CHAPTER 2 TYPES OF BRIDGE SUPERSTRUCTURES.doc

Dept. of Civil & Urban Eng., IoT

Hawassa University

CHAPTER 2 TYPES OF BRIDGE SUPERSTRUCTURES In this chapter, different types of super structures are introduced and their structural behavior, suitability, relative advantages, and disadvantages are discussed. Culverts Culvert is a cross drainage work whose length (total length between the inner faces of abutments or extreme vent-way boundaries when measured at right angles to the axis of vent-way) is less than 6 meters. Mostly, culverts are constructed over streams which remain dry for most part of the year. In any highway or railway project, the majority of cross drainage works fall under this category. Hence culverts collectively are important in any project, though the cost of individual structures may be relatively small. Culverts are mainly of four types such as pipe culvert, box culvert, arch culvert and slab culvert. Pipe culvert For small streams crossing the road or railway embankments, one or more pipes may be placed to act as the culvert. The diameter of pipe is kept not less than 300 mm. The exact number of pipes and their diameters will depend on the discharge the discharge of water in the stream. Generally pipe culverts are preferred when the discharge in the stream is low (say, about 10 m 3/sec.

Components of pipe culvert: 1. Concrete bedding: It is necessary to provide the concrete bedding of suitable depth below the pipes. 2. Construction at the ends: At both ends of the pipe culvert, it is preferable to provide masonry head walls with arch at top when the depth of earth filling above it is small. The construction of head walls at the ends of the road formation width assists to retain the earth and prevents the stream water to damage the embankment. If the depth of earth filling is more, it will not be economical to provide high head walls and in such cases, the length of the culvert should be increased in such a way that the embankments, with its natural side slopes with revetments, is accommodated with out high retaining walls. In both cases, the splayed wing walls may be provided along with the head walls at the ends.

________________________________________________________________________ Fundamentals of Bridge Design – CEng 552 – Chapter 2

Prepared by: M. K. Chandrasekar

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3. Earth cushion: An earth cushion of minimum specified depth should be provided at the top of the pipes. 4. Material of pipe: The pipes may be of RCC, cast iron, steel or wood. Cast-iron pipes are suitable upto a diameter of 750mm and RCC pipes are suitable up to a diameter of 1800mm. Box culvert Reinforced concrete box culverts are used for square or rectangular openings with spans up to about 4m. The height of vent rarely exceeds 3m. The top of the box section can be at the road level or can be at a depth below road level with a fill depending on the site conditions.

Components of box culvert: 1. Barrel of box section of sufficient length to accommodate the carriage way and the curbs, 2. Wing walls splayed at 45 degrees to retain the embankments and also to guide the flow of water into and out of the barrel, 3. Earth fill over the top of the box barrel if necessary and 4. Concrete foundation bedding Arch culvert Stone masonry arch culverts can be constructed for spans from 2m to 4m. This type of culvert also requires the components similar to the other types. Slab culvert Reinforced concrete slab culvert is economical for shorter spans and the construction method is also easy to complete without much difficulty. If the span is limited to 6m, the term culvert is suitable to be used. If the span is more than 6m, it has to be called as Slab Bridge. The slab generally has to be designed as a one way spanning slab. The components such as deck slab, abutments, wing walls, concrete foundations for abutments, and bedding concrete are common in the construction activities.



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Arch Culvert

Slab Culvert

T – Beam bridge/girder-slab bridge or girder bridge The T-beam bridge is by far the most commonly adopted type in the span range of 10 to 25 meters. The structure is so named because the main longitudinal girders are designed as T-beams integral with part of the deck slab, which is cast monolithically with the girders. Simply supported T beams of greater spans are now-a-days prestressed to reduce deflection and vibration and also to reduce the material. This type of structures can be continuous over some number of piers also. For the longer spans, the longitudinal T beams are laterally supported by cross beams called as diaphragms at designed intervals. Diaphragms offer resistance against the lateral torsional buckling of the longitudinal T-beams.

T- Beam Bridge Hollow box girder bridges Reinforced concrete hollow box girder bridges are economical in the span range of 25 to 30 meters. The closed box shape provides torsion rigidity, and the depth can be varied conveniently along the length as in continuous deck or in balanced cantilever layout. The cross section can consist of a single cell or can be multi-cellular. The extra torsion stiffness of the section makes this form particularly suitable for grade separations, where the alignment is normally curved in plan. The cells can be rectangular or trapezoidal, the latter being used increasingly in pre-stressed concrete elevated highway structures. Reinforced concrete hollow girder bridges are currently not favored, whereas pre-stressing is gaining appreciation.



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A typical cross section of a reinforced concrete hollow box girder super structure suitable for two lane traffic on a National highway for a simply supported clear span of 30 m is shown in the figure annexed. The components of the super structure are, the cantilever portion including the curb, the top slab carrying the roadway, the webs, in this case two exterior webs and one central web, and diaphragms, typically two end diaphragms and three intermediate diaphragms.

Continuous Girder Bridge Rectangular openings are provided in the diaphragms to enable removal of formwork from inside the cells after casting. Detailing of reinforcement should ensure that the edges are duly strengthened. It is desirable to provide one access opening of 750 mm diameter in the soffit slab for each cell near one of the abutments. This opening is for maintenance purposes. Continuous Girder Bridges Continuous girder bridges, not connected monolithically to supports, are suitable when unyielding supports are available. The spans can be equal, but usually the end spans are made about 16 to 20 % smaller than the intermediate spans. The decking can be in the form of slab, T beam or box section. The bending moments and shears at the various sections of the bridges are evaluated by using influence lines. As the bending moment is more in the support portion, a haunch profile or a curved soffit is normally preferred and the section at the support is strengthened with compression reinforcement besides provision of thickened webs and a cross beam or diaphragm. The length of haunch portion is generally 0.2 to 0.25 of the span. All but one of the bearings will be of the expansion type. Continuous girder bridges have the following advantages over simply supported girder bridges.  Less number of bearings over the piers  Reduced width of pier, thus less flow obstruction and less amount of material  Requires less number of expansion joints due to which both the initial cost and maintenance cost become less  Rigidity quality over the bridge is improved  Lesser depth of girder, hence economical  Better architectural appearance  Lesser vibration and deflection Disadvantages:  Not suitable on yielding foundations  Analysis is laborious and time consuming.



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Balanced Cantilever Bridges Unyielding supports are required for continuous construction. But if the soil conditions in the site is poor to bear the load and if supports are expected to be yielding, for medium spans in the range of about 35 to 60 meters, a combination of supported spans, cantilevers (or overhangs) and suspended spans may be adopted as shown schematically in the figure annexed. The bridge with this type of superstructure is known as balanced cantilever bridge. The connection between the suspended span and the edge of the cantilever is called an articulation. The bearings at articulations should be alternately of fixed and expansion types and can be in the form of sliding plates, roller-rocker arrangement or elastomeric pads. The cantilever span is usually about 0.2 to 0.25 of the supported span. The suspended span is designed as a simply supported span with support at the articulations. Generally, while designing the main span, the maximum negative moment at the support would occur when the cantilever and suspended spans are subjected to full live load with no live load on the main span. The maximum positive moment at the mid span would occur with full live load on the main span and no live load on the cantilever or suspended span. Similarly the governing shears shall be obtained using influence lines. Bearings at the pier supports shall be alternately fixed and expansion types. The cross section of a balanced cantilever bridge can be a T beam or hollow box girder type. The soffit at the support can be hauched or parabolic profile.

The advantages of balanced cantilever bridges over simply supported girder bridges are listed below.  Less concrete steel and form work are required for cantilever bridge constructions  The reactions at the piers are vertical and central permitting slender piers  Requires only one set of bearings at every pier  Fewer expansion joints are needed for the full structure, resulting less initial cost and less maintenances cost. Disadvantages:  Requires more skill on the part of designer.  After settlement of supports the super structure will not maintain the same level at all the cross sections for the free traffic flow.



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Rigid frame bridges Rigid frame bridges are structures consisting of a number of parallel girders (or slab instead of girders) which are rigidly connected to the supporting columns or piers. Usually the decking and substructure are cast monolithically. The types of rigid frames normally used in bridge construction are shown in the figures given below. Type (a) is suitable for single span opening as in the case of bridges over railway tracks. Type (b) shows a two span bridge with the base of the column fixed. If the base is hinged, which is a more common condition, the column is tapered downward. This type can also be used for bridges with greater number of spans by adding the required number of intermediate columns. Type (c) offers an aesthetically pleasing structure over restricted access highways as has been used extensively over expressways in USA and Germany.

Rigid frames possess the advantages listed for continuous bridges and have the following additional advantages:  No bearings are needed at the supports  The rigid connections result in more stable supports, than possible with independent piers of comparable dimensions  Due to the slender dimensions, the supporting piers or columns cause least obstruction for the flow of traffic below the bridge Arch Bridges Girder bridges of reinforced concrete will be uneconomical for spans beyond 35 meters. Arch type can be used advantageously in the span range of 35 to 200 meters and has been applied up to 305 meters as in the case of Gladesville Bridge in Sydney, Australia. A strong point in favor of arch bridges is their pleasing appearance and aesthetic elegance. The arch form is best suited to deep gorges with steep rocky banks which furnish efficient natural abutment to receive the heavy thrust exerted by the ribs. In the absence of these natural conditions, the arch usually suffers a disadvantage, because the construction of a suitable abutment is expensive and time consuming. Concrete arch for bridges can be in the form of arch slab or arch ribs. In the case of slab type for short spans, the space between the deck slab or road way and the arch slab (called spandrel) is usually filled with earth and the filling is retained by spandrel walls. Such an arch is called a spandrel filled arch. For spans longer than about 25



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meters, the deck is supported on columns or walls resting on the arch and an arch of this type are known as open spandrel arch.

All arches develop thrust at the supports and the thrust is to be taken by unyielding abutments. This thrust tends to reduce the bending moment at any section of the arch. The aim of the designer will be to maximize this reduction, so that the arch will have only compression in the section. While it is possible to nearly eliminate bending moments due to dead load by choosing the arch axis to coincide with the thrust line for bending moment, live load will cause net bending moments. The arch axis is generally governed by three considerations: (a) span and rise from the road gradient and navigation or traffic clearance below, (b) the economical shape from point of view of saving materials and (c) the beauty of the intrados. The most important parameter is the rise-span ratio, the economical value of which lies between 0.30 and 0.20. A large rise reduces the thrust and leads to thinner arch section. The usual profiles adopted in practice are parabolic, segmental and elliptical. Parabolic arches will be admirably suited in rugged country with exposed solid rock or abutments. In plains and particularly for a spandrel filled arch, a segmental profile may be more satisfactory. Elliptical shape is not much favored, except in cases where clearance requirements need an almost uniform soffit height Arches are designed by trial and error. First the preliminary dimensions are assumed. Influence lines for horizontal thrust and bending moments are constructed using first principles. The resulting stresses are then checked against allowable stresses, and the sections are redesigned, if necessary. In case of steel arch bridges, if it is a trussed one, fabrication and erection pose more difficult problems than a girder bridge. The arch rib can be in the form of a box section, tubular section or a trussed structure. Stone masonry arch bridges are by nature strong and they require very little maintenance. Their inherent reserve strength is evident from the many stone arch bridges existing in service now, carrying many times their originally designed load. Their disadvantage is the slow pace of construction; it takes time to build them, piece by piece; each stone requires dressing and individually matching with the surrounding stones. Hence in affluent countries with high labor costs stone masonry bridges are not in favor. The components of arch bridges are shown in figure annexed. The arch consists of voussoirs rising between the back of the abutment level and the keystone at the crown. The exterior and the interior faces of the arch are called the extrados and intrados, respectively. The space between the extrados and the top of the key stone is called spandrel.



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To serve as a cushion, an earth fill is provided above the extrados up to road or sleeper level, usually for a minimum depth of 900 mm. Spandrel walls are provided along the edges of the arch vault to retain the earth fill. Masonry arches may be of semi-circular, segmental, pointed, semi-elliptical, parabolic or multi-centered in shapes. The rise of an arch should not ordinarily be less than one-third of span. For spans up to 12 m clear, a simple segmental or circular curve may be adopted. For spans over 12 m clear, the shape of the arch axis should be designed such that the axis conforms as nearly as possible to the equilibrium polygon for dead load plus 50 per cent of the live load taken as equivalent uniformly distributed load covering the entire span. Steel Truss bridges Steel truss girder bridges have been used economically in the past over a span range of 100 m to 500 m. A bridge truss derives its economy from its two major structural advantages, (a) the primary forces in the members are axial forces (stress resultants are less), and (b) greater overall depths permissible with its open web construction leads to reduced self-weight when compared to solid web systems. Warren truss is most common for short span bridges, while other types like Pratt truss, N-girder and K-girder trusses are also common. The main components of the truss girder bridges are (a) two main truss girders, (b) floor beams connecting bottom chord joints, (c) stringer beams connected to floor beams, (d) decking or flooring, (e) top tie beams parallel to the floor beams and (f) lateral bracings provided at the top and bottom chord levels to resist horizontal transverse loads. Truss girder bridges can be of different further structural classifications, like cantilever truss bridge, arch truss bridge, suspension bridge with steel truss deck etc.

Omo river truss girder bridge ________________________________________________________________________ Fundamentals of Bridge Design – CEng 552 – Chapter 2


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Plate girder bridge Since the days of early steel bridge construction there has been a marked preference for the plate girder bridge system due to its aesthetic value and easiness of maintenance. Deck type or half through type are most common. A plate girder highway bridge usually consists of the deck slab (normally reinforced concrete) and stringers running longitudinally and resting on transverse floor beams, which in turn rest on the plate girders. In modern highway bridges of moderate and long spans, the concrete deck slab is replaced by a stiffened deck plate over which a thin layer of asphalt concrete wearing course is directly laid. The steel deck plate is stiffened in two orthogonal directions, longitudinally by closed rib system and transversely by the floor beams. Since the stiffness in the two orthogonal directions of the deck is different, it is said to be orthotropic deck. Cross bracings are provided to firmly fix in position the two plate girders or plate girder assembly.

Plate Girder Railway Bridge near Modjo Dry Port Bow String Girder Bridge Bow String Girder Bridge consists of two arches which are tied horizontally at the springing by tie members, so that the reactions at the supports will be only vertical. Thus the structure eliminates the main difficulty of arch supports (i.e. horizontal thrust is eliminated). This type of bridge is normally used in the span range of 30 to 35 m and is applicable in situations where unyielding abutments required for arches are not available and good headroom is required under the bridge adjacent to the abutments. Usually, the girder is supported on pin bearings at one end to permit rotations and roller bearing at the other end to provide expansion.

The decking is usually between and monolithic with the tie members. Footpaths, if provided, are cantilevered outside the ties. The deck slabs are cast monolithic with cross girders, spanning from tie to tie. Each cross girder would be carried directly by a pair of hangers (also called as suspenders). Curb of at least 150 mm height and 300 mm width should be provided to prevent damage to hangers from vehicles. Bracings between the two arches are to be provided, where possible, to cater to the wind load. The bow string girder bridge uses less quantities of materials than an



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alternate design with reinforced concrete girder bridge, but it would require more costly formwork. Hence the overall economy is not quite apparent. However, the enhanced aesthetic appearance of a bow string girder bridge merits attention. Cable stayed bridges General: A cable stayed bridge is a bridge whose deck is suspended by multiple cables that run down to the main girder from one or more towers. The cable stayed bridge is specially suited in the span range of 200 to 900 m. It was developed in Germany in the post war years in an effort to save steel which was then in short supply. Cable stayed bridges are economical over wide range of span lengths and they are aesthetically attractive. Main components of a cable stayed bridge are (a) inclined cables, (b) towers and (c) orthotropic deck. (Orthotropic means having different elastic properties in two mutually perpendicular directions. It evolved from steel deck plate in an effort to reduce the dead weight of highway bridges during the post second world war period. The most developed form consists of deck plate stiffened by a shallow grid work of closely spaced welded ribs, the stiffened plate then acts as the top flange of the stringers) The cables connecting the deck and towers would permit elimination of intermediate piers facilitating a larger width for purposes of navigation.

Abay River Cable Stayed Bridge When the number of stay cables in the main span is between 2 and 6, the spans between the stay supports tend to be large (between 30 m and 60 m) requiring large bending stiffness. The stay forces are large and the anchorages of cables become complicated. On the other hand, the use of multiple stay cables would facilitate smaller distances between points of supports (between 8m and 15m) for the deck girders, resulting in reduced structural depth and facilitating erection by free cantilever method without auxiliary supports. Cables can be arranged in a fan form or in a harp form. Fan type configuration results in minimum axial force in deck girders. The harp form requires larger quantity of steel for the cables induces higher compressive axial forces in the deck and causes bending moments in the tower. Fan shape is superior from structural and economical points of view. Harp configuration of cables also warrants erection of the tower and the deck to progress at the same



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time. The cables may be arranged in one vertical plane, in two vertical planes or in two inclined planes. The single plane system has the advantage that the anchorage at deck level can be accommodated in the traffic median resulting in the least value of required total width of deck. With the two vertical plane system, additional widths are needed to accommodate the towers and deck anchorages. Aesthetically, the single plane system is more attractive as this affords an unobstructed view on one side of the motorist. In the case of a two plane system of cables, a side view of the bridge would give impression of intersection of cables. The two inclined plane system of cables with the cables radiating from the apex of an A-frame tower facilitates the three-dimensional structural performance of the superstructure and reduces the torsional oscillations of the deck due to wind. The deck structure can be of reinforced concrete slab with ribs along the edges, if the width is less than 15 m and the span is relatively small. For width greater than 15 m and spans longer than 500m, the all steel plate deck becomes mandatory in order to reduce the dead loads. Other combinations using pre-stressed concrete decks and composite decks have also been used. Shallow box sections with wind nose at either end have been used successfully for long spans. Single free standing tower may be employed when the cables are in one plane. In this case the piers should be sufficiently wide to accommodate the bearings. For bridges with cables in two planes, the towers can be a free standing pair or a portal frame with a slender bracing. An additional bracing may be introduced below the deck. The height of towers should preferably be in the range of 0.2 to 0.25 Lm (where Lm = the main span). The higher the tower, the smaller will be the quantity of steel required for the cables and the compressive forces. But it is not advantageous to increase the height beyond 0.25 Lm. The steel used for cables have ultimate tensile strength (UTS) of the order of 1600 MPa. High carbon fibre cables having UTS of about 3300 MPa are under development. Carbon fibre cables have negligible corrosion and possess high fatigue resistance. Cables are used either as parallel bundle strands of wire or coiled bundle of strands enclosed in a polyethylene tube. Anchorage of cables at the deck is fixed and has a provision for a neoprene pad damper to damp oscillations. Suspension bridges Suspension bridge is currently the only solution for spans in excess of 900 m. Suspension bridges consist of two large, or main, cables that are hung (suspended) from towers. The main cables of a suspension bridge drape over two towers, with the cable ends buried in enormous concrete blocks known as anchorages. The roadway is suspended from smaller vertical cables that hang down from the main cables. In some cases, diagonal cables run from the towers to the roadway and add rigidity to the structure. The main cables support the weight of the bridge and transfer the load to the anchorages and the towers.



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The world’s longest span bridge at present is the Akashi-Kaikyo bridge (1998) across Ambassador Bridge Akashi strait in Japan which is a suspension bridge with a main span of 1991m. The Brooklyn Bridge, which was the world’s longest suspension bridge at the time of its completion in 1883, crosses the East River in New York City and has a main span of 486 m 31 cm (1,595 ft 6 in). While suspension bridges can span long distances, this design has a serious drawback: It is very flexible, and traffic loading may cause large deflections, or bending, in the bridge roadway. Suspension design is rarely used for railroad bridges, because trains are heavier and can travel faster than highway traffic. Apart from the above drawback of suspension bridges, the collapse of Tacoma Narrows bridge, Washington, due to heavy wind of about 67 kmph, on November 7, 1940 (opened to traffic on July 1, 1940), taught us the need to study the aerodynamic stability of bridges. In an attempt to slenderize the bridge the designer provided shallow plate girders for stiffening the deck. The span to depth ratio was 350, where the earlier bridges of this span range provided around 90. The span to width ratio was 72, as against the normal value of 35. The result was that the bridge was extremely flexible. Also, instead of allowing the wind to pass through, the plate girders caught the wind, to buck and roll. The use of solid plate stiffener, which helped in visual enhancement, rendered it aerodynamically unstable. The bridge was oscillating vertically and earned a little galloping. At last the vertical oscillation turned into violent twisting motion with amplitude reaching 9.2m. Soon the whole deck twisted itself to pieces and fell into the Narrows. As improvement in designing for aerodynamic stability of suspension bridge deck, the objects set were to widen the deck, leaving cantilevered foot paths, fin like, increasing voided depth by providing trussed stiffeners for the deck and as a whole making the deck orthotropic, including diagonal suspenders in addition to the vertical suspending cables and incorporating artificial damping devices against wind action.



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Akashi Kaikyo Bridge - Japan



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CHAPTER 2 TYPES OF BRIDGE SUPERSTRUCTURES.doc

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