Clauses Administratives particulières [SAC]) specifies the length of the preparation period and indicates whether or not this is included in the contract period. Further details concerning this point are given in article 10.4.2 of this chapter. 1 0 .5 .3 -

Examination period for constr uctio n surveys

As conflicts between the Contractor and Construction Manager often arise over compliance with acceptance deadlines, it is recommended that Article 8.2 of the Special Administrative Clauses should contain a text which clearly mentions:

•

The documents considered by the Construction Manager to form an indivisible whole

•

The deadlines that the Construction Manager agrees to observe, in the framework of the first and subsequent evaluations of the documents.

Although it could probably be improved, the following text could serve as an example for this text: Delays for the examination and approval of construction documents

The contractor must submit the construction surveys for each part of the structure to the Construction Manager for approval in the form of homogeneous groups of documents (e.g. formwork plans, reinforcement plans and design calculations for the section concerned), accompanied by the corresponding construction procedures. The Construction Manager shall notify the contractor of his findings in writing and within a maximum period of forty-five (45) working days for the first examination of the "longitudinal flexion of the bridge deck" and “transverse flexion of the bridge deck" groups and twenty-five (25) working days for the first inspection of the other groups of documents. These deadlines are reduced to fifteen (15) and five (5) working days for the subsequent examinations of these groups of documents. It should be noted that in the event of the staggered arrival of documents within the same group, these periods start on the arrival date of the last document. 1 0. 5 .4 -

Operating const raints in the publi c domain

Many bridges are built immediately next to or even above other traffic-bearing routes. Under these conditions and for safety reasons, it is necessary to wait until the traffic on these arteries has been interrupted before carrying out certain operations. As traffic can normally only be interrupted at certain times of the day, work on site may be disrupted. It is important to mention these constraints clearly in articles 8.4 and 8.5 of the SAC, either by describing the constraints directly in these articles, or by referring the reader to another of the documents included in the Contractor Tender Document.


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1 0 .5 .5 -

Duration of hold point s

Article 91 of the SAC outlines the main hold points on the site as well as the time necessary for the Construction Manager to cancel these hold points. The following table gives a non-exhaustive list of the hold points likely to apply to the bridge deck of a structure built by the cantilever method in addition to the average delay before they can be lifted. Hold point s

Time required

Casting and form work removal Acceptance of the quality control element Authorization for the casting of a part of the structure Authorization to move formtravelers forward Authorization to remove falsework from a section of bridge deck Prestressing Authorization for tensioning of prestressing Acceptance of tensioning before reinforcements are cut Authorization for the injection of prestressing ducts Bridge bearings Acceptance of bearing blocks Acceptance at moment of delivery Acceptance of the fitting of bridge bearings (adjustment and positioning) Fittings Acceptance of waterproofing substrate Acceptance of waterproofing; authorization to apply surfacing materials Acceptance of a quality control element for the prefabricated concrete cornice Acceptance of the adjustment of cornices before sealing Acceptance of pavement joints before fixing or sealing Acceptance of retaining systems before sealing Testing Authorization to performloading tests

1 day 1 day 1 day 1 day 1 day 1 day 1 day 1 day 1 day 1 day 1 day 1 day 1 day 1 day 1 day 1 day 1 day

10.6 - Special Techni cal Claus es 10.6.1 -

Preamb le

There now follows a list of points that must be clearly specified in the Special Technical Clauses (STC) for the Contractor Tender Document. These points may be broken down into two categories. The first concerns elements of additional information to that provided in the GTC and by the applicable standards, either because these documents are incomplete, or because none of them cover the field in question. The second category concerns the options proposed by these texts. In line with the standard STC included in the CAPT-DCE-OA software published by Sétra, none of these points repeats the requirements laid down in the GTC and the applicable standards, because they are already binding for the contractor.As these points concern conventional types of box girders built by the cantilever method, additional information may be required for more sophisticated box girder designs. 1 0 .6 .2 -

Construct ion survey progr am for the struc ture

As we have already mentioned, great care should be taken with regard to the quality and execution of the surveys. With this in mind, it is advisable to include an article entitled “Programme des études d’exécution” (Construction Survey Program) in the Special Technical Clauses. This can be worded in the following manner: Construction Survey Program

The Contractor must submit a construction survey program which includes a list and a provisional schedule of the documents to be drawn up.


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The list enumerates the documents that must be submitted for the construction of both temporary and permanent structures. It shall be drawn up in accordance with the framework of surveys specified in the contract. The provisional schedule includes the deadlines for the submission of documents and the fixed or tentative deadlines for approval by the Construction Manager, in accordance with the minimum deadlines established by Article 8.2 of the SAC. It presented in the form of a bar chart clearly showing the critical tasks and the time allowed. 1 0 .6 .3 -

Construct ion surveys for the struc ture

10.6.3-1 -Actions

The STC must state all of the actions to be considered in the verifying calculations for the structure. Many of these actions are defined in fascicule 61 Titre II of the french CPC and in the BPEL 91 revised in 1999. These actions, which are normally listed and described in the STC are:

•

Selfweight of the bridge deck (specify the unit mass of the concrete)

•

Weight of the fittings added to the bridge deck

•

Prestress forces

•

Concrete shrinkage and creep (specify the area in which the structure is situated)

•

General thermal effects (gradient and uniform variation)

•

Thermal effects (uniform variation) to be considered for the expansion joints

•

Predicted overloads from road vehicles and pedestrians on the structure, including fatigue trucks, if used

•

Impacts on restraint systems.

These actions must be accompanied by certain actions which are specific to box girder bridge decks and the method of construction. These must be specified in the STC and relate toconcern:

•

Selfweight of temporary structures (form travelers, launch beam, etc.)

•

Site overloads defined in Chapters 3 and 5 of this guide, only to be used for calculations in the construction phase before the closure of a cantilever

•

Specific site actions (e.g. an unbalanced segment, collapse of one of the form travelers, forces transmitted by launch beam supports, etc.), also described in Chapter 5 of this guide.

Depending on the situation, it may also be necessary to mention certain additional actions such as:

•

Impacts on cetain supports

•

Thrust due to water or ice

•

Wind and/or ice to be considered for certain very exposed sites


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•

Seismic effects

•

Etc.

10.6.3.2 – Combined actions

The STC usually mentions the different combined actions that have to be considered. The combinations to be used for the in-service verification of the structure are specified in the BPEL 91 revised in 1999. Combinations to be used for verifying the structure with regard to the risks of cantilever instability are given in Chapter 5 of this guide. It should be noted that they differ from those mentioned in the Directives Communes (Common Guidelines) of 1979. 10.6.3.3 –Verification of the bridge deck

The STC must specify:

•

The verification categories, as described in Article 1.3 of the BPEL 91 revised in 1999, used to verify the longitudinal flexion of the bridge deck and the transverse flexion of the upper slab on structures with transverse prestressing (see Paragraph 3.3.2)

•

The cracking categories, as described in Article A 4.5.3 of the BAEL 91 revised in 1999, for the verification of reinforced concrete parts (transverse flexion, deviators, cross beams, etc.)

•

The thermal gradient and the thermal expansions to be adopted in the construction phase (see Paragraph 3.3.3.2).

As differences of opinion exist between Construction Managers, the STC must specify whether Article 6.1.2.3 of the BPEL 91 revised in 1999 is applicable in the strict sense, or whether different permissible stresses must be adopted. (For example, Paragraph 3.3.3 of this guide recommends the application of the comment in Article 6.1.2.3 in construction situations). The STC must also state whether it is compulsory to perform a range calculation for prestress effects. If not, the STC must specify the values of k and k’ to be adopted for the application of Article 4.10 of the BPEL 91 revised in 1999. These values are directly related to the requirements formulated by the Construction Manager with regard to empty ducts and the measurement of transmission factors (see our recommendations in 3.2.3). As the BPEL 91 revised in 1999 does not include any rules for the accumulation of non-prestressed reinforcement bars designed to compensate for transverse flexion on the one hand, and the tangent general flexion and distribution forces on the other, it is essential to include a rule of this type in the STC (see Chapter 4 of this guide). It would also be advisable to add a rule governing how to allow for different construction phases in the calculation of stresses at the ULS (see our proposals in paragraph 3.3.5). In accordance with Paragraph 3.2.5, it is important to note that an as-built calculation must be performed after the completion of the construction work. This calculation takes account of the actual phasing of the work carried out on a bridge built by the cantilever method, with the actual dates of casting and any incidents that occurred on site (e.g. a broken strand that was not replaced). It may also consider the estimated coefficients of friction given by measurements taken on site. Finally, depending on the structure in question, the following requirements may also be specified in the STC:

•

Allow for the curvature of the structure on the horizontal plane in the calculations of longitudinal flexion


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•

Perform a finite elements calculation for a section of the bridge deck

•

Perform a finite elements calculation for a specific part of the structure (segment on pier, segment on abutment, ribs, etc.)

•

Perform a second-order calculation for one or more piers if they are very slender (in this case, the STC must specify the design assumptions for the horizontal forces in the pier heads)

•

Etc.

Chapter 3 of this guide gives a lot of valuable advice on choosing the coefficients and rules mentioned in this paragraph. 1 0. 6 .4 -

Brid ge deck form work

10.6.4.1 - Formwork

The STC must specify the category of facing to be used for the inner and outer surfaces of the box girder. This category must conform to the definitions provided in Article 52 of Fascicule 65A of the french CCTG. 10.6.4.2 – Small control slabs or test segment

The STC must specify whether the contractor must manufacture a test segment for the suitability tests or whether small control slabs or elements will suffice. In all cases, the STC must clearly mention:

•

The dimensions of these elements

•

The ducts and non-prestressed reinforcement bars that they must contain.

•

Whether continuity cable anchor blocks must be fitted.

Furthermore, the STC must clearly define the acceptance conditions for these tests: inspection of facings, core sampling in the sheath areas, measurement of flatness of facings, etc. 10.6.4.3 – Removal of formwork

It is advisable to establish a minimum strength requirement for the concrete at the moment the formwork is removed. This must not be less than 15 MPa. 1 0 . 6. 5 -

Prestr essin g

10.6.5.1 – Cantilever tendons

The STC must precisely state the number and type of strands to be used for the cantilever tendons, along with their strength and relaxation categories. It must also mention the type of duct and, of course, the type of injection product to be used. 10.6.5.2 - Continuity tendons

The characteristics mentioned in the STC for the continuity tendons must be the same as those specified for the cantilever tendons. The Owner’s requirements for the permanent protection to be used on the anchorages (e.g. The “Les outils” collection – Sétra

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conventional sealing, metal caps, etc.) must also be specified. Certain Construction Managers demand rigid ducts for the ducts at the level of the anchor blocks, guaranteeing a consistent alignment without any concentration of radial tendon forces. 10.6.5.3 – Transverse tendons

If the project calls for the installation of transverse tendons in the upper slab or ribs, the STC must clearly state the applicable requirements, which are identical to those for the continuity tendons. 10.6.5.4 – External tendons

The characteristics mentioned in the STC for the external tendons must be the same as those specified for the cantilever tendons. The type of deviator tubes and corrosion inhibitors to be used in the structural deviators, in addition to the types of anti-vibration systems to be adopted, if any, must also be specified. Finally, the Owner’s requirements with regard to additional prestressing, and more specifically, to the capacity of this prestressing, must also be specified. 1 0 .6 .6 -

Special fitti ngs in the box girder

As these fittings are not always clearly marked on the plans included in the Contractor Tender Document, the STC must clearly describe the maintenance fittings that the Owner has requested for the structure. These fittings include:

•

The electrical system for the box girder, including the internal lighting

•

Stairways or ramps for passing the cross beams and deviators

•

Closing devices (doors, grids, plates, etc.) for the access points to the bridge deck (manholes in cross beams on abutments, manholes in the lower slab, on piers or close to abutments)

•

Anchor rails fixed to the upper slab or webs, which may be used for the immediate or subsequent installation of networks in the box girder

•

Handling rails fitted in the upper slab, in line with any hatches that may be built into the lower slab

•

Rails used in the deployment of tensioning jacks for the external prestress tendons

•

Inspection equipment for hollow piers.

Chapter 9 of this guide contains numerous guidelines for the drafting of the corresponding clauses. 1 0 .6 .7 -

Inspection of work

As mentioned in Chapter 7, the inspection of work carried out on a structure built by the cantilever method is an important and complex task which the Construction Manager must start to organize quite a long time in advance. The Contractor Tender Document (SAC, STC) must clearly specify these inspections, which usually correspond to hold points for contractors.


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In addition, it may not be possible for the Contract Manager and his or her laboratory to carry out certain inspections. If this is the case, it will be necessary to use the services of an external inspection company. These inspections must be clearly defined and paid for separately.

10.7 - Pric e sch edul e A standard price schedule which is perfectly suited to the construction of all standard structures is appended to fascicule 65A of the french CCTG. In addition to the prices normally shown in this appendix, the price schedule for a prestressed concrete structure built by the cantilever method using cast-in-situ segments must also include prices for the following elements:

•

Cantilever stabilization systems

•

Form travelers

•

Formwork tool for segments on piers

•

Centering for end sections.

The price for the cantilever stabilization system is normally given as a “fixed price” or a “fixed price per pier”, covering the supply, installation, and eventual removal of the temporary stability blocks, the temporary prestressing and, if necessary, the temporary pilings or cable-stays. The price also covers any jacking operations required during construction, especially during the transfer onto permanent bearings. The price for the form travelers usually includes the design, construction, transportation, adjustments, successive movements during construction and the final dismantling of form travelers used on site, in addition to cost of external inspections. This is either a fixed price or, for very large structures, a “per pair” price. In view of the large sums of money involved, this price is normally paid in instalments as the work advances on site. The price of the formwork tool for the segment on pier is given as a fixed or “fixed per pier” price. This normally includes the design, construction, transportation, adjustments, successive movements during construction and the final dismantling of structures used for centering and encasing the segments on piers, in addition to the cost of external inspections. The price for the centuring of end sections includes the design, assembly and removal of the centuring used for the sections of bridge deck not built by form travelers, plus the costs of external inspections. Payment is often on a fixed-price or fixed price per abutment basis, although a price per square meter may also be charged according to the area of bridge deck to be shored up.


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A1 Determination example Example of the determination of a bridge deck built by the cantilever method This appendix presents an example of the calculation of the bridge deck for a structure built by the cantilever method. It also provides information about specific aspects of the design.

A 1.0 – Pu r p o s e o f t h i s ap p en d i x Based on a concrete example, this appendix describes a simplified method for roughly calculating the formwork and prestressing for a bridge deck of a cast in-situ structure built by the cantilever method. Considering the complexity of the calculations to be performed, this method is not designed to determine the definitive formwork and cabling. Instead, it is used to provide the minimum amount of information (e.g. formwork, breakdown into segments, cantilever tendons) required for the preparation of a more sophisticated computerized design model. Like the rest of this guide, this appendix has been drawn up in accordance with the BPEL 91 revised in 1999 and fascicule 61 Titre II of french CPC.

A 1.1 - Rem i n d er s A 1.1.1 - No t at i o n

Figure A1-1 describes the notation used. We take: G to be the centre of gravity of the cross-section v to be the distance from G to the upper axis v’ to be the distance from G to the lower axis h = v + v’ to be the total height of the section eo to be the off-setting (off-centring?) of the mean tendon d to be the minimum distance from the mean tendon to the upper axis in order to guarantee the adequate coating of the tendons d’ to be the minimal distance from the mean tendon to the lower axis in order to guarantee the adequate coating of the tendons


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Fig. – A1.1 – Notations

Furthermore, we take: B to be the area of the section I to be its moment of inertia in relation to a horizontal axis passing through G ρ =

I Bvv '

to be the geometric output of the cross-section

c = ρv to be the ordinate (in relation to G) of the highest point of the central core c’= ρv’ to be the ordinate of the lowest point of the central core Mg to be the moments due to the selfweight Mq `to be the moments due to the fittings Ms1 and Ms2 to be the maximum and minimum extreme moments respectively (counted algebraically) due to the imposed loads Ml and M2 to be the maximum and minimum extreme moments respectively applied to a cross section M = M1 - M2 F to be the prestress force (effective traction). Finally, we use the same indices for shear forces (T) as for the moments. A 1.1.2 – Rem i n d er s ab o u t p r es t r es s i n g A1.1.2.1 – Limit stresses

In Class II of the BPEL, the limit stresses are: - in compression: σ c = 0.6 f cj − k f c with k = 0.02 for such a structure


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- in traction: − σ t = − ( f tj − k f c ) Given that the hyperstatic forces due to prestressing are ignored, in each section we should have: below Mmini = M2

σ sup

σ inf =

below Mmaxi = M1

=

P P e0 + M 2 − v ' ≤ σ c B I

=

σ sup

σ inf

P P e0 + M 2 + v ≥ σ t B I

=

P B

+

P e0 + M 1 I

(1)

(2)

v ≤ σ c

(3)

P P e0 + M 1 − v' ≥ σ t (4) B I

This can be expressed as: − c ' −

M 2 P

−

I σ t v P

≤ e0 ≤ c −

M 1 ¨P

+

I σ t v ' P

A1.1.2.2 – Sub-critical sections

These are sections in which we are not restricted by the coating requirements for the tendons; P can therefore be chosen so that: − c ' −

M 2 P

−

I σ t v P

= c −

M 1 ¨P

+

I σ t v ' P

so P =

ΔM − Bσ t c + c '

and

e0

= c −

M 1 P

+

I σ t v ' P

= − c ' −

M 2 P

−

I σ t with v P

− (v '−d ')≤ e0 ≤ v − d

In so far as we can choose the thicknesses of the two members, we can choose them in such a way that I I Δ M = = (with Δσ being the range of stress variation σ c v v' Δσ

+ σ t ).

Thus, under extreme loading cases, we may obtain the four limit stresses (Fig. A1.2a), but if an excessively thick member is dictated (e.g. an upper member for which

I > v

ΔM normally applies) we will only be able to Δσ

obtain three limit stresses (Fig.A1.2b). - σ t - σ


0

0

²σ σ

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0

0

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Fig. A1.2 – Limit stress diagrams for a sub-critical section A1.1.2.3 – Over-critical sections

These are sections in which we are restricted by the coating requirements for the tendons. Based on compliance with the minimum stress, this gives: P ≥

ΔM ΔM − B σ t = − Bσ t ρ h c +c '

P must therefore be at least equal to the higher of the two values below (according to the structurally significant moment): M 1 I c + v '−d ' + σ t v

M 2

and −

c ' + v − d +

I σ t v '

As in the previous situation, the number of limit stresses that can be obtained depends whether or not it is possible to modify the thickness of the members. Let us consider the example of a section that is mainly subjected to negative moments (e.g. a section on an intermediate support); therefore M2 (the minimum algebraic value) is structurally significant. If we are able to modify the thickness of the upper structural member, it will be possible to obtain three limit stresses (Fig. A1.3a), with: P = −

M 2 I c ' + v − d + σ t v '

and

I = v

ΔM Δσ

(compliance with the change of stress σ b, in the upper axis).

If, as is usually the case, the size of the upper member is imposed (excessively thick), we will only be able to obtain two limit stresses (Fig. A 1.3b) with: P = −

M 2 c ' + v − d +

I σ t v '

- σ t

0

- σ t

σ

≤ σ

0

sous M1

sous M1

sous M2

- σ t

sous M2

σ 0

σ

0

(a)

(b)

Fig. A1.3 – Limit stress diagrams for an over-critical section


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1 0 .7 .1 -

Focal point method for the design of a conti nuous beam

Using the notation adopted by M. Courbon, the focal point method consists of using the mechanical constants ai, bi and ci for each span to calculate the focal point ratios for the left, (φi) and right, (φ’i). Both groups of equations are given below:

ϕ1 ψ1 = 0

ϕ‘n = 0 b2 ϕ 2

bi +1 ϕ i +1

bn −1

= a2 + c1 - b1 ϕ1

bi

= ai+1 + ci - bi ϕi bn ϕ n

= cn-1 + an - bn ϕ‘n

ϕ ' n −1

ϕ ' i

b1

= an + cn-1 - bn-1 ϕn-1

ϕ '1

= ci + ai+1 - bi+1 ϕ‘i+1

= c1 + a2 - b2 ϕ‘2

Having ascertained the the focal point ratios and the rotations ω’i and ω"i for the end of span i, which is considered to be independent and subjected to the loading case in question, we can calculate the bending moments on the supports Ai-1 and Ai when span Ai-1 Ai is loaded. ω ' i

Mi-1

ϕ ' i

=

bi (

+ ω " i

1 ϕ i ϕ ' i

ω ' i +

Mi

=

bi (

− 1)

ω " i ϕ i

1 ϕ i ϕ ' i

− 1)

The moments on the other supports are determined by the following focal point ratios: Mi-2 = -ϕi-1 Mi-1

Mi-3 = -ϕi-2 Mi-2

etc.

Mi+1 = -ϕ‘i+1 Mi

Mi+2 = -ϕ‘i+2 Mi+1

etc.

A 1.1.4 – Un i t s o f m eas u r em en t

The following units of measurement are systematically used in this example (unless otherwise specified) : - Length: meter (m) - Mass: tonne (t) - Force: MegaNewton (MN) - Stress: MegaPascal (MPa), which is also equal to 1 N/mm² or 103 kN/m². In addition, we shall adopt a value of g = 9.81 m/s2 for gravity acceleration.


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A 1.2 – So u r c e d at a A 1.2.1 – L en g t h an d d e p t h o f b r i d g e d e c k

Fig. A1.4 – Longitudinal section of the structure

The example involves the determination of a structure with three asymmetrical spans totaling 214.50 m between the centers of the end supports. There are two end spans of 53.375 m and 63.375 m and one central span of 97.75 m. The bridge deck is cast in-situ due to its moderate length (Fig.A1.4). Given the need to comply with clearance dimensions, the height of the box girder varies parabolically between: - On pier: h1 = 5.30 m , giving a slenderness ratio of approximately 1/18 - At the crown ho = 2.30 m, giving a slenderness ratio of approximately 1/43 (*). The segments on piers are 8.00 m long and the closing segment for the central span is 2.00 m long. (*) This is quite a low slenderness ratio but it is imposed due to the fact that this structure is designed to double the capacity of an existing bridge. A 1.2.2 – Fu n c t i o n al c r o s s s ec t i o n

The supported platform is illustrated in Figure A1.5. However, only one side of the traffic lanes is shown, as the complete structure consists of two bridges fastened together.

Fig. A1.5 – Cross-section of the structure

According to fascicule 61 Titre II of french CCTG, the distributed overload A is given as: Usable width = 10.15 m  first category bridge Loadable width = 10.15 - (2 x 0.50) = 9.15 m Number of loadable lanes = entire section (9.15/3) = 3 lanes


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a1 = 0.9 as the bridge features three lanes Width of lane = 9.15 / 3 = 3.05 m a2 = 3.5/3.05 = 1.15 Considering the third comment in Article 4.21, the linear loads according to the length of loaded section is given by:

≤ 200 m with A( ) =

s = 9.15 A2( ) = 9.15 x 0.90 x 1.15 A( ) = 9.45 A( ) 230 +

36000 (A(l) in kg/m² and l in m - units used in fascicule 61 Titre II). l + 12

A 1.2.3 – Wei g h t o f f i t t i n g s

The probable linear weight of the fittings is taken to be q = 47.4 kN/m. A 1.2.4 – Ch ar ac t er i s t i c s o f m at er i al s

The characteristics of the materials used are as follows: - For the concrete: compressive strength f c28 = 35 MPa, for a permissible in-service compression stress = 0.6 x 35 = 21 MPa - For the cantilever and continuity tendons, we use 12T15S prestress units, giving an effective mean in-service force estimated at 1.95 MN; the overall diameter of the ducts is 80 mm -For the external tendons, we use 19T15S prestress units, giving an effective mean in-service force estimated at 3.1 MN; the overall diameter of the ducts is 100 mm. A 1.2.5 – Cr o s s s ec t i o n s o f t h e b r i d g e d ec k


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Fig. A1.6 – Cross sections of the bridge deck

As the total width of the slab is equal to 10.95 m, including the longitudinal anchoring girders for the safety barriers, it is logical to design a box girder with two webs positioned according to the findings of a summary analysis of the transverse behavior. This analysis was also used to establish the thickness of the upper slab between the webs at 25 cm, whereas the thickness of the webs (36 cm) is determined by the casting conditions, allowing for the diameter of the ducts. Vertical webs were chosen for aesthetic reasons. The thickness of the cantilevers varies between 25 cm on the BN4 side and 30 cm on the web side. The mean thickness of the upper slab is therefore 27 cm. Figure Al.6 shows the cross-sections used as the basis for the first approximate calculations. Finally, we know that there must be room to accommodate gussets in the slabs. Although we do not yet know the precise dimensions of the formwork for these elements, we must allow for them in our calculations. As an initial estimate, we shall use the dimensions given in Figure A1.7.


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Fig. A1.7 – Detail of the gussets in the box girder

The transverse surface area represented by the gussets is:

⎡1.202 − π x 0.602 0.402 0.40 x 0.08 0.40 x 0.20 ⎤ + + + + 0.4 x 0.08⎥ = 0.4905 m² S=2 x ⎢ 4 2 2 2 ⎣ ⎦ In the central spans, the total height of the box girder varies parabolically according to following the law: 2 x ⎞ h(x) = h0 + (h1 - h0) ⎛ ⎜ ⎟ ⎝ l ⎠

2

using the notation shown in Figure A1.8 below:

Fig. A1.8 – Variation in height of bridge deck in central span

In the end spans (Fig. A1.9), the total height varies parabolically at first (section closest to pier), and then it becomes constant and equal to h0, close to CO and C3.


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Fig. A1.9 – Variation in height of bridge deck

We can schematize the sections according to Figure A1.10.

Fig. A1.10 – Calculation of the mechanical characteristics of the bridge deck

We then have to establish the thickness ε (x) of the lower slab; in fact, for this, we shall use a parabolic variation law similar to the one used for h(x), with ε0 and ε1 the thicknesses at the crown and on the pier: 2 x ⎞ ε(x) = ε0 + (ε1 - ε0) ⎛ ⎜ ⎟ ⎝ l ⎠

2

As an initial estimate, we shall use ε0 = 24 cm: a value which is imposed by the casting conditions, and ε1 = 45 cm: a value chosen in view of the span lengths and the slenderness ratios. A 1.2.6 – Ch ar ac t er i s t i c s o f t h e s ec t i o n s

A computerized calculation gives us the mechanical characteristics of the sections on pier and at the crown. h(x) ε(x) B v v’ I

ρ

Section at the crown

Section on pier

2.300 m 0.240 m 6.0965 m2 0.860 m 1.440 m 4.620 m4 0.612

5.300 m 0.450 m 9.3129 m2 2.420 m 2.880 m 41.532 m4 0.640

A 1.2.7 – Seq u en c e o f c o n s t r u c t i o n o p er at i o n s

In the following calculations, we shall use the sequence of construction operations shown below: 1 2 3 4 5

Construction of cantilever on P1 Construction of the cast-on-falsework section near to C0 Closure of C0-Pl, stressing of continuity tendons for C0-Pl and transition to permanent bearings for P1 Construction of cantilever on P2 Construction of cast-on-falsework section near to C3


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Prestressed concrete bridges built using the cantilever method – Design guide 6 7 8 9

Closure of P2-C3; stressing of tendons for P2-C3 and transition to permanent bearings for P2 Closure of P1-P2 and stressing of continuity tendons for Pl-P2 Stressing of external tendons Installation of fittings and commissioning

A 1.3 – Can t i l ev er p r es t r es s i n g d es i g n A 1.3.1 - Mo m en t o n p i er d u r i n g t h e c as t i n g o f t h e f i n al s eg m en t s

The complete cantilever is shown in Figure A1.11 below. In order to simplify the calculations, we consider that the bridge deck is supported on the centerline of the pier. The moment resulting from this assumption is slightly higher than if the calculation were to be performed in line with the temporary stabilizing blocks.

Fig. A1.11 – Determination of the support moment during the casting of the final segments

The calculation of Mg is shown hereafter (with x defined in Figure A1.8): 2 2 ⎡ ⎡ x ⎞ ⎤ x ⎞ ⎤ ⎛ ⎛ B( x) = 10.23 x 0.27 + 0.72 ⎢2.30 + 3.00⎜ ⎟ ⎥ + 5.03 ⎢0.24 + 0.21⎜ ⎟ ⎥ + 0.4905 48 . 875 48 . 875 ⎝ ⎠ ⎝ ⎠ ⎢⎣ ⎥⎦ ⎢⎣ ⎥⎦

Hence, with a unit mass of 2.5 t/m3, therefore a unit weight of 0.024525 MN/m3: g (x) = 0.024525 B (x) = 0.149990 + 33.02.10-6 x2 x

∫

- Tg(x) = g (ξ )d ξ = 0.149990 x + 11.01.10-6 x3 0

x

∫

- Mg(x) = − T g (ξ )d ξ = 0.074995 x2 + 2.75.10-6 x4 0

For the section on pier, i.e. for x = 48.875 – 1.00 m (½ closing segment), this gives Mg = - 186.34 MNm. At this moment Mg, it is necessary to add the moment due to the known site loads (Qc1) in addition to the moment due to random site loads (Qc2 and Qc3). Qc1 Moment due to the weight of the form traveler, assumed to be equal to approximately 40 t, i.e. 0.40 MN, and exerted at 3.00 m / 2 + 1 m = 2.5 m from the crown:


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MQc1 = Mform traveler = - 0.40 x 46.375 = - 18.55 MNm Qc2 Moment due to a distributed load of 200 N/m² over a half-cantilever Qc3  Moment due to a concentrated load of (50 + 5b) kN, with b being the width of the upper slab of the box girder: MQc2 = - (200 x 10.95 x 47.875) x 0.000001 x (47.875 / 2) = - 2.51 MNm MQc3 = - [(50 + (5 x 10.95) 1 x 0.001 x (47.875 – 3.00) = - 4.70 MNm We can therefore consider that: Mg = - 186.34 – 18.55 – 2.51 – 4.70 = - 212.10 MNm. The shape of the moment curve in cantilever Mg is shown in Figure A1.12 below.

Fig. A1.12 – Bending moment in the cantilever under selfweight + form traveler

A 1.3.2 – Cal c u l at i o n o f c an t i l ev er c ab l i n g

Background: Cantilever tendons: 12T15S Effective force: 1.95 MN Sheath diameter: 81 mm Concrete: B35 We consider the bridge deck to be cast in-situ with certain precautions taken (presence of empty ducts, measurement of transmission factors). The strength of the concrete in the segments on piers at the moment of the casting of final pair of segments is taken to be f cj = 35 MPa. The allowable stress in the upper axis is therefore:

σt = 0.7 f tj - k f cj with k = 0.02 σt = 0.7 x (0.6 + 0.06 x f cj) – 0.02 x f cj = 0.7 x (0.6 + 0.06 x 35) – 0.02 x 35 = 1.19 MPa The force N developed by the cantilever tendons in the section on pier must satisfy the following condition:


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N M v N e0 v + + ≥ σ b B I I with B = 9.3129 m2, I = 41.532 m4 ,v = 2.420 m and M = -212.10 MNm e0 = 2.258 m  position of the mean tendon supposing its distribution over two beds e0 = v - 2 φg with φg = 0.081 m

N ≥

Mv I 1 e v + 0 B I

− σ b −

n tendons ≥

=

212.10 x 2.42 41.532 = 46.74 MN 1 2.258 x 2.42 + 9.3129 41.532

− 1.19 +

46.74 ≈ 24 tendons 1.95

to which we add two tendons in order to prestress the final pair of segments. We therefore have 13 pairs of 12T15S tendons, allowing us to divide the cantilever into 2 x 13 standard segments of 3.375 m in length (see Figure A1.13 below).

Fig. A1.13 – Breakdown of cantilevers into segments

The shape of the moment curve in the cantilever under selfweight and prestressing is given in Figure A1.14 below.

Fig. A1.14 – Bending moment in the cantilevers

A 1.3.3 - Ver i f i c at i o n d u r i n g t h e c as t i n g o f t h e P1-P2 c l o s i n g s eg m en t

At this stage in the design, it is advisable to make sure that the cantilever tendons are strong enough to take up the weight of the bridge deck and form traveler during the construction of the central closing segment. To this end, two vertical forces are applied in a downward direction at each end of the cantilevers (figure A1.15). These The “Les outils” collection – Sétra

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forces represent the effect of the weight of the form traveler and the concrete for the closing segment when still wet. These forces are exerted on two independent and isostatic structures because, at this stage, the two halves of the bridge have not yet been connected.

Fig. A1.15 – Effects of the weight of the form traveler and of the closing segment

In this new phase, the forces and stress in the upper horizontal plane along the centerline of the piers can be expressed as follows: M

= - 186.34 - (18.55/2) – 16.84 = - 212.45 MN.m

Ncantilever = 2 x 13 x 1.95 = 50.7 MN Misocantilever = 50.7 x 2.258 = 114.48 MN.m

N M . v 50.7 (−212.45 + 114.48) x 2.42 + = + B I 9.3129 41.532

σ s

=

σ i

= − 0.26 MPa ≥

σ b

= − 1.19 MPa ==> The dimensioning is therefore correct.

A 1.4 – Pr es t r es s i n g d es i g n f o r t h e c l o s i n g s eg m en t A 1.4.0 - Pr eam b l e

We have seen in A1.2.7 that the closing segments were constructed in the following order: - Closure of C0-Pl and stressing of continuity tendons for C0-Pl - Closure of P2-C3 and stressing of continuity tendons for P2-C3 - Closure of P1-P2 and stressing of continuity tendons for P1-P2.


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The continuity tendons must be determined in reverse order. By starting at the final closing segment, the hyperstatic effect of the continuity tendons stressed during the previous closing operations is not an unknown. A l .4.1 – Cl o s i n g s eg m en t b et w een PI an d P2

The continuity tendons for the span P1-P2 must take up the selfweight of the closing segment, the forces resulting from the removal of the form traveler and the effects of the thermal gradient. In contrast to the other continuity tendons, they develop both isostatic and hyperstatic moments. Figure Al. 16 below represents the structure at the moment of the P1-P2 closure.

Fig. A1.16 – Detail of central closing segment

In order to determine the number of tendons necessary for the closing segment in the central span, we shall study each phase of the closure and determine its effect on the structure. A1.4.1.1 - Effect of the selfweight of the cl osing segment and the form traveler

As mentioned previously, the first stage of the closure can be simulated by applying two vertical forces in a downward direction at each end of the cantilevers (figure A1.15). In this phase, the bridge deck still consists of two independent isostatic structures. Therefore, this phase has no bearing on the calculation of the continuity tendons. A1.4.1.2 – Effect of the removal of the form traveler

This phase can be broken down into two parts. In the first part, two vertical forces are applied in an upward direction at each end of the cantilevers (Figure A1.17). These forces represent the effect of the removal of the form traveler and the concrete used for the closing segment (fresh concrete). In the second part, these forces will be replaced with a uniformly distributed load directed in a downward direction and representing the set concrete (Figure A1.18). In this phase, the structure is hyperstatic.

Fig. A1.17 – Diagram of moments due to the removal of the form traveler


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Fig. A1.18 – Diagram of moments due to weight of closing segment

At the end of these two stages, the moment to be considered for the calculation of the continuity tendons for the span P1-P2 is: MG = -7.75 + 3.40 = - 4.35 MN.m A1.4.1.3 – Effect of the thermal gradient

Figure A1.19 below represents the bending moment due to a thermal gradient of 12° C.

Fig. A1.19 – Diagram of the moments due to a thermal gradient of 12°C

The value of the moment at the crown used for the calculation of the continuity tendons for P1-P2 is therefore: Mtherm = 12.65 MN.m A1.4.1.4 – Isostatic moment of continuity tendons for P1-P2

The isostatic moment is an unknown and is proportional to the prestressing that is used (Figure A1.20). The isostatic moment is calculated in the following way: Miso = N . e0 with N = normal force due to the tendons in the section and e0 = offsetting of the mean tendon In our example: N = 1.95 x ntendons and e0 at the crown = 1.318 m The “Les outils” collection – Sétra

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Fig. A1.20 – Diagram of isostatic moments for continuity tendons A1.4.1.5 – Hyperstatic moment for P1-P2 continuity tendons

As the structure is hyperstatic when the continuity tendons are stressed, a hyperstatic moment is set up in the central span which is of the opposite sign to the isostatic moment developed by the tendons. This hyperstatic moment is an unknown and depends on the intensity and the distribution of the prestressing in the span (alignment and length of tendons) and on the characteristics of the structure in question (whether or not there is variation of inertia). The diagram for this is shown in Figure Al.21 below.

Fig. A1.21 – Diagram of the moments due exclusively to the hyperstatic effect of the P1-P2 continuity tend ons

At the pre-design stage, it is possible to use a simplification which consists of considering the span to be embedded on its supports (Figure Al.22). In this case, the hyperstatic moment can be evaluated in the following way:

M hyper = −


area of isostatic moment diagram length of span

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Fig. A1.22 – Diagram of moments exclusively due to the hyperstatic effect of the continuity tendons according to a simplified calculation A1.4.1.6 – Calculation of P1-P2 continuity tendons

Background: Cantilever tendons: 12T15S Effective force: 1.95 MN Sheath diameter: 81 mm Concrete: B35 As before, we consider the bridge deck to be cast in-situ with certain precautions taken (presence of empty ducts, measurement of transmission factors). The strength of the concrete in the segments on piers at the moment of the removal of the form traveler used for the closing segment is taken to be f cj = 16 MPa. The allowable stress in the bottom axis is therefore: σ b

= 0.7 f tj – k f cj with k = 0.02

σ b

= 0.7 x (0.6 + 0.06 x f cj) – 0.02 x f cj = 0.7 x (0.6 + 0.06 x 16) – 0.02 x 16 = 0.77 Mpa

The force N developed by the continuity tendons in line with the closing segment must satisfy the following condition: N B

−

M G v ' I

−

N e0 v ' I

−

M Therm v ' I

−

M Hyper v ' I

≥ − σ b

with B = 6.0965 m2 I = 4.620 m4 v’= 1.440 m M = 13.57 MN.m e0 = -1.318 m  position of mean tendon e0 = - (v – 1.5 φg) with φg = 0.081 m As we do not know the number of tendons required, we also do not know their position in the box girder or their isostatic moment diagram. Therefore, an iterative calculation is performed, starting with a single pair of tendons 22.25 m long, spanning six segments of 3.375 m and the closing segment of 2 m.


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The distribution of the prestressing produces an isostatic moment which is distributed almost uniformly if a single pair of tendons is used (subject to the variation in e0 and the variation of inertia of the section). The hyperstatic moment is therefore: ntendons = 2

f u = 1.95 MN F p = 3.90 MN e0 = -1.318 m at the crown L = 97.75 m lc = 22.25 m Miso = F p x e0 = -5.14 MN.m at the crown Mhyper = - (lc x Miso) / L = 1.17 MN.m

hence

σ b

= − 0,71 MPa

⇒

σ b

≥ − σ b

We will therefore choose one pair of 12T15S tendons. In the event of one pair of tendons being insufficient, it would be necessary to repeat the calculation with two pairs of tendons and recalculate the hyperstatic moment with the newly tendons arrangement. A 1.4.2 – Cl o s i n g s eg m en t b et w een C0 an d P1

The continuity tendons for the span C0-Pl must take up the selfweight of the cast-on-falsework section. Figure Al.23 below shows the structure at the moment the span is closed.

Fig. A1.23 – Detail of the structure during placement of the closing segment on the C0 side A1.4.2.1 – Calculation of support reactions, shear force and bending moment

As the cross-section of the bridge deck is 6.0965 m² close to the abutments (h = 2.30 m), the load due to the selfweight of the cast-on-falsework section is equal to Q = 6.0965 x 0.02453 = 0.14955 MN/m. As the structure in question is isostatic, we can easily calculate the support reactions and the longitudinal bending moment using static equations (Fig.A1.24):


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Fig. A1.24 – Determination of the forces on the C0 side due to the closure of the span

Support reactions: ⎡∑ F = 0 ⎢ M = 0 ⎣∑

⇒

R P1 = 0.04238 MN and R C0 = 0.78015 MN

Shear force diagram:

Fig. A1.25 – Diagram of shear forces due to closure on C0 side

Bending moment diagram: For the 5.50 m long section between C0 and the closing segment, the bending moment is equal to: M(x)

= R C0 . x −

Q . x2 2

so for x = 5.217 m

⇒ M = 2.03 MN.m

Fig. A1.26 – Bending moment on C0 side due to the closure of the span A1.4.2.2 – Calculation of continuity tendons between C0 and P1

Background: Cantilever tendons: 12T15S Effective force: 1.95 MN


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Duct diameter: 81 mm Concrete: B35 As before, the bridge deck is considered to be cast in place with certain precautions taken (presence of empty ducts and measurement of transmission factor) and the strength of the concrete is taken to be the value obtained when the falsework is removed f cj = 16 MPa. The allowable stress in the lower axis is thus: σ b

= 0,7 f tj – k f cj with k = 0.02

= 0.7 x (0.6 + 0.06 x f cj) – 0.02 x f cj = 0.7 x (0.6 + 0.06 x 16) – 0.02 x 16 = 0.77 Mpa The force N developed by the continuity tendons in line with the closing segment must satisfy the following condition: N B

−

M v ' I

−

N e0 v ' I

≥ − σ b

with B = 6.0965 m2, I = 4.620 m4, v’ = 1.440 m, M = 2.03 MN.m e0 = -1.318 m  position of the mean tendon e0 = - (v – 1.5 φg) with φg = 0.081 m

N ≥

M v' I = 1 e0 v' − B I

− σ b +

2.03 x 1.440 4.620 = − 0.19 MN 1 1.318 x 1.440 + 6.0965 4.620

− 0.74 +

In principle, no tendons are necessary. However, in order to prevent any cracking due to parasitic phenomena (obstructed shrinkage, etc.), we shall specify one pair of 12T15S tendons. A1.4.2.3 - Verification of stresses after closure of P1-P2

The calculation that we have just performed is intended to determine the amount of continuity cabling needed to consolidate the structure after the removal of falsework from the cast-on falsework section. However, somewhat higher forces develop in this area after the closure of P1-P2. Indeed, as the structure is now hyperstatic, the bridge deck becomes sensitive to the effects of thermal gradients, to which is added the hyperstatic effect of the continuity tendons for P1-P2. In this new phase, the forces and stress in the lower axis are: Mg = 2.03 MN NcontinuityC0P1 = 2 x 1.95 = 3.9 MN McontinuityC0P1 = 3.9 x -1.318 = -5.l4 MN.m McontinuityP1P2 = 1.17 MN.m x 5.50 / 53.375 = 0.121 MN.m MΔt = 0.5 x 12.65 MN.m x 5.50 / 53.375 = 0.65 MN.m The “Les outils” collection – Sétra

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σ s

=

σ i

N M . v' 3.9 − = B I 6.097

= − 0.09 MPa ≥

σ b

1.44 ⎞ - ⎛ ⎜ (2.03 - 5.14 + 0.12 + 0.65) x ⎟ 4.62 ⎠ ⎝

= − 2.0 MPa (with j = 28 days)

The continuity cabling is therefore adequate. A 1.4.3 – Cl o s i n g s eg m en t b et w een P2 an d C3

The continuity tendons for the P2-C3 span must take up the selfweight of the cast-on-falsework section. Figure A1.27 below represents the structure at the step when the span is closed.

Fig. A1.27 – Determination of forces on C3 side due to the closure of the span A1.4.3.1 – Calculation of support reactions and bendi ng moments

A calculation similar to the one performed for the C0-Pl closing segment gives the support reactions and the bending moment: ⎡ ∑ F = 0 ⎢ M = 0 ⎣∑

⇒

R P2 = 0.28374 MN et R C3 = 2.03456 MN M = 13.57 MN.m

A1.4.3-2 – Calculation of the continuity tendons for the P2-C3 span

The force N developed by the continuity tendons in line with the closing segment must satisfy the following condition: N B

−

M v ' I

−

N e0 v ' I

≥ − σ b

with B = 6.0965 m2, I = 4.620 m4, v’ = 1.440 m, M = 13.57 MN.m e0 = -1.318 m  position of the mean tendon e0 = - (v – 1.5 φg) with φg = 0.081 m

N ≥


M v' I = 1 e0 v' − B I

− σ b +

13.57 x 1.440 4.620 = 6.07 MN 1 1.318 x 1.440 + 6.0965 4.620

− 0.74 +

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n tendons ≥

6.07 ≈ 3 1.95

Therefore, we will opt for two pairs of 12T15S tendons. A1.4.3.3 – Verification of stresses after closure of P1-P2

As before, prior to the stressing of the external tendons, it is important to verify the suitability of the cabling after the closure of the span P1-P2 and the stressing of the continuity tendons for P1-P2. In this new phase, the forces and stressing in the lower horizontal plane are: Mg= 3.57 MN NcontinuityP2C3 = 4 x 1.95 = 7.8 MN McontinuityP2C3 = 7.8 x – 1.318 = - 10.28 MN.m McontinuityP1P2 = 1.17 MN.m x 15.50 / 63.375 = 0.286 MN.m MΔt = 0.5 x 12.94 MN.m x 15.50 / 63.375 = 1.58 MN.m The continuity cabling is therefore sufficient.

A 1.5 – Ex t er n al p r es t r es s i n g d es i g n A 1.5.0 - Pr es en t at i o n o f t h e d es i g n m et h o d

The external prestress tendons must take up the forces due to the fittings, traffic loads A(l), thermal gradient and creep. In order to calculate the number of tendons required, we shall proceed in stages: - Determination of geometry of tendons - Computerized calculation of the envelope of longitudinal moments due to A(l) - Choice of critical sections to be dimensioned or verified - Computerized calculation of effects of thermal gradient - Calculation of creep effects - Determination of prestressing. A 1.5.1 – Det er m i n at i o n o f f o r c es A1.5.1.1 - Preamble concerning the geometry of the external tendons between CO and C3

For an initial estimation, we can use certain rules to define the mean alignment of the external prestress tendons.


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Longitudinally, we shall place the intermediate deviators for the central span one third of the way along the span, i.e. at 32.583 m from the centerline of the piers. We shall also place a deviator on the end spans, also at 32.583 m from the centerline of the piers (Fig. Al.28). Remember that the deviators on piers position the external prestress tendons as close as possible to the upper axis of the bridge deck (negative longitudinal bending moment, whereas the intermediate deviators position these same tendons close to the lower axis (positive longitudinal bending moment).

Fig. A1.28 – Longitudinal geometry of alignment for external tendons at end of segments

Transversally on the section on pier, we shall make the minimum distance from the mean tendon to the upper axis equal to the thickness of the upper slab, i.e. 0.25 m. For the section at the crown, the minimum distance from the mean tendon to the lower axis shall be equal to the thickness of the lower slab plus 0.15 m. This value allows a gap of 0.10 m to be left between the sheath of the prestressing tendon and the top of the lower slab (Fig.A1.29).

Fig. A1.29 – Transverse geometry of external tendons A1.5.1.2 - Overload A(l)

A computerized calculation gives us the envelope of the longitudinal moments due to the imposed loads A(1) (Fig.A1.30).


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Fig. A1.30 – Diagram of moments due to the imposed load A(1)

The characteristic values of the enveloped curve below are used to determine the dimensioning sections and the verification sections for the external prestressing (Fig.A1.31).

Fig. A1.31 – Position of the sections used to analyze the external prestressing

The following table summarizes the forces due to A(l) and the type of calculation to be performed in each section. Sections

Σ1 Σ2 Σ3 Σ4 Σ5 Σ6 Σ7

Moments Closing segment C0-P1 Pier P1 Closing segment P1-P2 Pier P2 Closing segment P2-C3

Type of calculation

Min

Max

- 4.75 MN.m - 19.23 MN.m - 45.71 MN.m - 6.36 MN.m - 43.50 MN.m - 15.50 MN.m - 9.42 MN.m

8.39 MN.m 19.87 MN.m 11.41 MN.m 21.62 MN.m 6.40 MN.m 23.63 MN.m 19.59 MN.m

dimensioning verification verification dimensioning verification verification dimensioning

A1.5.1.3 - Fittings

A computerized calculation is used to obtain the curve of the bending moment due to the fittings. (Remember that the linear weight of the fittings is equal to q = 47.4 kN/lm).


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Fig. A1.32 – Diagram of the moments due to the weight of the fittings

The table on the left summarizes the forces due to the fittings in each section. Sections

Σ1 Σ2 Σ3 Σ4 Σ5 Σ6 Σ7


1.94 MN.m -0.98 MN.m -41.25 MN.m 12.97 MN.m -45.16 MN.m 4.44 MN.m 6.37 MN.m

A1.5.1.4 – Thermal gradient

A computerized calculation is again used to obtain the curve of the bending moment due to the thermal gradient of 12° C.

Fig. A1.33 – Diagram of the moments due to a thermal gradient of 12° C

The table below summarizes the forces due to the thermal gradient in each section. Sections

Σ1 Σ2 Σ3 Σ4 Σ5 Σ6 Σ7



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1.27 MN.m 5.18 MN.m 12.36 MN.m 12.65 MN.m 12.94 MN.m 5.23 MN.m 3.16 MN.m

septembre 2007


A1.5.1.5 - Creep

As an initial estimate, we reserve a margin of 2 MPa for the stress in the lower axis. This reserve is used to calculate a creep moment in the central span: σ b

= −

M v' I

⇒ M = − σ b

I v'

Thus, for the section Σ4: v’ = 1.44 m I = 4.62 m4 M creep = 2 x

4.62 = 6.42 MN.m 1.44

In the end spans, the moment corresponding to this compression reserve will be determined by a linear interpolation between M = 0 MN.m on the abutment and M = 6.42 MN.m on the pier. A 1.5.2 – Di m en s i o n i n g o f ex t er n al p r es t r es s i n g

As the calculations are similar and repetitive, we shall only dimension the external tendons close to the closing segment sections. A1.5.2.1 – Calculation in section

4 (closing

segment between P1 and P2)

V a l u e o f l o n g i t u d i n a l m o m en t s

Moment MG1 (state after construction, see Figures Al.17 and A1.18)  MG1 = - 4.35 MN.m Fittings

 MG fittings =

Creep

 Mcreep =

Overload A(1)

 MA(1) min

Thermal gradient

 MΔt =

12.97 MN.m

6.42 MN.m = -6.36 MN.m and M MA(1) max = 21.62 MN.m

12.65 MN.m

C a l c u l a t i o n o f M Q

MQ =1.2 MA(1) + 0.5 MΔt MQ min = -7.63 MN.m

or

MQ = MΔt with the values above gives: MQ max = 32.27 MN.m

C al c u l a t i on o f M m i n a n d M m a x

M min = MG min + MQ min = (MG1 + MG equip) + MQ min = -4.35 + 12.97 – 7.63 = 0.99 MN.m M max = MG max + MQ max = (MG1 + MG equip + MG creep) + MQ max = -4.35 + 12.97 + 6.42 + 32.27 = 47.31 MN.m


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C a l c u l a t i o n o f i so st a t i c a n d h y p er st a t i c m o m e n t s o f c o n t i n u i t y t en d o n s

f u

= 1.95 MN

effective force for one 12T15S tendon

e0

=-1.318 m

for the continuity tendons

n

=2

number of 12T15S continuity tendons

Miso

= N . e0 = f u . n . e0 = 1.95 x 2 x -1.318 = - 5.14 MN.m

Mhyper

=

5.14 x [(6 x 3.375) + 2.00] 97.75

= 1.17 MN.m

M peci =Miso +Mhyper =-5.14 + 1.17 =-3.97 MN.m C a l c u l a t i o n o f i s o st a t i c a n d h y p er s t a t i c m o m e n t s o f ex t e r n a l t en d o n s

Fig. A1.34 – Longitudinal geometry of external tendons in the central span

e01 = - (1.440 – 0.24 – 0.10 - (0.10 / 2)) = -1.05 m e02 = 2.420-0.25 = 2.17 m f u = 3.1 MN effective force for a 19T15S tendon n=

number of 19T15S external tendons

Me1 = n . fu . el = n . 3.1 . (-1.05) = -3.26.n MN.m Me2 = n . fu . e2 = n . 3.1 . (+2.17) = 6.73.n MN.m


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Fig. A1.35 – Diagram of isostatic moment of external tendons in central span

x =

6.73 x 32.583 = 21.958 m 6.73 + 3.26

2 x S1 = 6.73 . n . 21.958 = 147.712 . n

Hyperstatic moment

2 x S2 = 3.26 . n . 10.625 = - 34.584 . n S3 = 3.26 . n . 32.584 = - 106.061 . n 7.067.n

M hyper = −

7.067 . n = − 0.072 . n 97.75

P ecl + P ext

M max .v '



MPext = Miso + Mhyper = - 3.26 . n – 0.07 = -3.33 . n Calculation of stress: σ i

σ i

=

(2 x 1.95) + 3.1. n

N B

−

M .v ' I

=

B

−

I

−

M P ecl + M P ext .v ' I

47.31 x 1.44 [(− 5.14 + 1.17 ) + (− 3.33. n )] x 1.44 − σi = (1.546 .n) – 12.87 4.620 4.620

= − 3.60 MPa < − σ b = − 2.0 MPa for n = 8 = − 0.50 MPa > − σ b = − 2.0 MPa We will therefore opt for 4 pairs of 19T15S tendons.

for n = 6 σ i

6.0965

−

=

σ i

A 1 . 5 . 2 . 2 – C a l c u l a t i on i n t h e

1 section (closing segment between CO and P1)


Moment MG1 (state after construction - see figures A 1.17,18)

 MG1 =

Fittings

 MGfittings =

Creep

 Mcreep =


Page 258

0.58 MN.m 1.94 MN.m

6.42 x 5.50 / 53.375 = 0.66 MN.m septembre 2007


Overload A(1) 8.39 MN.m

 MA(1) min

Thermal gradient

 MΔt =

= -4.75 MN.m et M MA(1) max =

1.27 MN.m


MQ =1.2 MA(1) + 0.5 MΔt or MQ = MΔt, which gives the following with the values above: MQ max = 10.70 MN.m

and

MQ min = -5.70 MN.m


M min = MG min + MQ min = (MG1+ MGfittings) + MQ min = 0.58 + 1.94 – 5.70 =-3.18 MN.m M max = MG max + MQ max = (MG1+ MGfittings + Mcreep) + MQ max = 0.58 + 1.94+0.66 + 10.70 = 13.88 MN.m C a l c u l a t i o n o f i so st a t i c a n d h y p er st a t i c m o m e n t s o f c o n t i n u i t y t en d o n s

f u

= 1.95 MN


e0

=-1.318 m


n

=2


Miso

= N . e0 = f u . n . e0 = 1.95 x 2 x -1.318 = - 5.14 MN.m

Mhyper

=0

Mhyper

= 1.17

for the end span tendons

5.50 5.50 − 0.576 = 0.06 MN .m for the continuity and exterior tendons of the span 53.375 53.375

P1-P2 M peci = Miso +Mhyper =-5.14 + 0.06 MN.m =-5.08 MN.m C a l c u l a t i o n o f i s o st a t i c a n d h y p er s t a t i c m o m e n t s o f ex t e r n a l t en d o n s


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Fig. A1.36 – Longitudinal geometry of external tendons in span CO-P1

e1 = - (1.440 – 0.24 – 0.10 - (0.10 / 2)) = -1.05 m e2 = 2.420-0.25 = 2.17 m f u = 3.1 MN effective force of one 19T15S tendon n = number of 19T15S external tendons Me1 = n .f u .e1 = n .3.1 .(-1.05) =-3.26.n MN.m Me2 = n . f u . e2 = n . 3.1 . (+2.17) = 6.73.n MN.m

Fig. A1.37 – Isostatic moment diagram for external tendons in span C0-P1

x =

3.26 x 32.583 = 10.625 m 3.26 + 6.73

S1 = (6.73 .n. 21.958)/2 = 73.856 . n

Hyperstatic moment on pier

S2 = (3.26.n .10.625)/2 = - 17.292 .n S3 = (3.26. n .15.292) = - 49.755 . n



M hyper =

2.163 . n . 2 = 0.081 . n 53.375

S4 = (3.26 . n . 5.500) / 2 = - 8.951 .n


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-2.163 .n

0.081× 5.50. n = 0.008 . n 53.375

M hyper (Σ1 ) =

MPext = Miso + Mhyper = -3.255 . n + 0.008 . n = -3.25 . n C a l c u l a t i o n o f st r ess i n σ i

σ i

=

1

=

N B

−

(2 x 1.95) + 3.1. n 6.0965

M .v ' I

−

P ecl + P ext

=

B

−

M max .v ' I

−


13.88 x 1.44 [(− 5.14 + 0.09 ) + (− 3.25. n )] x 1.44 − 4.620 4.620

σi = (l.524.n)-5.26 for n = 2

σi =-2.20 MPa <-σt =-2.00 MPa

for n = 4

σi = 0.84 MPa >-σt =-2.00 MPa

We will therefore opt for two pairs of 19T15S tendons. A1.5.2.3 – Calculation in section

7 (closing

segment between P2 and C3)


Moment MG1 (state after construction - see Figures A1.17 and A1.18) MG1 = 1.11 MN.m Fittings

 MGfittings =

Creep

 Mcreep =

Overload A(1) 19.59 MN.m

 MA(1) min

Thermal gradient

 MΔt =

6.37 MN.m

6.42 x 15.50 / 63.375 = 1.57 MN.m = -9.42 MN.m et M MA(1) max =

3.16 MN.m


MQ=1.2 MA(1) + 0.5 MΔt or

MQ = MΔt

MQ max = 25.09 MN.m

and

which, using the values above, gives:

MQ min =-11.30 MN.m


M min = MG min + MQ min = (MG1 + MGfittings) + MQ min = 1.15 + 6.37 – 11.30 = -3.78 MN.m


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M max = MG max + MQ max = (MG1 + MGfittings + Mcreep) + MQ max = 1.15 + 6.37 + 1.57 + 25.09 = 34.18 MN.m C a l c u l a t i o n o f i so st a t i c a n d h y p er st a t i c m o m e n t s o f c o n t i n u i t y t en d o n s

f u

= 1.95 MN


e0

= -1.318 m


n

=4


Miso

= N e0 = 1.95 x 4 x -1.318 = - 10.28 MN.m

Mhyper

=0

Mhyper

= 1.17

for the end span tendons

15.50 15.50 − 0.576 = 0.145 MN.m for the continuity and exterior tendons of the span 63.375 63.375

P1-P2 M peci = Miso +Mhyper =-10.28 + 0.15 MN.m = -10.13 MN.m C a l c u l a t i o n o f i s o st a t i c a n d h y p er s t a t i c m o m e n t s o f ex t e r n a l t en d o n s

Fig. A1.38 – Longitudinal geometry of external tendons in span P2-C3

e01

= - (1.440 – 0.24 – 0.10 - (0.10 / 2)) = -1.05 m

e02

= 2.420-0.25 = 2.17 m

f u

= 3.1 MN effective force for a 19T15S tendon

n

= number of 19T15S external tendons

Me1

= n . f u. e1 = n . 3.1 . (-1.05) = -3.26.n MN.m

Me2

= n . f u.e2 = n .3.1 . (+2.17) = 6.73.n MN.m


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Fig. A1.39 – Isostatic moment diagram of the external tendons in span P2-C3

x =

3.26 x 32.583 = 10.625 m 3.26 + 6.73

S1 = (6.73 .n. 21.958) / 2 = 73.856 . n

Hyperstatic moment on support:

S2 = (3.26.n .10.625) / 2 = - 17.292 .n S3 = (3.26. n .15.292) = - 49.755 . n



M hyper =

18.417 . n . 2 = 0.581 . n 63.375

S4 = (3.26 . n . 15.500) / 2 = - 25.226 . n -18.417 . n

0.581. 15.50 . n = 0.142 . n 63.375

M hyper (Σ 7 ) =

MPext = Miso + Mhyper = - 3.26 . n + 0.142 . n = - 3.113 . n C a l c u l a t i o n o f st r ess σ i

σ i

=

=

N B

−

M .v ' I

=

P ecl + P ext B

−

M max .v ' I

−


(4 x 1.95) + 3.1. n − 34.18 x 1.44 − [(− 10.28) + (− 3.113. n )] x 1.44 6.0965

4.620

4.620

σi = (l.480.n)-8.404 for n = 4

σi =-2.49 MPa <-σt = -2.00 MPa

for n = 6

σi = 0.47 MPa >-σt = -2.00 MPa

We shall thus opt for two pairs of 19T15S tendons. The “Les outils” collection – Sétra

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A2 Monographs of cast-in-situ bridges


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NAME OF STRUCTURE Situation, Contractors, Construction dates

REF.

SCHEMATIC LONGITUDINAL SECTION

Beaumont sur Revue Travaux Oise bridge Contractor: Dragages et Travaux Publics Feb. 1986 - Dec. 1987 A1

Oct. 1988

The piers are supported by six Ø1.40 m piles anchored in the limestone bedrock. The right bank abutment is built directly on the ground. Due to the presence of a thick layer of peat, the left bank abutment is built on four Ø 1.10 m diameter piles. Revue Travaux Oct. 1988 Consortium: Revue DTP (Dragages Générale et Travaux des Routes Publics) - CITRA et des May 1984 - Sept. Aérodromes 1986 Oct. 1989 A2 June 1986

Poncin viaduct (A40)

The supports are all built on the bedrock, either directly on superficial footings (CO, PI, P3, P4, P5, C6 - 13m x l4m x 3 m), or on deep foundations: P2: 12.5m diameter box girder built from diaphragm wall elements. P3 : Four 5 m x 3 m H-shaped rectangular foundation piles Revue Champ du Comte viaduct Travaux Apr. 1989 (RN 90) Consortium: - SOGEA - GTM-BTP Dec. 1986 - Jan. 1989 A3

Revue Générale des Routes et des Aérodromes Feb. 1990

Champ du Comte viaduct (1,040 m)

Supports and foundations: double portal frame configuration “Puits marocains” (deep pile foundation shafts) 4 to 5.60 m in diameter and 6.00 to 22.00 m in depth

The “Les outils" Collection – Sétra

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CROSS-SECTIONS OF BRIDGE DECK

CONSTRUCTION METHOD and weight of segments

HEIGHTS OF BRIDGE DECK Slenderness ratios

QUANTITIES OF MATERIALS USED

COMMENTS

Cast-in-situ segments. Length of standard segments 3.53 m: End spans built on falsework

- On support: h = 6.60 m s (slenderness) = 1/18 - At the crown: h = 3.00 m s = 1/40

For the bridge deck: - Surface area of bridge deck :1,950 m² - Concrete: 1,793 m3 including 585 m3 of lightweight concrete - Non-prestressed reinforcement bars: 225,860 kg (126 kg/m3) - Prestressing bars: 72,650 kg (41 kg/m3) (37 kg/m2) - Mean thickness of bridge deck: 0.92 m over entire structure, 0.77 m in the central span.

- Bridge deck prestressing: half using 9T15 tendons and 12T15 internal tendons & the other half using replaceable 19T15 tendons – End span fixed to a weighted abutment acting as a counterweight with a solid 9.00m long rectangular section at the end.

Cast-in-situ segments of variable depth. Length of standard segments: 3.30 m (weight: 120 to 220 tonnes) Incremental launching of end sections (constant depth)

- On support PI or P2: h = 10.00 m s= 1/15.5 - At crown of main span: h = 4.00 m s = 1/39

For the bridge deck: - Formwork: 27,850m² - Concrete: 11,270m3 - Non-prestressed reinforcement bars: 1,266,900 kg (112 kg/ m3) - Longitudinal prestress bars: 516,400 kg (45.8 kg/ m3) - Transverse prestress bars: 77,450 kg

Internal prestressing: - cantilever: 2 x 27 19T15 tendons - continuity: 2x7 12T15 tendons –External prestressing: 2 x 10 19T15 tendons

Main quantities: - B30 concrete: 8,500 m3 - B45 concrete: 17,000 m3 - Non-prestressed reinforcement bars: 3,800,000 kg - Longitudinal prestress bars: 850,000 kg

Internal prestressing: -cantilever tendons: 2 x 13 12T15 2 x 1 19T15 tendons - continuity tendons: 2 x 5 12T15 External prestress tendons: 2 x 3 19T15 2 x 1 12T15 tendons

- Thickness of webs in central span: 0.36 m – Thickness of webs in end span: 0.36 m on the first 3 segments then 0.50 m, 0.70 m and finally 1.00 m before reaching the solid section. Solid rectangular section inside the counterweighted abutment

ON SUPPORTS P1 and P2 CONSTANT DEPTH AT PI-P2 CROWN

Thickness of webs near P3, P4 and P5: 1.20 – 1.10 – 0.90 m respectively Viaduct with 2 separate bridge decks

Cast-in-situ segments On support:

h = 5.80 m s = 1/17 At crown of main span: h = 2.90 m s = 1/34.5

Cross-section of 1 box girder


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NAME OF STRUCTURE Situation, Contractors, Construction dates Nantua viaduct (A40) Consortium: -GTM - Campenon Bernard June 1983 Nov. 1986

REF.

Revue Travaux Jan. 1986 Revue Générale des Routes et des Aérodromes Oct. 1989 June 1986

A4

Lalleyriat viaduct (A40)

Revue Générale des Routes et des A5 Aérodromes Nov. 198


The variabile quality of the terrain required a wide range of foundations: P7 and P9: Four Ø2.20 m“puits marocains” (deep pile foundation shafts) P8: one Ø 6 m deep pile foundation shaft PI, P2 and P3: Ø 10 m diaphragm walls P4, P5 et P6: superficial footings of approx. 10 m x 10 m x 2.5 m

The structure consists of two bridge decks of different lengths with a height difference of 5 to 7 m,: * North deck: 130 m, 3 spans * South deck: 194 m, 4 spans and different widths: * North deck: 8.75 m * South deck: 11.75 m Length of central span for both decks: 58 m The majority of the foundations consist of footings built on sound bedrock.

Tacon viaduct Contractor: CITRA France

A6

Revue Générale des Routes et des Aérodromes Nov. 1989 Revue Travaux Jan. 1986

The structure consists of two independent bridge decks with a height difference of approximately 6 m. Deep pile foundation shafts (P5: four Ø 2.40 m shafts) or cast-in-situ piles (P2, P3, P4 & P6: four piles of Ø 1.20 m to Ø 1.50 m)


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CONSTRUCTION METHOD and weight of s egments

HEIGHTS OF QUANTITIES BRIDGE DECK OF Slenderness MATERIALS ratios USED Cast-in-situ - On support: For the bridge segments. h = 6.65 m deck: Length of segments: s = 1/17 - Concrete: 2.50 to 3.50 m (60 to - Fixed support of 9,800m3 85 t). ½ cantilever: - Nonh = 8.44 m prestressed Internal prestressing reinforcement using 12T15 tendons - At the crown bars: 1,250,000 h = 3m kg (126kg/m3) s = 1/38 - Prestress bars: 430 000 kg (44 kg/m3)

COMMENTS

The viaduct ends in a 124 m span supported in the tunnel. To build & balance this span, a counterweighted span of approx. 3,000 t was built on centuring. This consists of two mobile sliding counterweights, each with a capacity of 1,500 t.

Cast-in-situ Constant depth of segments (method 2.,80 m dictated by site s = 1/21 access problems). Internal prestressing with possibility of adding extra external prestressing.

At the c row n

Cast-in-situ segments. Each segment measures 3.50 m long. Internal prestressing (12T15 tendons) with possibility of adding extra external and internal prestressing.

- On support: h=6m s = 1/15 - At the crown and for segments of constant depth: h = 3m s = 1/30

For the bridge deck: - Concrete: 4,820 m3 - Non-prestressed reinforcement bars: 554,000 kg (115 kg/m3) - Prestress bars: 185,000 kg (38 kg/m3)

On support (for the spans of variable depth): - height of bridge deck = 6 m - Thickness of bridge deck = 0.55 m


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NAME OF STRUCTURE Situation, Contractors, Construction dates Cheviré bridge over the River Loire

REF.


Revue Travaux Apr. 1989

Consortium: Concrete package: Quillery et Vezin Metal package: Baudin Châteauneuf et ACP Dec. 1987 – Oct. 1990

A7 Presence of joints between N5 and N6 & between S5 and S6

Pont à Mousson viaduct over the Moselle

Revue Travaux Jan. 1985

The viaduct consists of two independent structures: - The main crossing of the River Moselle and its lateral canal; - The SNCF (French state rail network) railroad crossing.

Consortium: Pertuy-Bouygues 1982

Cast-in-situ pile foundations: P2, P3, P7 : Four Ø 1.30 m piles P4, P5, P6: Six Ø 1.30 mpiles P1 : Six Ø 0.80 m piles

A8 Chinon bridge AFPC in Indre-et(French Loire Construction Association) Entreprise: Study Day, GTM 12 June 1986 Sept. 1984 Revue A9 Travaux Jan. 1986

The piers are built on footings within a cofferdam. The abutments are built on four Ø 0.80 m piles.


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CONSTRUCTION METHOD and weight of segments Cast-in-situ segments. Length of standard segments: 2.70 m (111 t) Length of segments on piers: 8.70 m. Segments on piers are cast horizontally then tilted to the required angle before the construction of the cantilever begins.

Segments of the 3 large cantilevers are cast in-situ. The rest of the structure is cast on falsework.

HEIGHTS OF BRIDGE DECK Slenderness ratios - On support: h = 9.00 m at level of NI and SI - Standard spans (constant depth): h = 4.55 m s = 1/14


COMMENTS

For the concrete bridge deck: - B35 concrete: 27,500 m3 -Non-prestressed reinforcement bars: 3,850,000 kg (140 - At end of cantilever kg/ m3)

- The first 6 meters of the end spans were cast on falsework. The central metal span was hoisted onto its permanent bearings in one operation.

above the River Loire: H = 5.20m

- On supports 4, 5 & 6: h = 4.50 m s = 1/17 - Beyond the length of the parabolically variable section (18 m) either side of these supports: h = 2.75 m s = 1/27.5

The initial pre-design did not feature any external prestressing. A summary analysis of mixed cabling was included but the dimensions of the structure were not modified due to a lack of time

½ -section at crown ½- section on pier

For the entire bridge deck (including the caston-falsework sections): - B35 concrete: 3,437 m3 - Non-prestressed reinforcement bars: 466,000 kg (136 kg/m3) - Prestress reinforcements: int.: 71,600 kg ext.: 63,000 kg (in total: 39 kg/m3) Comment: The structure supports oversize Class E loads.

Cast-in-situ segments. The end of the end spans is cast on falsework.

With its oversized dimensions, this bridge was designed to support thirty-meter long EDF (Electricité de France –French electricity supply utility) convoys weighing 650 tonnes


-Prestress reinforcements (longitudinal and transverse): 1,216,000 kg (44 kg/ m3)

- On support: h = 4.50 m s= 1/15.5 - At the crown: h = 2.80 m s = 1/25

For the bridge deck: - B35 concrete: 1,385 m3 - Non-prestressed reinforcements: 205,000 kg (150 kg/m3) - Prestress reinforcements: internal: 28,500 kg external: 21,500 kg (in total: 50 t or 36 kg/m3)

- Prestressing: int: (cantilever & continuity) 19T15 ext.: 27T15 ½ section on span

Large spans: cantilever: 24 x 12T15 continuity: 6 x 12T15 external: 8 x 19T15 Given the complex 2dimensional alignment of the tendons and the thinness of the deviators, strong radial tendon forces of the deviators close to piers have resulted in the latter being reinforced.

Mixed prestressing: - int. tendons: cantilever: 2 x 9 12T15 continuity: 2 x 4 12T15 - ext. tendons: 2 x7 12T15 Comment: external tendons are somewhat limited in power

Equivalent thickness: 75 cm

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NAME OF STRUCTURE Situation, Contractors, Construction dates Bridge over the Loch d’Auray

REF.


La Technique Française du Béton Consortium: Précontraint , - Campenon 11th FIP Bernard (International - E.T.P.O. Nov. 1986 – Mar. Prestressing Federation) 1989 Congress, Al0 Hamburg 1990 Revue Travaux June 1991

La Technique Française du Béton Précontraint , Contractor: 12th FIP DODIN Sud (International Aug. 1989 - June Prestressing 1991 Federation) Congress, A11 Washington 1994 Revue Travaux Oct. 1991 July 1992 La Bridge over Technique the SaintDenis river on Française du Béton Reunion Précontraint , Island 12th FIP (International Contractor: Prestressing SBTPC Federation) Aug.1989 - Aug. Congress, 1991 Washington A12 1994 Revue Travaux July 1992

Bourran viaduct at Rodez


Engineering geology: risk of landslides Deep pile foundation shafts: one Ø3 m or Ø4 m shaft for each pier sunk to depths of 11 to 15 m.

Abutment C0, situated directly above a rocky spur embedded in the sand, is supported by two Ø2 m deep pile foundation shafts, one of which is extended by micropiles. The three piers are built on superficial footings. The ground had to be injected under P1.

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COMMENTS

The large span has 2 cantilevers (3 m segments) built using form travelers. Two temporary pilings supported the portal legs before placement of the final segment. The rest of the structure was built on falsework.

- On support: h = 5.20 m s = 1/16 - At the crown: h = 2.50 m s = 1/33.5

For the cantilever sections: Mixed prestressing: - Internal tendons Cantilever: 12T15 Continuity: 2 x 6 12T15 - External tendons (from portal leg to portal leg) 2 x 2 12T15 The ribs are transversally prestressed.

In addition to the usual tendons In the end spans, rectilinear external prestressing, extending from the abutment to portal leg, has been added in order to restore a normal force virtually equal to that created by the thrust of the portal legs in the large span.

Construction using “from below” type of form travelers. Length of standard segments: 3.70 m, segments on piers: 9 m. Both ends of the bridge deck were built on falsework (17.70m in span S1 and 6.00 m in span T5).

- On support: h = 6.00 m s= 1/16.5 - At the crown: h = 3.00 m s = 1/33

For the bridge deck: - Concrete: 3,000 m² - Non-prestressed reinforcement bars: 365,000 kg (122 kg/m3) - Longitudinal prestress bars: 119,190 kg (40 kg/m³) Largecantilevers: - Cantilever 2 X 16 12T15 - Continuity 2x3 12T15 - External tendons 2x4 19T15 - Mean thickness of concrete: 0.71 m

- Possibility of adding extra tendons. - Vertical prestressing of segment on P1 - Given the flexibility of the piers in rotation (2 bent pier shafts for P2 & P3), the winds effects were calculated very accurately, but in the end, no bracing was provided.

Construction using “from below” type of form travelers. Length of standard segments: 3.45 m (max. weight: 120 t), segments on piers: 7.80 m. Both ends of the bridge deck were built on falsework.

- On support: h = 4.50 m s = 1/17 - At the crown: h = 2.50 m s = 1/31

For the bridge deck: - Concrete: 3,100 m3 - Non-prestressed reinforcement bars: 437,100 kg (141 kg/m3) - Longitudinal prestress bars: 59 t 12T15 62 t 19T15 (in total 39 kg/m3) Prestressing of upper slab via sheathed & greased single strands: 20,000 kg (6.5 kg/m3)

- Prestressing: cantilever : 2 x 14 12T15 continuity : 2 x 2 12T15 external tendons : 2 x 6 19T15 - provisional bracing of curved pier shells to take up forces due to cyclonic winds - Highly problematical construction of foundations.

Directly above portal legs

Thickness of webs in segments on P1 and P4: 60 cm. Slightly asymmetrical box girder: the transverse slopes begin at the centerline of the pavement which is not aligned with the centerline of the box girder. To avoid any difference in the height of the webs, the entire box girder has been tilted transversally (gradient of 0.69 %).


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REF.


La Technique Française du Béton Consortium: Précontraint , - Quillery 12th FIP - Chantiers (International Modernes Prestressing - Borie SAE Federation) Congress, A13 Washington Foundations: via footings for PI, P2 and P5 (with stitched reinforcement of the footings of the very 1994 tall pier P2), via 7 m shafts for P3 and P4

Piou and Rioulong viaducts

Rioulong viaduct: 5 spans (45, 72, 81,81, 63 m), 6 % slope Limay viaduct in Yvelines Consortium: - Nord France TP - Holzmann 1992

Revue Travaux Apr. 1993

A14 Foundations via cast-in-situ piles: - Six Ø 1.40 m piles for P2, P3, P6, P7 - Four Ø 1.40 mpiles for P4 and P5 - Four Ø 1.20 m piles for C8 - Eight Ø 0,80 m piles for C1 La Technique Française du Béton Consortium: Précontraint , - SPIE T.R 13th FIP - DODIN Sud (International - SOGEA Prestressing Feb. 1995 - Dec. Federation) 1997 Congress, Amsterdam A15 1998

Viaur valley viaduct (RN 88)

- P1 and P2 are erected on superficial foundations. Their footings were temporarily anchored to the rock via 10 stressed 12T15 ties designed to take up the wind effects during construction. - P3 and P4 are supported on footings anchored to the rock by four 3 m shafts excavated using explosives - The bridge deck is embedded on P3 and P4.


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Half-section of a standard section Half-section of sections close to piers




COMMENTS

Construction using Constant depth “from below” type of h=5m form travelers. Length s = 1/18 of segments: 3 m (6.8 m on pier), with a stagger in the construction of the cantilevers to allow the incorporation of metal struts.

Piou viaduct: - Concrete: 6,500 m3 -Non-prestressed reinforcement bars: 819,000 kg (126kg/m3) - Longitudinal prestress bars 228,000 kg (35 kg/m3) int. (cantilevers and crowns) super 19K15 ext.: 27K15 super transversal: 92,000 kg (14 kg/m3) 3C15 - equivalent thickness: 0.73 m

Incoprporation of bracing: - When casting segment n, 4 fixing sockets were sealed in the bottom flange of the web, using a false strut to give the required accuracy. - The real strut was attached to segment n + 1 by bolting on the lower part and casting on the upper part.

Construction using form travelers. Length of segments on piers: 9.60 m Length of standard segments: 3.90 m.

- On support: h = 5.30 m s = 1/17 - At the crown and in spans of constant depth: h = 2.70 m s = 1/33

For the bridge deck: - Concrete: 4,680 m² - Non-prestressed reinforcement bars: 769,000 kg (164 kg/m3) -Internal longitudinal prestress bars: 119,000 kg External: 103,000 kg (in total: 47 kg/m3)

Mixed prestressing: - Cantilever tendons: 20 (piers on the island) to 30 (piers in the river) 12T15S. Continuity: 14 (in the 90 m spans) to 2 (for spans on the island) 12T15S. - Ext. tendons: 12 (at mid-span) to 8 (on pier) 19T15S

Construction using form travelers. Length of standard segments: 2.76 m. The extremities of the end spans are built on falsework.

- On support: h= 12m s = 1/16 - At the crown and in the spans of constant depth. h = 4.50 m s = 1/42

Main quantities: - Concrete: 21,000 m3 - Non-prestressed reinforcement bars: 3,900 t - Prestressing: 650 t

- Mixed prestressing: Int.tendons 19T15 Ext. tendons 27T15 - Transverse prestressing via single strands - Movements of major spans due to wind effects during construction limited by the use of vertical prestress tendons.

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NAME OF STRUCTURE Situation, Contractors, Construction dates Doubling of the Général Audibert bridge over the Loire, in Nantes

REF.


Revue Travaux April 1989

Contractor: ETPO Jan. 1987 - Mar. 1989 Two independent but connected bridge decks, made from prestressed concrete box girders of variable depth. Length: 163 m (3 spans: 51 m + 67 m + 45 m). Width: 17.40 m.

A16

Bridge over the Truyère at Garabit Consortium: - GTM-BTP - Dumez July 1990 - Feb. 1993

Revue Ouvrages d’art n° 16 Nov 1993

A17 Portal bridge with a prestressed concrete box girder. Bridge deck embedded on the portal legs and on simple supports on the abutments. Length: 308 m (3 spans: 82 m + 144 m + 82 m). Width:21 m.

Auxonne and Revue Travaux Maillys Jul./Aug.1994 viaducts in the Côte d’Or Consortium of contractors: - Dodin Sud : - SOGEA : - Dodin Ouest Sept. 1991 – Nov. 1993

Structure with two separate bridge decks. Usable width of each of the two decks: 11m. 8 spans: 52 m + 77 m + 136 m + 77 m + 3 x 55 m + 45 m.

A18


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SEGMENTS ON PIERS SEGMENTS AT CROWN UPSTREAM

DOWNSTREAM




COMMENTS

Cast-in-situ segments. Length of segments on piers: 9 m. Length of standard segments: 3.50 m. Closing segments: 2m. Weight of standard segments: 45 tonnes.

- On support: h = 3.80 m s = 1/18 - At the crown: h = 1.80 m s = 1/37

For the bridge deck: - Concrete: 2,150 m3 - Prestressing: 85,190 Kg - Non-prestressed reinforcement bars: 338,000 Kg Usable area of the structure: 2,840 m3

Prestress tendons inside concrete: 12T 13 20.50

Cast-in-situ segments.

- On the abutment supports: 10 m. - At the crown: 3 m. -At the level of the portal legs: 8 m.

- Concrete: 16,000 m3 - Non-prestressed reinforcement bars: 13,700 m3 - Prestressing: 362 t.

- Int. prestress tendons: cantilever (2 X 35 per cantilever) and continuity (2 X 7 on end span and 2 X 2 on central span): FUC 19-620 - Ext. prestress tendons: FUC 19620, remobvable on end spans - Transverse prestressing of upper slab: FUC 4-620

Large span and 2 adjacent ½ spans: successive cast-insitu cantilever segments built using 2 x 2 form travelers. Cast-on-falsework end spans

For the segments built by the cantilever method - On support: h = 6.50 m s = 1/21 - At the crown: h = 3.20 m s = 1/42 For the sections built on falsework: h = constant = 3.20 m s= 1/17

For the bridge deck: - B40 concrete: 10,300 m³ - Int. prestressing: 260 tonnes. - Ext. prestressing: 325 tonnes. - Non-prestressed reinforcement bars: 1,520 tonnes.

- Cantilever tendons: 12 T 15. - Continuity tendons: 12T15. - Continuity tendons outside concrete: 19T 15.

The box girder features: - a 20.50 m upper slab which is transversally prestressed by 4T15 tendons at intervals of 0.60 m; - two webs angled at 30%, with vertical thickness of 0.60 m (1.20 m towards the portal legs); - a lower slab whose thickness varies between 1.20 m directly above the portal legs to 0.25 m at the crown. SECTION THROUGH P3-P4 CANTILEVER On support In span


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REF.


Revue Ouvrages d’art Consortium: Nov. 1994 - Campenon Revue Bernard Régions Générale des - Roux SA Routes et Feb. 1993 - Apr. des Length: 496 m (48 m + 5 x 80 m + 48 m). 1995 Aérodromes Width: 14.50 m. Nov. 1995

Corniche bridge in Dole

A19

Revue Travaux Jul./Aug.1997

Rivoire viaduct in Isère Consortium: - Razel - FougerolleBallot - Royans Travaux July 1995 - Nov. 1996

A20

Vecchio bridge in Corsica Contractor: Razel

Two independent structures. Two concentric bridge decks with radii at the left edge of 984.50 m and 1001.15 m: center distance of 24.10 m. Length: 247 m ( 64 m + 113 m + 70 m). VIVARIO

Revue Ouvrages d’art Dec. 1998

VENACO

A21

3-span prestressed concrete structure (42.25 m + 137.50 m + 42.25 m), 10 m wide.


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CONSTRUCTION METHOD and weight of segments Cast-in-situ segments. Length of standard segments: 3.20m. Length of segments on piers: 6.40 m.

HEIGHTS OF BRIDGE DECK Slenderness ratios - On support: h =5.50 m e = 1/14 - At the crown: h = 2.50 m s = 1/32

Length of segments on piers: 8.50 m. Length of standard segments: 16 x 3.20 m on central span and 17 x 3.20 m on end span. Standard segments built using “from below” type of form travelers. Construction of central span in halfcantilevers using successive 3.60 m long cantilever segments using form travelers into which the prefabricated webs were placed. These are of variable depth and angle, due to the variation in width of the lower slab.


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COMMENTS

For the bridge deck: - B35 concrete: 4,100 m3 - Prestressing: 190 tonnes.

First bridge with corrugated webs to be built by the cantilever method in France.

For the bridge deck: - B35 concrete: 4,000 m² - Formwork: 15,200m² -Reinforcements: 690t - Prestressing: 174 t

- On support: h= 11 m s= 1/12 - At the crown: h = 3.50 m s = 1/39

Prestressing: - cantilever tendons in the upper slab; - continuity tendons outside the concrete stretching from abutment to abutment; - internal continuity tendons in central span; - prestressing bar in web panels.

septembre 2007



REF.


Bridge over the Revue Travaux Rhine in n° 783 Strasbourg Consortium: - Bilfinger - Berger and Max Früh

FebThe piers. 2002

Length: 457 m (121 m + 205 m + 131 m). Width: 14.75 m.

2001 - 2002

A22 Bridge over the Seine at Gennevilliers Consortium: - SPIE Batignolles - GTM - Fougerolles 1990 - 1992

Length: 568 m (110 m + 169 m + 96 m + 169 m + 114 m). Width: 18.06 m.

A23


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Prestressed Prestressed concrete bridges built using the t he cantilever method – Design Design guide gui de


SECTION ON PIER PILE

SECTION AT CROWN

The “Les outils” outils” collection collection – Sétra




Cast-in-situ segments. Standard segments vary in length from 3.50 to 5.00 m. Length of segments on piers: 9.50 m.

- On pier: h = 9.00 m s = 1/23 - At the crown: h = 4.50 m s = 1/45 - On abutment: h = 3.20 m Variation according to a parabolic curve for the central span and a cubic curve for the end spans.

For the bridge deck: - B65 concrete: 7,750 m3 - Non-stressed reinforcement bars: 913,000 kg (118 kg/m3) - Prestressing: Prestressing: 598,000 kg (77 kg/m3)

Cast-in-situ segments. Length of standard segments: 3.30m for the 96 m span and 3.60 m for the other spans.

- On pier: h = 9.00 m - At the crown: h = 3.50 m

For the bridge deck: - Concrete: 13,900 m3 - Non-stressed reinforcement bars: 1,711,000 kg (124 kg/m3) - Prestressing: Prestressing: External 241 t Internal 728 t Temporary 22 t

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COMMENTS

septembre septemb re 2007


A33 Bi A B i b l i o g r ap aphh y This appendix firstly mentions all of the official french texts relating to the engineering and design of bridges built by the cantilever method. It then goes on to list a large number of documents and articles dedicated to this technique.

A 3 - 1 – O f f i c i a l t e x t s F r e n c h t e x t s r e l a t ed ed t o a l l s t r u c t u r e s

- Fascicule 61 Titre II of the french CPC: "Programme de charges et épreuves des ponts-routes" (Loading and testing program for road bridges). - Circular no. R/EG3 of July 20 1983 entitled "Transports exceptionnels, définition des convois types et règles pour la vérification des ouvrages d’art" (Oversize loads, definition of standard convoys and rules for the verification of civil engineering structures) published structures) published by the Direction the Direction des Routes (Highways Routes (Highways Department), for structures supporting these types of vehicles - Fascicule 6l Titre IV Section II of the french CCTG: "Actions de la neige sur les constructions" (Effects of snow on constructions) (DTU constructions) (DTU P 06-006 of September 1996) - Fascicule 62 Titre I - Section I of the french CCTG: "Règles techniques de conception et de calcul des ouvrages et constructions en béton armé suivant la méthode des états limites" (Technical engineering and design rules for reinforced concrete structures and constructions according to the limit states method) [BAEL 91 revised in 1999)] - Fascicule62 Titre I - Section II of the french CCTG: "Règles techniques de conception et de calcul des ouvrages et constructions en béton précontraint suivant la méthode des états limites" (Technical engineering and design rules for prestressed concrete structures and constructions according to the limit states method) (BPEL 91 revised in 1999) - Fascicule 65-A of the french CCTG and its supplement: "Exécution des ouvrages de génie civil en béton armé ou précontraint" (Construction of reinforced or prestressed concrete civil engineering structures) - Fascicule62 Titre V of the french CCTG: "Règles techniques de conception et de calcul des fondations des ouvrages de génie civil" (Technical engineering and design rules for the foundations of civil engineering structures) - Fascicule 68 of the french CCTG: "Exécution des travaux de fondation des ouvrages de génie civil" (Construction of the foundations for civil engineering structures) - Standard NFP 95-104: "Réparation et renforcement des ouvrages en béton et en maçonnerie; spécifications relatives à la technique de précontrainte additionnelle" (Repair and reinforcement of concrete and masonry structures; specifications relating to the additional prestressing technique)


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Texts exclusively related to structures situated in earthquake zones

- Decree no. 91-461 of May 14 1991 relating to the prevention of earthquake risks and the Order of September 15 1995 relating to the classification and rules for antiseismic constructions applicable to bridges in the “normal risk” category - Guide AFPS 92 for the seismic protection of bridges, bridges, edited by the Association Française du Génie Parasismique (French Parasismique (French Seismic Engineering Association), published on the presses of the École Nationale des Ponts et Chaussées (National Chaussées (National School of Civil Engineering) - The french standard NE P 06-013, more commonly called the "Règles de construction parasismique – règles applicables aux bâtiments - PS92" (Seismic Construction Rules – Rules Applicable to Buildings – PS92), PS92) , with regard to foundations.

A 3 - 2 – S é t r a , S é t r a - L C P C , S é t r a - S N C F G u i d e s - Sétra Technical Bulletin no.7 "Ponts construits par encorbellements successifs" - This Design Guide - Sétra Technical Guide "Précontrainte Extérieure" (External Prestressing) - Sétra Technical Guide "Appareils d’appui en caoutchouc fretté" (Laminated rubber pot bearings) - Sétra Technical Guide "Appareils d’appui à pot de caoutchouc" (Rubber pot bearings) - The guide to the ordering and management of studies for civil engineering bridges - Fascicule 32.2 of the technical instruction for the monitoring and maintenance of civil engineering structures, published in 1979 and modified modified in 1995 - The document entitled "l’Image de la Qualité des Ouvrages d’Art (Iqoa); catalogue des principaux défauts, aide à leur classification - ponts à poutre caisson en béton précontraint" (Image of the quality of civil engineering structures [IQOA]; catalog of the major defects and support for their classification – prestressed concrete box girder bridges) published bridges) published by Sétra in 1997.

A 3 - 3 – O t h e r p u b l i c a t i o n s P r e ss ss e d e l ’ Éc o l e N a t i o n a l e d e s P o n t s e t C h a u s sé sé es

- La conception des ponts (Bridge ponts (Bridge design) [J.A. Calgaro and A. Bernard-Gely] - Analyse structurale des tabliers de ponts (Structural ponts (Structural analysis of bridge decks) decks) [J.A. [J.A. Calgaro and M.Virlogeux] - Maintenance et réparation des ponts (Bridge ponts (Bridge maintenance and repair) T h o m a s T el el f o r d , L o n d o n

- Ponts en béton précontraint par post tension (Post-tensioned tension (Post-tensioned prestressed concrete bridges) H bridges) HA - T RL - L CPC – Sétra The “Les outils” outils” collection collection – Sétra

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L a co c o l l e ct ct i o n d e l ’ I r e x

- Projet National Qualité (National Qualité (National Quality Project) - Opération du Plan Génie Civil (Operation (Operation of the Civil Engineering Plan) – Documentation booklet "Parements en béton" (Concrete facings) Éd i t i o n s E y r o l l e s

- Procédés généraux de construction construction (General (General construction process ) – Vol.1 Vol.1 (J. Mathivat and C. Boiteau)

A 3 - 4 - A r t i c l e s i n m i s c e l l a n e o u s p u b l i c a t i o n s There now follows a list of articles relating to construction using the cantilever method, published between 1990 and mid - 2002 in the main French civil engineering journals and in certain foreign publications. Each of these articles is followed by a code indicating the topic(s) covered. The following table shows the meanings of these codes:



Research, calculations



Design & construction of a bridge with cast-in-situ segments



Design and construction of a bridge with prefabricated segments



Design and execution of a repair or reinforcement project



Materials

A F P C / A F G C p u b l i c a t i o n s f o r F I B ( Fé dé r at i o n d e l ’ In d u s t r i e du d u B Bé t o n Concrete Indus I ndus try Fe Federation) deration) congr esses - [VIR 90.1] M. 90.1] M. Virlogeux: La Virlogeux: La résistance à l’effort tranchant des ouvrages ouvrages constitués de voussoirs préfabriqués, La technique française du béton précontraint (The resistance to shear force of structures made from prefabricated segments, The French Prestressed Prestressed Concrete Technique), Hamburg Technique), Hamburg (1990)  - [MOS 90] J. 90] J. Mossot: Le viaduc du Champs du Comte, La technique française du béton précontraint (The Champs du Comte viaduct, The French Prestressed Concrete Technique), Technique), Hamburg (1990)  - [BOU 90] J. 90] J. Boudot: Le Boudot: Le viaduc de Sylans et des Glacières - Les structures triangulées en béton précontraint, La technique française f rançaise du béton précontraint (The (The Sylans et Glacières viaducts – Triangulated structures made from prestressed concrete, The French Prestressed Concrete Technique), Technique), Hamburg (1990)  - [SER 90] C. 90] C. Servant, R Gallet, Ph. Lecroq, R Barras: Le Barras: Le viaduc de l’Arrêt-Darré, La technique française du béton précontraint (The (The Arrêt-Darré viaduct, The French Prestressed Concrete Technique), Technique), Hamburg (1990)  - [VIR 90.2] M. 90.2] M. Virlogeux, G. Lacoste, M. Legall, RY. Bot, J-R Runigo, J. Combault, M. Duviard, G. Suinot and P. Fraleu: Le Fraleu: Le pont sur le Loch d Auray, La technique française du béton précontraint (The (The bridge over the Loch d’Auray, The French Prestressed Concrete Technique), Technique), Hamburg (1990) 


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- [VIR 94] M. 94] M. Virlogeux, E. Bouchon, J.C. Martin, J. Lefevre, Y. Maury, T. Guyot, M. Pottier, A. Heusse, P. Fraleu, J. Ryckaert, J. Mathivat and B. Lenoir : Pont : Pont de Cheviré, La technique française du béton précontraint (The Cheviré bridge, The French Prestressing Concrete Technique), Technique), Washington (1994)  - [FUZ 94] J-R 94] J-R Fuzier, C. Adib: Poutres de lancement: le pont de Baldwin, La technique française du béton précontraint (Launch beams: the Baldwin bridge, The French Prestressed Concrete Technique), Technique), Washington (1994)  - [BOU 94.1] 94.1] E. Bouchon, D. Lecointre, M. Virlogeux, R. Gachiteguy, G. Viossanges, R. Gai, M. Boy, P. Ballester, M. Roudanes, P. Fraleu : Le viaduc de Bourran à Rodez, La technique française du béton précontraint (The (The Bourran Viaduct in Rodez, The French Prestressed Concrete Technique), Technique), Washington (1994) 

- [BOU 94.2] E. 94.2] E. Bouchon, E. Conti, D. de Matteis, E Pero, M. Virlogeux, R. Damour, A. Abastado, E Edon, M. Tassone, A. Demozay, P. Jacques, F. Veyres, C. Lavigne: Le Lavigne: Le pont de la rivière Saint-Denis à la Réunion Réunion (Océan Indien), La technique française du béton précontraint (The (The bridge over the Saint-Denis river on Reunion Island [Indian Ocean], The French Prestressed Concrete Technique), Technique), Washington (1994)  - [CHA 94] 94] P. Chassagnette, J.J. Lagane: Doublement du pont sur la Seine à Gennevilliers, La technique française du béton précontraint (Doubling the width of the bridge over the River Seine at Gennevilliers, The French Prestressed Concrete Technique), Technique), Washington (1994)  - [CRO 94] A. 94] A. Crocherie, G. Gillet, B. Canitrot, F. Edon, P. Kirschner, B. Fournier, F. Renaud, T. Thibaux, P. Doguet : Les viaducs du Piou et du Rioulong, La technique française du béton précontraint (The Piou and Rioulong viaducts, The French Prestressed Concrete Technique), Technique), Washington (1994)  - [CAN 94] B. 94] B. Canitrot, G. Gillet, B. Bouvy, A. Palacci, B. Raspaud: Le Raspaud: Le pont de l’autoroute A75 sur la Truyère à Garabit, La technique française du béton précontraint (The (The A75 highway bridge over the Truyère at Garabit, The French Prestressed Concrete Technique), Technique), Washington (1994)  - [LEB 94] 94] J-D Lebon, A. Leveille: Le pont de la Corniche à Dôle, La technique française du béton précontraint (The (The Corniche bridge at Dôle, The French Prestressed Concrete Technique), Technique), Washington (1994)  - [COM 94] J. 94] J. Combault, J.P Teyssandier, N.D. Haste, P Richard, M.S. Fletcher, Y. Maury, J. Mac Farlane: Le second pont sur la Severn, La technique française du béton précontraint (The second Severn Bridge, The French Prestressed Concrete Technique), Technique), Washington (1994)  - [MUL 94] J. 94] J. Muller, G. Causse: Le pont à voussoirs préfabriqués de l’autoroute H3 à Hawaï, La technique française du béton précontraint (The H3 highway bridge made from prefabricated segments in Hawaï, The French Prestressed Concrete Technique), Technique), Washington (1994)  préfabriqués en Asie - L’exemple du projet du - [GAS 94] C. 94] C. Gasaignes, J. Boudot, O. Martin: Ponts Martin: Ponts à voussoirs préfabriqués KwunTong By-Pass, La La technique française du béton précontraint (Bridges (Bridges built from prefabricated segments in Asia – The example of the KwunTong By-Pass project, The French Prestressed Concrete Technique, Technique, Washington (1994)  - [ABE 94] H. 94] H. Abel, G. Causse, C. Outteryck, D. de Matteis, H. Capdessus, J. Bouillot, B. Grezes, G. Perez, J. Combault, A. Leveille,Y. Faup, F. Zirk: Le Zirk: Le pont d’Arcins sur la Garonne à Bordeaux, La technique française f rançaise du béton précontraint (The Arcins bridge over the Garonne in Bordeaux, The French Prestressed Concrete Technique),, Washington (1994)  Technique)


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- [BAR 94] P. Barras, D. Poineau: Réparation du pont de Blagnac - Études, projet et suivi des travaux, La technique française du béton précontraint (Repair of the Blagnac bridge – Studies, design and monitoring of works, The French Prestressed Concrete Technique), Washington (1994)  - [VIR 96] M.Virlogeux, J.M. Lacombe, A. Le Bourdonnec: Practical design of cantilever tendons in bridges built by the balanced cantilever method, FIP Symposium, London (1996)  - [SER 98] C. Servant (Serf), E. Bouchon (Sétra), R. Gachiteguy (Aveyron DDE [Departmental Public Works Directorate]), J.J. Lagane, P. Chassagnette, V. Preyssas (Spie-BatignoUes TP): Record de portée dans la vallée du Viaur, La technique française du béton précontraint (Span length record in the Viaur valley, The French Prestressed Concrete Technique), Amsterdam (1998)  - [COM 98] J. Combault (Dumez-GTM): Le pont de la Confédération (Ile du prince Edouard - Canada), La technique française du béton précontraint (The Confederation Bridge [Prince Edward Island - Canada], The French Prestressed Concrete Technique, Amsterdam (1998) / - [JAC 98] P Jacquet, M. Duviard: Le viaduc de Rogerville, La technique française du béton précontraint (The Rogerville viaduct, The French Prestressed Concrete Technique), Amsterdam (1998)  - [MEU 98] P. Meurisse, X. Pham, K. Gharbi, J.P. Viallon: Une nouvelle génération de ponts mixtes: les viaducs du Boulonnais, La technique française du béton précontraint (A new generation of composite bridges: viaducts in the Boulogne area, The French Prestressed Concrete Technique), Amsterdam (1998)  - [BOUS 98.1] C. Bousquet, J.M. Cussac, A. Fauvelle, B. Radiguet : TGV Méditerranée - Lot 2H -Viaducs sur le Rhône, La technique française du béton précontraint (The TGV Méditerranée high-speed rail line – Package 2H – Viaducts over the Rhône, The French prestressed Concrete Technique), Amsterdam (1998)  - [BOUS 98.2] C. Bousquet, J.P. Jung, F. Valotaire: Le viaduc TGV de Vernegues, La technique française du béton précontraint (The Vernegues TGV high-speed rail line viaduct, The French Prestressed Concrete Technique), Amsterdam (1998)  - [DEWI 98] M. De Wissocq, L. Paulik, M. Placidi, J. Vassord: Pont du Vecchio, La technique française du béton précontraint (The Vecchio bridge, The French Prestressed Concrete Technique), Amsterdam (1998) 

Revue tr avaux - [VIR 91.2] M. Virlogeux, G. Lacoste, P Fraleu, M. Legall, P.Y. Bot , J.P Runigo, J. Combault, M. Duviard, G. Suinot, M. Le Corre: Le pont sur le Loch d’Auray (The bridge over the Loch d’Auray) (June 1991)  - [JOU 91] A. Jouanno, G. Gillet, B. Bouvy, J.C. Foucriat, J. Goyet: Autoroute A75 dans le Cantal: Les études du pont sur la Truyère (Studies for the bridge over the Truyère) (October 1991)  - [BOU 91] E. Bouchon, D. Lecointre, M. Virlogeux, R. Gachiteguy, G. Viossanges, R. Gai, M. Boy, R Ballester, M. Roudanes, R Fraleu: Le viaduc de Bourran à Rodez (The Bourran viaduct at Rodez) (October 1991)  - [BOU 92] E. Bouchon, E. Conti, D. de Matteis, E Vacher, M. Virlogeux, R. Damour, A. Abastado, E. Edon, M.Tassone, M. Bustamante, A. Demozay, P.Jacques, E Veyres, C. Lavigne: Le pont de la rivière Saint-Denis à la Réunion (The bridge over the Saint-Denis river on Reunion Island) (July-August 1992) 


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- [RIC 93] C. Ricard, J.Jouves, B. Bouvy, Ph. Dhiver: Remise en état du pont de la RD220 sur le canal d’amenée de la chute de Bourg-lès-Valence (Renovation of the RD220 road bridge over the head race canal for the Bourg-lès-Valence waterfall) (November 1993)  - [HUM 93] E. Humbert, J. Hooghe, X. Durand, Y. Picard: Le viaduc de Limay, Yvelines (The Limay viaduct, Yvelines) (1993)  - [BER 95] A. Bernardo: Le pont du Rambler Channel à Hong Kong (The Rambler Channel bridge in Hong Kong) (April 1995)  - [CON 95] E. Conti, H. Oudin-Hograindleur: Passerelle tournée sur l’autoroute A4 à Noisy-le-Grand (Rotated footbridge on the A4 highway at Noisy-le-Grand) (April 1995)  - [RIC 96] D. Richard, G. Frantz, R Jacquet: Le viaduc de Rogerville (The Rogerville Viaduct) (April 1996)  - [COM 96] J. Combault, J. Hervet, V.Vesval: Le second franchissement de l’estuaire de la Severn (The second crossing of the Severn estuary) (April 1996)  - [BOI 96] A. Boisset, J. Combault, M. Lefebvre, D. Maire: Le pont de l’île du Prince-Edouard (The Prince Edward Island Bridge) (July and August 1996) / - [MAG 97] H. Magnon-Pujo, B. Deberle: Le viaduc de la Rivoire (Isère). Une construction anticipée pour faciliter la circulation de chantier (The Rivoire viaduct [Isère] Anticipatory construction in order to facilitate the movement of site traffic). (July-August 1997)  - [DEL 98] G. Delfosse, P. Eaure, G. Perez: Confortement par précontrainte additionnelle du pont de la Seudre en Charente-Maritime (Reinforcement of the Seudre bridge in the Charente-Maritime by additional prestressing) (February 1998)  - [QUI 98] D. Quivy, Y. Deleporte, B.Vincent: A39 - Les viaducs sur le Doubs et la Loue (A39 Highway – viaducts over the Doubs and the Loue) (February 1998)  - [DOG 98] P. Doguet: Un grand ouvrage sur l’Agout - Le viaduc de Castres (A major structure over the Agout – the Castres viaduct) (February 1998)  - [MON 99] J.Y. Mondon: Le Hung Hom by-pass à Hong Kong (The Hung Hom by-pass in Hong Kong) (January 1999)  - [ROI 99] D. Poineau,J.M. Lacombe, G. Desgagne, C. Creppy, H. Marneffe, L. Duflot, R Ribolzi, B. Vandeputte, R. Zanker: La réparation du pont de Châlons-en-Champagne (The repair of the Châlons-enChampagne bridge (April 1999)  - [PAU 00] L. Paulik: Le pont du Vecchio en Corse (The Vecchio Bridge in Corsica) (January 2000)  - [DIEU 00] R Dieuaide: Le viaduc de Digoin (The Digoin viaduct) (January 2000)  - [DEM 00] A. Demare, G.Tréffot: Le projet du second pont sur le Rhin au sud de Strasbourg (The design of the second bridge over the Rhine to the south of Strasbourg) (January 2000) 


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- [JAE 00] J.M. Jaeger, S. Nunez, JJ. Blanchi, D. Primault: A10 - Le viaduc de la Dordogne (A10 Highway – The Dordogne viaduct) (January 2000)  - [DEW 01] V. Dewilde, E. Dallot, E. Tavakoli, D. Guio, D. de Matteis: Passage à l’Euro(code) pour le second viaduc de Pont Salomon (Transition to the Eurocode for the second Pont Salomon viaduct) (January 2001)  - [DEM 01.1] A. Demare, G.Tréffot: Second pont sur le Rhin au sud de Strasbourg: Les travaux sont commencés (Second bridge over the Rhine to the south of Strasbourg: Work has begun) (May 2001)  - [GIA 01] D. Giacomelli, L. Marraci: Conception de la réhabilitation du pont de Saint-André-de-Cubzac (Design of the renovation of the Saint-André-de-Cubzac bridge) (November 2001)  - [POR 01] T. Portefaix, C. Roude, L. Rosset: Le viaduc sur la Medway (The Medway viaduct) (December 2001)  - [BOUT 01] L. Boutonnet: Kingston Bridge à Glasgow (The Kingston Bridge in Glasgow) (December 2001)  - [CAY 01] F. Cayron, P. Cote: Un nouveau viaduc ferroviaire dans les nouveaux territoires de Hong Kong (A new railroad viaduct in the New Territories of Hong Kong) (December 2001)  - [LAC 02] J.J. Lacaze, D. Giacomelli, M. Duviard, V. Vesval, P. Charlon, C. Sandre: A89 - Le viaduc du Pays de Tulle (A89 Highway – The Pays de Tulle viaduct) (January 2002)  - [CHU 02] J.R Chuniaud, T. Jamet, J.M.Tanis, F. Menuel, E. Barlet, R Chatelard, J.R Viailon: Le pont sur le Bras de la Plaine (Ile de la Réunion): un ouvrage d’exception dans un site grandiose (The bridge over the Bras de la Plaine [Reunion Island]: an exceptional structure in a magnificent setting) (January 2002)  - [DEM 02] A. Demare, G.Tréffot: Second pont sur le Rhin au Sud de Strasbourg: La grande travée au-dessus du fleuve est achevée (Second bridge over the Rhine to the south of Strasbourg: The large span above the river is finished) (February 2002) 

Sétra Bulletin ouvrages d’ art - [CON 91] E. Conti, E. Vacher: Le pont sur la rivière Saint-Denis à la Réunion (The bridge over the SaintDenis river on Reunion Island) (July 1991)  - [ABE 91] H. Abel-Michel, C. Outteryck, B. Grèzes, G. Ferez: L’exécution du pont d’Arcins (The construction of the Arcins bridge) (July 1991)  - [LEC 92] D. Lecointre, D. Lefaucheur: Ferraillage passif des bossages (Non-prestressed reinforcement of anchor blocks) (January 1992)  - [VIO 93] P.Vion: L’exécution du pont de Villeneuve-sur-Lot (Construction of the Villeneuve-sur-Lot bridge) (July 1993)  - [COM 93] J. Combault, B. Flourens: Le pont de la corniche à Dole, de nouveaux plis dans le Jura (The Corniche bridge at Dole, new undulations in the Jura) (March 1993)  - [BAR 93] R Barras: La réparation du pont de Blagnac: études, projet et suivi des travaux (Repair of the Blagnac bridge: studies, design and monitoring of work) (November 1993)  The “Les outils” collection – Sétra

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- [GIL 93] G. Gillet, B. Canitrot,A. Palacci, D. Froissac, P. Gernigon, B. Bouvy,J. Goyet: Le pont sur la Truyère à Garabit (The bridge over the Truyère at Garabit) (November 1993)  - [PER 94] G. Ferez, B.Taimiot: Le renforcement du pont de Bergerac (The reinforcement of Bergerac bridge) (July 1994)  - [JEH 94] J.C.Jehan, J.L. Bernard: La démolition du pont de Beaucaire sur le Rhône (Demolition of the Beaucaire bridge over the Rhône) (July 1994)  - [BOU 94.3] E. Bouchon, J. Lefevre: Pont de Tanus, les effets du vent (Wind effects on the Tanus Bridge) (November 1994)  - [REl 94] J.M. Reinhard: Le Pont de la corniche à Dole (The Corniche Bridge at Dole) (November 1994)  - [DEL 94] G. Delfosse, G. Ferez: Renforcement par précontrainte extérieure (Reinforcement via external prestressing) (November 1994)  - [GIL 96] G. Gillet: Contournement de Marvejols: les viaducs du Piou, du Rioulong et de la Planchette (The Marvejols By-pass: the Piou, Rioulong and Planchette viaducts) (July 1996)  - [JAC 96] R Jacquet: Le viaduc de Rogerville sur l’autoroute A29 (The Rogerville viaduct on the A29 highway) (July 1996)  - [TAV 96] F.Tavakoli: Modélisation STl d’un pont construit par encorbellements successifs (STL modeling of a bridge built by the cantilever method) (November 1996)  - [GAC 98] R. Gachiteguy, G.Viossanges: Le viaduc du Viaur, des fléaux sous haute surveillance (The Viaur viaduct: cantilevers under close surveillance) (March 1998)  - [BAR 98] P. Barras: Réparation de l’ouvrage sur le quai Deschamps à Bordeaux (Structural repair on the Quai Deschamps in Bordeaux) (August 1998)  - [PAU 98] L. Paulik: Le pont sur le Vecchio (The bridge over the Vecchio) (December 1998)  - [DAL 00] F. Dallot, D. de Matteis, V. Dewilde, F.Tavakoli: Passage à l’Euro(code) pour le second viaduc de Pont Salomon (Transition to the Eurocode for the second Pont Salomon viaduct) (August 2000)  - [GOD 00] B. Godart, L. Divet: Une nouvelle réaction de gonflement interne des bétons: la réaction sulfatique (A new internal swelling reaction in concrete: ettringite formation) (May 2000)  - [TAV 00] F. Tavakoli: Renforcement du pont sur la Saône à Lyon (Reinforcement of the bridge over the Saône in Lyon) (December 2000)  - [DEM 01.2] A. Demare, G.Tréffot: Second pont sur le Rhin au Sud de Strasbourg: études de faisabilité des BHP (Second bridge over the Rhine to the south of Strasbourg: HPC feasibility studies) (March 2001)  - [DEM 01.3] A. Demare, G. Tréffot: Second pont sur le Rhin au Sud de Strasbourg: des piles et des fondations profondes dans le fleuve pour résister aux séismes et aux chocs de bateaux (Second bridge over the Rhine to the south of Strasbourg: piers and deep foundations in the river to resist earthquakes and impacts from boats) (June 2001)  The “Les outils” collection – Sétra

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- [LEF 02] D. Le Faucheur: Cumul des aciers de cisaillement et des aciers de flexion (Cumulation of shear and flexion reinforcements) (July 2002) 

Bul leti n des l aboratoi res d es ponts et chaussé es - [DIV 98] L. Divet, F Guerrier, G. Le Mestre: Risque de développement de réactions sulfatiques dans les pièces en béton de grande masse, le cas du pont d’Ondes (Risk of the development of ettringite formation in massive concrete parts, the example of the Ondes bridge) (January-February 1998)  - [DIV 00] L. Divet: État des connaissances sur les causes possibles des réactions sulfatiques internes au béton (Extent of the understanding of the possible causes of ettringite formation in concrete) (July-August 2000) 

T BTP Annales de l’ I - [VIR 81] M. Virlogeux: Analyse de quelques problèmes spécifiques du calcul des ponts construits par encorbellements successifs (Analysis of certain problems specific to the design of bridges built by the cantilever method) (February 1981)  - [POI 92] D. Poineau, J. Theillout, F. Cusin: Réparation et renforcement des structures de bâtiments et d’ouvrages d’art; application des techniques de tôles collées et de précontrainte additionnelle (Structural repair and reinforcement of buildings and civil engineering structures) (February 1992) 

Techni ques de l’ ingé ni eur - [GEO 96] J.M. Geoffray: Le béton hydraulique - Mise en œuvre (Hydraulic concrete – Application) (May 1996) 

P CI JOURNAL - [SCH 95] J. Schlaich, K. Schaefer, M. Jennewein: Temperature induced deformations in match cast segments (July-August 1995)  - [ROB 97] C.L. Roberts-Woolman, J.E. Brein, M.E. Kregle: Towards a consistent design for structural concrete (May-June 1997) 

Revue l’Indus tri a italiana del cemento ( IIC ) - [SMI 97] Dennis R.Smith, PhD: Attraversando la baia di Narragansett, Rhodes Island USA, la costruzione del ponte Jamestown - Verrazzano (mars 1997)  - [ITA 99] Ing. Salvatore Giuseppe Italiano: II ponte sul fiume Ticino nei pressi di Pavia (January 1999)  - [ROS 00] Marco Rosignoli: Ponti in C.A.P. ad anime reticolani (May 2000)  - [REN 00] Ing. Marco Renga: Il ponte di Chivaso sulla S.S. 458 di Casalborgone (Torino) (July-August 2000) 


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