ABSTRACT
Precast concrete is a construction product produced by casting concrete in a reusable mold or "for "form" m" whic which h is then then cure cured d in a cont contro roll lled ed envi enviro ronm nmen ent, t, transported to the construction site and lifted into place. In contrast, standard concrete is concrete is poured into site-specific forms and cured on site. Precast stone is distinguished from precast concrete by using a fine aggregate aggregate in in the mixture, so the final product approaches the appearance of naturally occurring rock or stone. Ancient oman builders made use use of concrete and soon poured the material into moul moulds ds to build build thei theirr comp comple lex x netw networ ork k of a!ueducts a!ueducts,, culverts culverts,, and and tunn tunnel els. s. odern odern uses uses for pre-ca pre-cast st techno technology logy includ include e a variet variety y of archit architect ectura urall and and structural applications featuring parts of or an entire building system. In
the
modern
world, ld,
pre-cast
panelled
build ildings
were
pioneered
in #iverpool #iverpool,, $ngland $ngland in %&'(. A process was invented by city engineer )ohn Alexander *rodie *rodie,, whose inventive genius also had him inventing the football goal net. +he tram stables at alton in #iverpool followed in %&'. +he idea was not taken up extensively in *ritain. owever, it was adopted all over the world, particularly in $astern $urope/01 and 2candinavia 2candinavia.. *y producing precast concrete in a controlled environment 3typically referred to as a precast plant4, the precast concrete is afforded the opportunity to properly cure and be closely monitored by plant employees. 5tili6ing a Precast 7oncrete system system offers offers many potential potential advantage advantages s over site casting casting of concrete. concrete. +he production process for Precast 7oncrete is performed on ground level, which helps with safety throughout a pro8ect. +here is a greater control of the !uality of materials and workmanship in a precast plant rather than on a construction site. 9inanc 9inancial ially ly,, the forms forms used used in a preca precast st plant plant may be reused reused hundr hundreds eds to thousa thousands nds of times times before before they have have to be replac replaced, ed, which allows allows cost cost of formwork per unit to be lower l ower than for site-cast production.
any states across the 5nited 2tates re!uire a precast plant to be certified by either the Architectural Precast Association 3APA4, :ational Precast 7oncrete Association 3:P7A4 3:P7A4 or Precast Prestressed 7oncrete Institute 3P7I4 for a precast precast producer to supply their product to a construction site sponsored by 2tate and 9ederal ;<+s. ;<+s. +her +here e are are many many diff differ eren entt type types s of prec precas astt conc concre rete te form formin ing g syst system ems s for for arch archit itec ectu tura rall
appl applic icat atio ions ns,, diff differ erin ing g in si6e si6e,, func functio tion, n, and and cost cost.. Prec Precas astt
architectural panels are also used to clad clad all all or part of a building facade facade free freestanding standing walls used for landscap landscaping, ing, soundproofing soundproofing,, and security security walls walls,, and some can be Prestressed concrete structural structural elements. elements. 2tormwate 2tormwaterr drainage, drainage, water and sewage pipes, and tunnels make use of precast concrete units.
any states across the 5nited 2tates re!uire a precast plant to be certified by either the Architectural Precast Association 3APA4, :ational Precast 7oncrete Association 3:P7A4 3:P7A4 or Precast Prestressed 7oncrete Institute 3P7I4 for a precast precast producer to supply their product to a construction site sponsored by 2tate and 9ederal ;<+s. ;<+s. +her +here e are are many many diff differ eren entt type types s of prec precas astt conc concre rete te form formin ing g syst system ems s for for arch archit itec ectu tura rall
appl applic icat atio ions ns,, diff differ erin ing g in si6e si6e,, func functio tion, n, and and cost cost.. Prec Precas astt
architectural panels are also used to clad clad all all or part of a building facade facade free freestanding standing walls used for landscap landscaping, ing, soundproofing soundproofing,, and security security walls walls,, and some can be Prestressed concrete structural structural elements. elements. 2tormwate 2tormwaterr drainage, drainage, water and sewage pipes, and tunnels make use of precast concrete units.
CONTENTS
Abstract Introduction Advantage Of Precast Concrete Precast Segment Manufacturing Site Selection And Preparation Casting Cell Construction Concrete Placing And Curing Storage And Finishing Loading And Transporting. The Construction Process Of Segmental ridges !evelopment Of Prestressed Segmental Segmental ridges !egree Of Prestressing Concrete ridge "rection Techni#ues Techni#ues
Cantilevering Method Precast Construction Cast$In$Place Construction alanced Cantilever Construction Progressive Placement Method The Linn Cove %iaduct Concluding The Cantilevering Process Form Travelers Launching &irders Launching &irder Slightl' Longer Than One Span Launching &irder Slightl' Longer Than T(o Spans Incremental Launching False(or) Stationar' False(or Span$'$Span "rection Consideration Of Construction Loads And Stresses T'pes Of Construction Loads And Influences Span Configuration And T'pical Sections Safet' And *ealth At Pre$Cast +ards Case Stud' Construction Of The Precast Segmental Structures For Sutong ridge
INTRODUCTION
+he popularity of precast concrete segmental bridge construction has grown worldwide in the last few decades. +hese types of bridges offer many benefits to
owners
like
reduced
costs,
reduced
construction
environmental impacts, and reduced maintenance of traffic.
time,
reduced
+hese benefits
can be achieved while utili6ing local labor and materials, better means of !uality control, and with minimum re!uirements for future maintenance. +hey also offer additional structural advantages of durability, fire resistance, deflection control, better rider serviceability, insensitivity to fatigue, and other redundancies. +hese bridges can accommodate highways, railways, and rapid transit, in both urban and rural environments. +hey can be straight or curved alignments, and can provide long spans for difficult obstructions and terrain. 2egmental *ridges are varied in types such as= incrementally launched, long line castings, cable stayed, precast segmental progressive placement, arches, cast-in-place segmental, short line match cast precast segmental construction, etc.
+his course will consider Precast 2egmental substructures and
superstructures utili6ing short line match casting of precast elements and both span by span, and balanced cantilever methods of erection. +he course will be broken down into four basic sections= Precast anufacturing, 2ubstructure $rection, 2uperstructure $rection > 2pan by 2pan ethod, and 2uperstructure
$rection > *alanced 7antilever ethod. $ach section will be further broken down by= set up and staging, construction, stressing and grouting, and completion.
A;?A:+A@$ <9 P$7A2+ 7<:7$+$
7olumn-9ree#ong 2pans ith fewer columns and more usable floor space, precast, prestressed concrete provides greater freedom for space utili6ation. 7onserves $nergy Prestressed concrete components can improve the thermal storage potential of a building. It effectively conserves energy re!uired for heating and cooling. aintenance 9ree Precast concrete does not re!uire painting and is free from corrosion. Its durability extends building life. esists 9ire ;urability and fire resistance mean low insurance premiums and greater personnel safety. +hose who investigate life cycle costing will appreciate the precast concretes excellent fire resistance characteristics.
apid 7onstruction Precast concrete construction gets the 8ob done sooner. +he manufacturing of prestressed members and site preparation can proceed simultaneously. $arly occupancy provides obvious benefits to the client. ?ersatility of ;esign Precast concrete buildings are not only functional but beautiful as well. :umerous panel configuration design possibilities are available. Precast concrete is 2A9$ $verybody knows that concrete does not burnB :ot only is the structural stability maintained for longer periods, but concrete construction prevents the spread of the fire from one building to another. It is sufficiently strong to resist impacts, blasts and natural catastrophes like earth!uakes, tornadoes and floods. Precast concrete is ?$2A+I#$ 9actory
production
allows
a
wide
choice
of surface
finishing,
colour
range and special shapes. Precast concrete has another advantage= its mouldability which entails designers to copy classical details like keystones and capitals or match the finish of materials like weathered stones. +he precast concrete industry can source a wide range of aggregates locally and offer a tremendous variety of colours and visual effects.
Precast concrete is $A#+C Indoor air !uality is a concern for all of us. Precast concrete is stable throughout its life and does not need chemical treatment to protect it against rot and insect attack= this means that there are no emissions in the internal environment. Precast concrete is
Advanced technologies used in the precasting plants create an improved !uality product 3i.e.
reduced
tolerances,
thinner
sections,
engineered solutions4
compared with cast-on-site concrete. Additionally this !uality can be checked before a unit is inserted into the structure or site workB Precast concrete is DURABLE 7oncrete lasts for years. $gyptian and 7hinese people used an ancient form of concrete for buildings and structures that still exist today. 7oncrete is used where the structural stability has to be maintained for long periods. $ffective design detailing helps to lengthen the life of a concrete buildingD precast manufacturers can offer guidance on designing for durability. Precast concrete is $7<#<@I7A# ade of natural raw materials 3stones, gravels, sand, cement4, locally available almost everywhere and in an enormous !uantity, precast concrete minimises the whole life cycle impact on the environment when compared with other construction materials. Precast concrete units can entirely be re-used or recycled 3almost %''E of a concrete building can be recycled, no matter how heavily reinforced4. Precast concrete is FAST +he top floor of a skyscraper can be cast in the factory when the foundations have not yet started. *ut the pro8ect re!uirements of a modern construction prefer a 8ust-in-time deliveryB
2ustainable
means a
F0-winG
situation for
the
three
Pillars
of
our
society= People,Profit and the Planet. If only one of this elements is FnegativeG, the solution canHt be considered sustainableB
Precast Segment Manufacturing
+he basic building blocks for the Precast 2egmental *ridge are the Precast 7oncrete 2egment $lements > superstructure or substructure. +here are different means of casting these segments. +his course will only consider short line match-casting. +he production of these segments is critical to the success of the pro8ect. +he segments are a ma8or controlling factor in the !uality, schedule, and profitability of the bridge and therefore re!uire a well prepared plan to fabricate, store, and transport. +he next five sections= 2ite 2election and Preparation, 7asting 7ell 7onstruction, 7oncrete Placing and 7uring, 2torage and 9inishing, and #oading and +ransporting will outline the fundaments for a manufacturing plan.
Site Selection an Pre!aration
%.+here are many decisions to be made when considering the segment casting site. Probably the most significant is whether to choose an already functioning pre-cast facility or to set-up and run your own. Although there will usually be several local pre- casters, their facilities and experience history may not be in manufacturing segmental bridge elements. +he site preparation details should be similar whether outsourcing or self-performing, so what factors influence the
choice 7ertainly past history of similar structures would be an important factor. +hese sites would have the speciali6ed e!uipment, trained personnel, and permitting needs for a !uick start up and timely production.
2econdly, distance and
transportation considerations 3local to highways, railways, or waterways4 would be evaluated. #astly, budget issues including tax-implications will affect the decision process 3the most experienced supplier may not be cost effective4.
J
K.
Availability of 7oncrete
3transit-mix delivered from an existing supplier or self-production from a mobile batch plant4, distance to the erection site and available transportation methods for delivery 3railways, waterways, and highways4, Permitting and Loning, Ade!uate 2torage Area, $nvironmental and @eotechnical ;esign 7riteria, Proximity to a 2killed orkforce, etcM
Casting Cell Construction
+he site should be arranged in an efficient organi6ed manner for producing the precast segments. +he number of casting cells constructed is directly related to the scheduling needs of the pro8ect.
9ast paced schedules will need
additional cells to achieve production re!uirements. $ach casting cell re!uires a si6able investment in time, property, and moneyD this must be balanced against the schedule to determine the most efficient pro8ect course.
At a
minimum, a bridge will re!uire cells for the pier columns 3if precast substructures are being used4, cells for typical superstructure segments and a cell for the pier and expansion segments 3span-by-span method of erection4, and a cell for variable depth superstructure segments 3balanced cantilever method of erection4. An engineer should design the casting cells and should considerD geotechnical data for foundation type 3each cell will need to support three segments plus formwork and e!uipment4, reinforced concrete design for the base slab of the cell, formwork design 3falsework, framing, and concrete forms4, walkways and access scaffolds, shelters, and miscellaneous electricalNmechanical. a8or components of a casting cell= *ase slab and foundation, forming system, rebar 8ig, survey towers and sites, and shelters.
2peciali6ed and general
e!uipment= 2team generators, chillers, straddle lifts, man lifts, gantries, conveyors,
forklifts,
+
cranes,
welders,
generators,
winches,
survey
e!uipment, etc. 2ome miscellaneous materials would include= dunnage, grout, form release, curing compound, bond breakers, epoxy, etc. Concrete Placing an Curing
9or this course the precast segments will be short-line match-cast.
+his
means the segments are cast se!uentially in a single stationary form system
where subse!uent segments are cast against their predecessor creating a matching pair. +he exact bridge geometry is established between the matched pairs such that the segment is uni!ue to a singular place in the structure. +he controlled setting of the precast yard allows production similar to an assembly line environment with the goal of completing a segment each day per cell.
+he first station in the assembly line is the ebar )ig. A plywood or steel replica of the form machine is erected at the casting cell for pre-tying the rebar cage. +he rebar is delivered and tied in the 8ig. $mbed items such as post-tensioning ducts and anchors are also rough installed.
+he cage is
lifted out of the 8ig to be placed in the forms for concrete placement. 2tandees and chairs are pre-attached to the cage to insure proper clearance and alignment when set in the form.
+he second station is the casting form.
+he previous dayHs production is
asbuilt by the survey crews to ensure the geometry was maintained while the concrete set.
7oncrete cylinders are broke to determine the concrete
strength and if acceptable, the formwork is loweredD the segment is rolled out of the forms and then set in the match-cast position for the next placement. +he forms are tightened around the match-cast segment, form oil is applied to the forms and a bond breaker is applied to the match surface. +he rebar cage is lowered into the forms and the core is slid into place. After posttensioning and embeds are secured, final survey and !uality control checks are performed, and the segment is ready for concrete. +he third station is placing, finishing, and curing the segment. *efore placing the concrete, !uality control tests must be performed both at the plant for production and at the placement. Air content, temperature, and slump testing, plus the casting of concrete cylinders for compressive strength testing are the minimum tests needed to ensure a !uality cast. +he concrete for the bottom slab is tremied through the core, then a stiff mix is placed down the walls 3care must be made to consolidate the mix without it FsloughingG down and out of the form to the bottom slab4. #astly, the top deck is placed 3care to consolidate around post-tensioning ducts and anchors4. +he deck is usually finished with a
roller screed and hand tools 3usually a post-erection deck treatment is applied for rideability, if so applied, the surface can be left somewhat rough4 and geometry control markers are set. +he segment is then cured overnight, steam and heat curing may be necessary to accelerate the initial strengthening of the concrete. +he procedure is repeated with the match-cast segment rolled out to storage, the casting rolled out to be the new match-cast, and the cell prepared for a new casting. Precast substructure piers are cast in a similar manner only the cells are oriented vertically.
Storage an Finis"ing
After a dayHs production, the previous dayHs match cast segment is r eady to be finished and set for storage. ;epending on the design, some segments can be lifted and placed in storage prior to any post-tensioning. +his will be a factor of strength gained during the initial cure of the segment and the dimensional properties of the bridge.
An organi6ed storage plan must be formulated early in the casting process.
:ot only should the location of each segment be established in an orderly manner for storage, but also for documenting the various stages of completion and acceptance, as well as, availability to deliver the segments to the bridge site when needed.
+ime and efficiency losses caused by searching for
segments will add up !uickly especially if multiple movements are needed for access. hile in storage any pointing, patching, and architectural finishes can be applied 3care must be taken when any repairs are made to the match-cast face to ensure the fit is not 8eopardi6ed4. +he post-tensioning rods and strands are stressed, anchored, and grouted.
+he Anchorages are sealed and poured
back with like concrete. Any bond- breaking agents applied during casting must be power-washed off and the match face must be clean. :ote= +he segment should be stored on stabili6ed grade using dunnage placed in a three point pattern to ensure the segment will not rack and lose shape.
Loaing an Trans!orting.
;epending on the location of the storage area to the bridge erection site, the method of transportation will differ. hether it is by trucks 3on and off road4, rail, or barge, several factors apply to all=
hauling restrictions > time and
weight, permits, environmental and noise ordinances, and distance. +he most direct routes might not be the most cost effective or available. A necessary decision will also include whether to purchase, rent, or subcontract the loading and transporting. +he lifting and handling of these large castings is speciali6ed work and any errors can be catastrophic therefore, the services of professionally experienced subcontractors are advised. :ote= the segments must be transported to the bridge for erection in the same relation as they were cast.
THE
CONSTRUCTION
PROCESS
OF
SEGMENTAL BRIDGES
The follo(ing Chapter , presents the important techni#ues for erection of concrete segmental bridges. Their characteristics are outlined so that understanding of the specific nature of each of these methods can be achieved. Apart from that this chapter deals (ith the most important issue of construction loads b' distinguishing the various t'pes of construction loads and sho(ing their relation to the erection method used for a specific pro-ect.
DEVELOPMENT BRIDGES
OF
PRESTRESSED
SEGMENTAL
Application of prestressed concrete for bridge construction (as developed b' French engineer "ugne Fre'ssinet/ as described in Section 0.1.2/ and has spread (idel' thereafter. Onl' prestressing made the slender/ long$span concrete bridges of toda' possible. The basic principle of prestressing is to induce an initial compressive force in the concrete that (ill balance tensile stresses that occur in the member under service conditions before an' tensile stresses occur in the concrete and cause crac)ing. Menn 31445/ p1026 names the t(o methods of inducing these stresses in the structure7 ' imposed forces from reinforcing steel that is prestressed to a certain degree8 ' imposed 9artificial displacements of the supports:/ e.g. bearings.
The second method according to Menn 314456 is much less used because of high losses of the prestressing force due to concrete creep and shrin)age. Prestressing tendons that are used for the first method consist of high$strength steel and are fabricated as (ires/ strands/ or bars 3;ilson and
beam on several supports/ most tension (ill occur in the lo(er fibers of the cross$section around midspan and in the upper fibers above intermediate supports. It is therefore most useful to place tendons in the locations (here tensile stresses (ill occur in the structure under service. This thought naturall' leads to the idea of implementing longitudinal tendons in the beam that are not simpl' straight but follo( a curve from the top above supports to the bottom at midspan and bac) to the ne>t support. In alanced Cantilever Construction the top cables in reaching out from the cantilever base to support the cantilever dead load are called cantilever beam cables8 the bottom cables in the middle of the span are called integration cables 3Mathivat 14=?6. Prestressed concrete/ compared (ith normal reinforced concrete has a higher degree of sophistication and causes higher cost for labor and for the prestressing tendons8 on the other hand it saves cost through more economical use of material. Onl' prestressing ma)es long and slender concrete spans possible at all.
Degree of Prestressing
Menn 314456 mentions that choice of the best prestressing profile for a certain pro-ect is not predetermined but is a tas) for the bridge designer. *e further gives an overvie( of the degree of prestressing. Full prestressing is supposed to (ithstand all tensile stresses under service conditions.
ceed a specified permissible value: 3Menn 1445/ p10@6/ so$called limited prestressing is performed. The last and most common method is partial prestressed/ (hich does not specificall' limit the concrete tensile stresses. Still/ calculation of 9behavior at ultimate limit state and under service conditions: 3Menn 1445/ p10@6 must be calculated/ also ta)ing into account the normal reinforcement. The purpose of the normal mild reinforcement is the control and distribution of crac)ing. ecause of the high prestressing force/ less conventional reinforcement is needed in the concrete/ and members can be thinner and lighter/ leading to more economical structures. The reduced susceptibilit' to crac)ing gives prestressed concrete
higher durabilit'. Some factors effectivel' contribute to initial and long$term reduction of the prestressing force. Immediate losses of prestress/ also called initial losses/ occur once the prestressing force is applied/ after the concrete has been placed and cured. Loss of prestress needs to be anticipated during design. Long$term losses in concrete depend on its design mi>ture/ curing/ the environmental climate/ and the member geometr'. Te>tboo)s give information on the reasons for prestress losses and provide man' formulas to calculate their effect. The follo(ing Table ,$1 based on ar)er and Puc)ett 3144@/ pp,$,226 summariBes these effects7 Table 4!" Infl#en$es Ca#sing Loss of Prestressing For$e Initial loss of %restress Slippage of strands in the anchorages "lastic shortening of concrete member Friction bet(een tendon and duct interior
Longter& loss of %restress ela>ation of steel strands 3loss of stress under constant strain6 Creep of concrete member 3plastic deformation under constant Shrin)age of concrete member 3volume change due to evaporation6
PreTensioning
Prestressing basicall' can be carried out as pre$tensioning and post$tensioning/ referring to the time (hen the prestressing force is imposed (ith respect to casting. In pre$tensioning the tendons are anchored to e.g. a stiff frame around the casting bed and are prestressed before the concrete is placed.
PostTensioning
Post$tensioning denotes the method of stressing the tendons onl' after the concrete has reached a specified strength. To allo( for the necessar' movement of the tendons inside the concrete the' are installed in tendon ducts that are made from steel or pol'eth'lene. The ducts need to be fi>ed to the normal
reinforcement to prevent misalignment during casting. After post$tensioning the ducts are filled (ith cement grout under pressure for and protection against corrosion of the tendons. &routing the ducts (ill introduce bond bet(een the steel and the surrounding grout. Dnbonded post$tensioning is less common. %er' similar to prestressing tendons are the techni#ues used for protection of sta' cables of cable$sta'ed bridges against corrosion/ as described e.g. b' Funahashi 31446. T(o different (a's of construction e>ist for post$tensioning. The prestressing tendons can be located either inside the concrete or outside of it. ">ternal post$ tensioning has the advantage of eas' accessibilit' for inspection/ maintenance (or)s and replacement. ;evertheless problems (ith corrosion protection are the reason for use of interior post$tensioning in most pro-ects. Post$tensioned tendons need special anchorages that are cast into the concrete structure. Anchorages have the shape of cones that are sitting on the end of the duct for better accessibilit' to single tendon strands (ith the prestressing -ac). Anchorages are mostl' surrounded b' spiral reinforcement/ (hich serves to distribute the compressive stresses into the concrete member. Small (edges around each strand or nuts 3Menn 14456 fi> the strands to the front plate of the anchorage. Special anchor bloc)s/ so$called blisters are cast into the structure to provide enough space for the anchorages/ e.g. on the inside of bo> girder segments of the second generation 3Podoln' and Muller 14=06. Previousl'/ tendon anchorages (ere also found in the -oint faces/ (here problems (ith accessibilit' occurred. Te>tboo)s on prestressed concrete structures provide more information on the la'out and calculation of prestressing s'stems.
CONCRETE BRIDGE ERECTION TEC'NI()ES Concrete segmental bridges have alread' been introduced in Section ?.2.1. The
follo(ing sections (ill present the important methods that are used in
erecting concrete segmental bridges no(ada's and the e#uipment emplo'ed. Special focus is put on constructabilit' issues/ pertaining to characteristics and re#uirements/ advantages/ and disadvantages of each method to prepare for the
case stud' that is presented in Chapter .
Cantile*ering Met+o,
efore used in construction of concrete bridges/ the cantilevering method had alread' been used in Asia for (ooden structures of earliest times/ as Podoln' and Muller 314=06 report. Amongst the ma-or steel structures that (ere erected (ith the cantilevering method are the Firth ail ridge and the Euebec ridge that are presented in Section 0.1.. "rection of concrete bridges (ith the cantilevering principle led to development of specialiBed se#uences that are discussed further belo(. As alread' introduced in Section ?.2.0/ cantilevering for concrete segmental bridges is a construction method (here segments/ either precast or cast$in$place/ are assembled and stressed together subse#uentl' li)e a chain to form the self$ supporting superstructure. Prestressing cables located in the upper part of the segment cross$section support the cantilever. In the variant of the progressive placement method sta' cables are often used to support the cantilever prior to closure of the span. Time$dependent material behavior of the segments under successive load steps re#uires comprehensive calculations for all construction stages. "ver' segment (ill develop strength (ith increasing age of the concrete. &overning for the structural behavior of the cantilever is that ever' segment carries and transfers loads from all follo(ing segments and construction loads until closure of the span. From these ver' basic facts in con-unction (ith geometr' and e>pected loads on the structure the calculation of moments and local stresses/ as (ell as calculation of the deflections that the' cause is possible. OptimiBation of geometr'/ prestressing/ and camber are then performed. !epending on the specific segment configuration and erection se#uence chosen for the cantilevering method the cantilever ma' never be e>actl' balanced so that the superstructure needs to be balanced to ensure stabilit'. It is possible to fi> the supports at the piers of cantilevering superstructures and
install vertical prestressing tendons. Furthermore it is ver' common to ma)e use of an additional temporar' pier (ith vertical prestressing that is located close to the permanent one 3Casas 144@6. This pier helps (ithstanding overturning moments from unbalanced load cases on the bridge superstructure. Several advantages have contributed to the success of the cantilevering method. Certainl' the most important one is that no false(or) or centering is re#uired/ leaving traffic under the spans (idel' unobstructed during construction. Access from the ground is onl' necessar' for construction of the piers and abutments and in preparation for the start of cantilevering/ (hich starts from these locations. Onl' relativel' little form(or) is re#uired due to the segmental nature of the superstructure. Cantilevering is a ver' feasible method if the bridge spans are too high above ground for e.g. economical use of false(or)/ and if the terrain under the spans is other(ise inaccessible or unfeasible/ being e.g. a deep gorge (ith danger of flood events. "speciall' in these cases rapid construction can be achieved (ith cantilevering. Fletcher 314=,/ p1?6 notes that especiall' in cantilevering 9complete calculations are re#uired for the construction stagesG and these are complicated as man' stressing effects are time$ dependent.: In addition to this/ the influence of step(ise construction needs to be considered. *o(ever/ the statical s'stem that needs to be anal'Bed is rather simple and in case of the cantilever prior to closure at midspan even staticall' determinate.
Precast Construction
Precast construction means that bridge members or segments are prefabricated at a location different that the site/ transported to the site/ and installed there. Mathivat 314=?6 gives the ma>imum economical span of bridges built in precast segment as about 15 m/ since cost for the placement e#uipment increase considerabl' the longer the spans are. Construction (ith precast segments has several advantages in comparison (ith cast$in$place segmental bridges. Casting of the segments can be performed under controlled/ plant$li)e conditions at
the precasting 'ard. This industrialiBed process allo(s eas' #ualit' control of segments prior to placement in the superstructure and saves mone' through reuse of the precasting form(or). Surface finishing (or)s/ such as te>turing/ sandblasting/ painting/ and coating can be performed on the ground level (ithout scaffolding (hen the segments are still accessible from all sides prior to installation in the superstructure. Another ma-or advantage mentioned b' Mathivat 314=?/ p0106 is that the complete casting of the superstructure can be removed from the critical path of the overall construction schedule/ since superstructure 9segments can be precast during construction of the substructure.: Assembl' of the
bridge
superstructure ta)es much less time than cast$in$place construction/ as precast segments do not need to cure on site before being prestressed together. Through the earl' casting of segments material properties are also influenced positivel'. As segments are usuall' stored at the precasting 'ard or on site for a (hile the concrete (ill have gained more strength until installation than cast$in$ place elements have (hen being loaded. The time$dependent effects of concrete shrin)age and creep (ill occur (ith reduced e>tent because of the increase age of the concrete segments 3Mathivat 14=?6 and (ill cause smaller deflections of the superstructure than (ith cast$in$place construction. *o(ever/
cost
for
the
precasting
'ard/
storage/
transportation/
and
installation of precast segments needs to be evaluated in comparison (ith cost for the form travelers for cast$in$place construction to achieve an economical solution. The precasting 'ard re#uires investment in e#uipment. Ad-ustable form(or) to form the bridge geometr' and alignment needs to be installed. Lifting e#uipment is also re#uired to put the segments into the storage area and later load them on truc) to be hauled to the construction site. It is common practice to use the match$cast method to achieve high accurac' in segment prefabrication. Match$casting means that the segments are cast in the form(or) bet(een a 9bul)head at one end and a previousl' cast segment at the other: 3Levintov 144/ p,26. Segment -oint faces need to be clean of an' dirt for
match$casting. Levintov 31446 distinguishes concrete segment prefabrication into short$line casting and long$ line casting. Short$line casting (ould comprise form(or) of the length of onl' one segment8 (ith the previousl' cast segment being moved into position for match$casting on a mobile carriage. Short$line casting can be carried out in the horiBontal position or (ith the segments tilted facing up(ard 3Podoln' and Muller 14=06/ ho(ever/ the normal horiBontal position facilitates match$ casting. The overall bridge alignment re#uires careful ad-ustment of the form(or) prior to each concrete placement. Short$line casting does not ta)e much (or)space. Long$line casting on the other hand means erection of form(or) for about a complete bridge span. According to Levintov 31446 the form(or) can be erected stationar' for the superstructure soffit onl'/ (ith smaller movable forms for (eb sides and interior form(or). This form(or) (ill be cheaper than the fle>ibl' ad-ustable form(or) for short$line casting/ but (ill re#uire much more (or)space. Levintov cautions that the long$line casting is feasible for straight superstructures are
or
superstructures
(ith
constant
curvature.
Segments
match$cast progressivel' on the long$line form(or) b' step$b'$step
advancement of the movable form(or) units and a movable bul)head. Phipps and Spruill 314456 describe the precasting c'cle that (as used in construction of the ilo>i Interstate I$115 viaduct. According to them/ the freshl' cast segments (ere steam cured in a movable shed covering the casting bed of the short$line form(or). The pretensioning strands (ere released b' cutting them/ #ualit' control and testing of concrete samples (as performed/ and internal form(or) units (ere removed from the ne( segment. After lifting the previousl' cast segment from its position for match$casting into the storage area/ the ne( segment (as rolled out of the form(or). It (as positioned for match$casting according to the re#uired overall alignment. Cleaning of the -oint face and the bul)head (as done prior to casting the ne>t segment. einforcement bars (ere preassembled in reinforcement cages to speed up placement. Pre$tensioning strands (ere used in the bo> girder segment/ being stressed prior to concrete placement. After concrete placement and
consolidation (ith vibrators the segment (as screeded and given a surface finish before the curing shed (as set up over the casting bed. ' agent is usuall' applied to the -oint faces shortl' before putting a segment into its location in the superstructure. Hoints are usuall' onl' a fe( millimeters thin. Podoln' and Muller 314=06 e>plain the functions of the epo>' agent that is applied to the -oint faces (hen placing precast segments. !uring segment placement the epo>' serves to lubricate the -oint faces/ (hich are cleaned b' sandblasting and 9compensate for minor imperfections in the match$cast surfaces: 3Podoln' and Muller 14=0/ p,=6. In the finished structure the hardened epo>' seals the -oints against moisture and thus additionall' protects the tendons in their ducts. Furthermore/ the epo>' is able to transmit compressive forces and shear forces. Information on mi>ing/ handling/ and properties of the t(o main ingredients/ the epo>' resin and the hardener/ is provided b' Podoln' and Muller 314=06. Interestingl'/ the epo>' agent can reach a higher final strength than the concrete itself. In addition to the epo>' transmitting shear forces bet(een segments the -oint faces are given a special shaping to transmit shear. So$called shear )e's are cast into the -oint faces to loc) the segments together. The' transmit shear forces and also help in e>act alignment of the segments during assembl'. Segments of the so$called second generation facilitate man' smaller shear )e's that are located not onl' in the bo> girder (ebs/ but also in top and bottom flanges 3Podoln' and Muller 14=06.
Cast-In-Place Construction
Podoln' and Muller 314=06 provide an e>ample for a t'pical casting c'cle. As outlined in Section ?.2.?.1/ an' previousl' cast segment needs to have developed at least the
specified strength to be prestressed to previous elements and support the subse#uent one. After finishing all (or) on a segment the form traveler is detached from the previous position and moved for(ards on rails that are mounted on the bridge superstructure. In order to remain balanced during advancement the form traveler ma' be e#uipped (ith a counter(eight. Dpon arrival at the ne( position it is ad-usted and anchored to the e>isting superstructure at its rear to be able to (ithstand overturning moments that (ill occur from the (eight of ne( concrete. The e>ternal form(or) is cleaned and aligned to the re#uired geometr' of the ne>t segment/ also incorporating the desired camber. girder ma' re#uire that the bottom slab is cast before internal form(or) for (ebs and top slab is advanced and aligned. After curing sufficientl' for strength and durabilit'/ the tendons in the ne(l' cast concrete segment can be prestressed. Finall'/ the casting c'cle starts all over again to cast the ne>t segment. Concrete placement can be carried out b' various means/ e.g. (ith buc)ets that are hoisted b' crane/ or b' pumping. ture occurs/ and that proper consolidation (ill be achieved. The most common method is to vibrate the concrete in the form(or) b' means of internal or e>ternal vibrating devices. Most important for the #ualit' of the concrete is curing to achieve strength and durabilit'. Dpon gaining enough strength/ the tendons in the ne(l' cast segment (ill be stressed to some degree and the c'cle starts all over again. ased on
Mathivat 314=?/ p0516 an overvie( of casting steps and t'pical values for their duration is be given in Table ,$0. It should be noted that the se#uence of steps given in this table is onl' a generic e>ample and (ould be bro)en do(n into more steps for planning an actual construction pro-ect7 Table 4-." T/%i$al D#ration of Casting Ste%s D#ratio 1 da'
0 da's 1 da' ? da's
A$ti*ities Post$tensioning tendons in previous segment Stripping form(or) Advancing form traveler Placing reinforcement/ ducts/ and tendons Placing concrete for bottom slab/ (ebs/ and top Curing concrete 3including Sunda'6
Mathivat 314=?6 also gives information on means of accelerating this process. Dse of special form travelers 9(ith lateral main beams or self$supporting carriages: 3Mathivat 14=?/ pp051f6 (ill leave the bottom slab (idel' unobstructed and ma)e construction easier. Secondl'/ 9increasing the length of segments: can be considered during the design phase (hile )eeping in mind the increasing (eight and cost of bigger form travelers. Step(ise construction of the bo> girder is also possible/ (ith relativel' simple form(or) for the top slab follo(ing a fe( segments behind the main form traveler. ample of a segmental bridge is given for combination of cast$in$ place and precast segment sections. girder of the rotonne ridge in France (ere precast and placed into the form travelers/ (hich (ere used to fabricate the remaining cast$in$place parts of the cross$section 3Mathivat 14=?6.
Balanced Cantilever Construction
alanced cantilever construction denotes building a bridge superstructure from both sides of the pier table in a scales$li)e fashion. This erection method is also )no(n under the name free cantilever construction 3Podoln' and Muller 14=06. Fletcher 314=,6 gives information that the pier table element/ serving as a base from (hich cantilevering is begun/ is usuall' bet(een 2 and 10 m long. In order to balance the (eight of both arms of the cantilever superstructure
the segments (ill be about e#uall' placed at both ends. Actual placement of ne( segments (ill hardl' proceed e>actl' at the same times as Mathivat 314=?6 e>presses. Therefore the pier can undergo overturning bending moments and needs to be designed accordingl'. Temporar' to(ers (ith vertical prestressing or counter(eights can provide additional support. Figure ,$1 schematicall' sho(s a t'pical construction stage in alanced Cantilever Construction.
alanced cantilevering can be carried out (ith cast$in$place or precast segments. For cast$in$ place balanced cantilevering a set of t(o form travelers is re#uired/ one for each arm of the cantilever. For multi$span bridges the form travelers can be dismantled after finishing cantilevering from one pier and can be set up for ne( use on the ne>t cantilever. In case of a bridge (ith variable bo> girder depth the pier table segment (ill be the most massive segment of the superstructure. This segment needs to be constructed prior to cantilevering to provide a (or)ing platform from (hich the t(o form travelers can start. It also includes diaphragms that facilitate the flo( of forces from the cantilever arms into the piers. ecause of siBe/ geometr'/ and construction separate from the rest of the superstructure the pier table segment (ill ta)e a considerable amount of time to construct. It can be put into place either (ith large precast segments or as cast$in$place (ith form(or) mounted on the pier shaft. An interesting pier design specificall' feasible for cantilevering is mentioned b' Fletcher 314=,6/ (ho points out that a pier consisting of transverse t(in (alls is
advantageous as it provides stabilit' for cantilevering but allo(s horiBontal movement of the superstructure from thermal elongation through fle>ing of the (all panels. Progressive Placement Method
The progressive placement method/ in comparison (ith the balanced cantilevering method/ is a one$directional process as sho(n in Figure ,$0. All cantilever segments are subse#uentl' placed at the tip of a cantilever that is built across all spans. oth cast$in$place and precast segmental construction can be used. Often sta' cables from the tip of a temporar' to(er on the superstructure support the cantilever. perienced. In comparison (ith incremental launching and balanced cantilevering/ a simpler flo( of forces ta)es place bet(een superstructure and the piers. ;o horiBontal forces are introduced in the piers and no unbalanced bending moments have to be (ithstood b' the piers. It is therefore possible to immediatel' install the permanent bearings 3Mathivat 14=?6.
Some disadvantages of the progressive placement method need to be dealt (ith during design and construction. As construction onl' progresses at the tip of one cantilever/ progress is slo(er than in balanced cantilevering. Progressive placement
resembles
incremental
launching
in
that the superstructure
undergoes stresses ver' different from the permanent service conditions/ including even stress reversals. In both cases the structure needs to incorporate temporar' prestressing tendons to account for these stresses. Mathivat 314=?6 also points at the difficult' in erecting the first span (ith progressive placement. Other construction methods ma' have to be emplo'ed for this stage.
The Linn Cove Viaduct
An interesting e>ample for use of the progressive placement method is the Linn Cove %iaduct 3Anon. 14=,6/ sho(n in Figure ,$?. Its location in an environmentall' sensitive area in ;orth Carolina/ the inaccessibilit' of the sloping site at the mountain face/ and the highl' curved alignment of the viaduct provided a set of difficult conditions for this pro-ect. Match$cast prefabricated segments for the highl' curved viaduct (ere delivered b' truc) directl' to the end of the alread' completed part of the structure/ (here the' (ere placed and attached
(ith temporar' thread bars and tendons. !eviating from the
method outlined above/ no cables (ere used above the spans/ but steel bents (ere installed under the spans during construction to provide support. Onl'
drilling so$called microshaft piles for the pier foundations needed to be done directl' on ground/ as the piers themselves also consisted of precast segments that (ere craned into position from above and post$tensioned verticall'. Implementation of the progressive placement method in connection (ith precast pier segments that (ere lo(ered into place from above/ called top do(n construction/ helped protecting the natural environment at the site as far as technicall' possible.
Fig#re 40" T+e Linn Co*e Via,#$t1 Nort+ Carolina1 )-S- 2ta3en fro& Ri*es !51 %4!6
Concluding the Cantilevering Process
The cantilevering process (ill finall' have reached its end (hen both girders meet at midspan and need to be connected. Three different (a's e>ist to achieve this connection in the structural s'stem 3Mathivat 14=?6. A hinged connection can be installed that allo(s horiBontal movements in the superstructure. As Mathivat 314=?6 (rites/ this s'stem is structurall' relativel' simple/ 'et the hinges are complicated details and the overall structural redundanc' of the s'stem is reduced. Podoln' and Muller 314=06 also mention the lo(er ultimate load$carr'ing capacit' of the hinged s'stem and the higher susceptibilit' to creep and rela>ation phenomena. Furthermore/ the t(o superstructure halves can
have a slight angle bet(een them as deflections occur/ (hich is detrimental to 9the appearance of the bridge and the users comfort: 3Podoln' and Muller 14=0/ p?26. Secondl'/ part of the midspan superstructure can be designed as a suspended span sitting on bearings bet(een the cantilevers. In this configuration the deflection angle bet(een the shorter cantilevers and the suspended span (ill be much smaller/ and 9differential settling of the supports: can better be accounted for 3Podoln' and Muller 14=0/ p?=6. Still/ the connections re#uire special details in the structural s'stem. Finall'/ the (hole superstructure can be made continuous at midspan. Achieving this staticall' indeterminate s'stem is the most common (a' in building cantilever bridges for several reasons. Mathivat 314=?/ p,16 names specificall' that the deflections in the stiffer continuous superstructures are 9indeed far smaller than those met in hinged structures: and both visual appearance and drivers comfort are better than in hinged superstructures. *e also notes the necessit'
for
e>pansion
-oints
in
ver'
long
continuous
multi$span
superstructures and advises to provide e>pansion -oints about 9?55 to 255 m apart: 3Mathivat 14=?/ p,6 in points of small moments in the superstructure. *oriBontal movements of the bridge superstructure can be accommodated 9b' the fle>ibilit' of the piers themselves/ or b' using elastomeric bearings or sliding supports: 3Mathivat 14=?/ p,6. Continuit' is generated b' casting a closure segment into the gap at midspan/ through (hich continuit' tendons/ the so$called integration cables as mentioned in Section ,.1 run in the bottom part of the bo> girder 3Mathivat 14=?6. Prior to casting this segment/ misalignments of the t(o superstructure halves are corrected (ith h'draulic -ac)s. It should/ ho(ever/ be tried to )eep these additionall' imposed stresses small b' pa'ing close attention to the correct alignment
including
camber
(hen
casting
the
superstructure
halves.
Additionall'/ as mentioned in Section ?.2.0.@/ the t(o girders are often -ac)ed apart to compensate for future effects 9of long$term creep and shrin)age of the superstructure on the substructure: as Matt et al. 314==/ p?@6 report. The' further mention casting the
closure segment of their bridge pro-ect at night to avoid problems from temperature gradients in the superstructure. For casting and curing of the midspan closure segment the girders need to be fi>ed in their position. Finall'/ continuit' tendons can be inserted into the ne(l' cast segment and post$ tensioned. Dpon closure of midspan internal stress redistribution ta)es place/ shifting the moments from the supports more to(ards midspan. The formerl' free cantilevers are no( restrained in deflection and rotation. Podoln' and Muller 314=06 provide a sample calculation for the effect of stress redistribution (ith consideration of time$dependent effects.
Cantile*er Ere$tion E7#i%&ent
!ifferent erection e#uipment is used in bridge construction.
Form Travelers
Cast$in$place cantilever construction re#uires form(or) that is attached to the tip of the gro(ing cantilever for casting. The follo(ing paragraphs (ill deal (ith girder cross$sections onl'. As the cantilever gro(s the forms travel are set forth in steps. These form travelers give shape to the segment/ support the (eight of the ne(l' cast concrete until it has gained enough strength to be post$ tensioned to the previous cantilever segments/ and transfer the segment (eight to the alread' e>isting superstructure. !etermining for the capacit' of the form travelers is the ma>imum siBe and initial (eight of the biggest segment in the bridge superstructure/ including other construction loads. Form travelers available in toda's construction industr' are made b' specialiBed manufacturers and are reusable and ver' fle>ible 3Levintov 1446 (ith respect to changing geometr' of the bridge superstructure and its alignment/ including camber. The' can be enclosed in a heated tent to enable concrete
placement and curing to proceed during adverse (eather conditions/ especiall' lo( temperatures. In comparison (ith a precasting 'ard/ form travelers often offer the less costl' solution/ since transportation and storage of prefabricated segments is avoided/ and the' integrate all the functions of the precasting plant into a relativel' small device. Fletcher 314=,6 notes that b' use of form travelers the form(or) is reused several times/ (hile ad-ustments to variable segment geometr'/ especiall' depth/ remain possible (ith relativel' little effort. !isadvantages of cast$in$place cantilevering (ith form travelers are discussed in Section ?.2.?.1. Figure ,$, sho(s a t'pical vie( of a form traveler. Form travelers consist of a sufficientl' stiff steel frame to (hich form panels for the bo> girder segments are attached at the front. According to Levintov 3144/ p,?6/ the steel frame is mostl' composed of t(o parallel 9diamond$ or triangular$shaped frames that are connected and stiffened b' diagonal bracing and transverse trusses at the upper front and rear.: girder/ so that construction loads can be transferred into the main load$carr'ing s'stem of the bridge directl'. The longitudinal main beams of the form travelers need not necessaril' be located above the (ebs. Form travelers are also used in configurations (ith the main beams in a lateral position/ leaving the bridge dec) free 3Podoln' and Muller 14=06.
Mathivat
314=?6
further
distinguishes
so$called
self$supporting
assemblies/ (here the stiffening effect of the form panels contributes to the stiffness of the (hole form traveler.
Suspended from the traveler are not onl' the adaptable forms for e>terior and interior of the concrete segment/ but also (or)ing platforms on different levels that can be accessed from above.
Launching Girders
Apart from various t'pes of cranes that can be used to place precast segments/ launching girders are (idel' used for this purpose. Levintov 31446 mentions limited access under the cantilever and great height of bridge superstructures above ground as reasons (h' launching girders (ould be used. The' are ver' feasible for bridges (ith several spans/ as due to their length the' can be advanced over gaps that are still to be bridged. !uring construction the' are moved for(ard on rails (henever a ma-or part of the bridge superstructure has been completed. Launching girders/ also called launching gantries/ are large trusses that are placed longitudinall' on the bridge superstructure. One or more movable crane devices for transportation of the precast segments can be attached to them/ running along the chords of the girders. Precast segments are delivered to the girder b' special heav'$dut' vehicles. If launching girders are built of high strength steel their (eight can be reduced considerabl'. At the same time/ ho(ever/ larger deflections occur that are limited b' additional support of the girders (ith a )ing post s'stem (ith sta' cables 3Mathivat 14=?6. Launching girder trusses can have triangular or rectangular cross$sections and can be constant in depth or higher to(ards the middle. The' can be disassembled into parts that are connected (ith high$strength friction bolts 3Mathivat 14=?6 for transportation/ ver' similar to to(er crane booms. Most launching girders are overhead trusses that have three leg supports. The three legs are called rear/ central/ and guide leg. Some of these legs/ often the guide leg/ are not permanentl' fi>ed to the girder to allo( the advancing
movement as (ill be described belo(. %er' often these legs form a bent above the superstructure/ leaving space for the precast segments that are turned 45J side(ard to be moved through the gap. Pivoting the (hole launching girder around
the
rear
support
leg
3Mathivat
14=?6
accommodates
bridge
superstructure curves in the horiBontal plane. A ma-or feature of launching girders is their length in comparison (ith the span length of the bridge superstructure. Levintov 3144/ p,6 (rites that launching girders composed of 9single or double trusses ma' range from slightl' longer than a span length to slightl' longer than t(ice a span length.: "rection se#uences for these t(o e>tremes shall be briefl' described in the follo(ing paragraphs.
Launc"ing #irer Slig"tl$ Longer T"an One S!an
Construction of the bridge superstructure (ith a launching girder about a span long is performed as follo(s. After advancement the girder rests (ith its rear leg on the cantilever tip at midspan and (ith its center leg on the ne>t pier. Around this pier ne( segments (ill be placed (ith balanced cantilevering/ filling the remaining half$span behind the pier and advancing the cantilever to the ne>t midspan. After(ards/ the ne>t pier table segment is placed and the cantilever advances half a span so that it comes to rest on guide leg and rear leg. The guide leg (ill remain on the pier as the girder advances further. t placement position has been reached. Figure ,$ sho(s the construction se#uence (ith a launching girder that is slightl' longer than one span.
A ma-or dra(bac) of this method is found in the previous description. The bridge superstructure (ill ta)e considerable loads during construction/ since the heav' launching girder rests (ith one leg at the midspan cantilever tip. Launc"ing #irer Slig"tl$ Longer T"an T%o S!ans
Construction of the bridge superstructure (ith a launching girder about t(o
spans long does not incur the aforementioned detrimental load condition. At all stages the load$carr'ing girder legs (ill be located above piers. This is sho(n in Figure ,$2. In the normal placement position the launching girder rests (ith its rear leg above a previous pier and (ith its central leg above a free pier. It is easil' possible to also support it at the guide leg once the third pier table has been placed. Placement of the segments (ill then proceed on both sides of the pier table in the middle of the girder. To speed up construction/ the girder can be e#uipped (ith t(o crane devices to place segments on both sides simultaneousl'. After the remaining gap in the superstructure has been closed and the cantilever has gro(n into the ne>t span/ the girder is advanced one complete span. !uring advancement the girder rests on the guide and central leg that remain on the ne(l' finished pier and the one that lies ahead. This second (a' of emplo'ing launching girders is the more recent techni#ue 3Mathivat 14=?6. "ven longer launching girders have been used in construction/ as reported b' Mathivat 314=?6. !ue to the long span re#uired for launching girders and the mechanical parts/ such as crane and advancement devices/ launching girders can be #uite costl'. If possible/ launching girders should be adaptable for reuse or should be rented. Launching girders can reach lengths of more than 15 m/ depending on the re#uirements of the bridge spans/ and (eights of up to about ,55 t 3Mathivat 14=?6. In addition to these specialiBed/ e>pensive and heav' pieces of construction e#uipment it is also possible to use simple lifting devices that are located at the cantilever tip. In the case of the Linn Cove %iaduct/ (hich has been presented in Section ,.0.1.,/ a derric) (as mounted to the bridge superstructure that placed segments as the' (ere delivered b' truc) 3Anon. 14=,6. Other t'pes of mentioned b' Levintov
dec)$mounted
e#uipment
are
imaginable
3144/ p,,6/ e.g. 9a longitudinal beam fitted (ith lifting tac)le and (inches.:
and
In$re&ental La#n$+ing
Incremental launching (as developed b' the &erman engineers FritB Leonhardt and
A casting
bed
(ith ad-ustable form(or) for the superstructure
segments is set up. This casting bed can also be enclosed in a heated tent so that controlled casting and curing conditions are achieved. The normal c'cle time/ regardless of segment length is one (ee). Segment lengths according to Liebenberg 314406 t'picall' range bet(een 1 and ?5 m. T(o different techni#ues for launching the bridge superstructure from the casting bed e>ist. *'draulic -ac)s can pull the superstructure (ith steel rods/ as it (as done for the io CaronK ridge 3Podoln' and Muller 14=06. The second/ more common method is to emplo' a pair of h'draulic -ac)s acting verticall' and horiBontall'. Continuous repetition of lifting the superstructure off the abutment and then pushing it for(ard as far as the -ac) allo(s (ill achieve the launching in incremental steps. Figure ,$@ sho(s the launching process schematicall'. Podoln' and Muller 314=06 caution to design the -ac) capacit' for
more than the usual friction coefficient of 0 because of imperfections that can occur during construction.
In front of the cantilevering superstructure a light(eight steel launching nose is attached (ith tendons that reaches the ne>t support before the bridge superstructure itself arrives. Its purpose is to )eep the bending moments in the superstructure smaller. Mostl' the launching nose has a length of about 25 of the bridge spans 3Podoln' and Muller 14=06. Another (a' of reducing the bending moments is to implement temporar' to(ers bet(een the bridge piers. These to(ers need to be able to ta)e the horiBontal forces that arise from launching. On top of all supports/ including abutments/ piers/ and temporar' to(ers temporar' sliding bearings are installed during construction that (ill later be replaced (ith the permanent ones. Stainless steel plates are installed on the bearings.
Several advantages ma)e incremental launching a ver' competitive erection
method. As (ith an' cantilevering method it leaves the site belo( completel' unobstructed during construction. Onl' for ver' long spans temporar' to(ers or cable sta's from above as supports are needed. ">cept for these the e#uipment necessar' is reduced to the -ac)ing mechanism/ the ad-ustable stationar' casting bed/ and temporar' sliding bearings/ all of (hich ma' possibl' be reused/ (hich reduces the capital investment considerabl'. Podoln' and Muller 314=06 furthermore mention the cost savings due to avoidance of segment transportation and heav' construction e#uipment. The' also point at less maintenance cost due to the higher prestressing of the superstructure. The controlled casting and curing conditions allo( stead' and #uic) construction progress. ridges that are erected (ith the incremental launching method should/ according to Podoln' and Muller 314=06/ have a constant cross$section/ especiall' in depth/ and have a straight superstructure. It is possible to accommodate small variations in alignment and horiBontal and vertical curvatures provided that the' have a constant radius. Close control of the bridge geometr' during casting and launching is ver' important. Sloping grades at the bridge site are also accommodated/ in this case 9the launch is usuall' in the do(n(ard direction:/ more than 0 slope (ould re#uire a retarding mechanism to stop the movement of the superstructure 3Liebenberg 1440/ p126. Liebenberg 31440/ p12,6 also gives a ver' clear statement of the main difficult' of the incremental launching method7 9!uring launching/ the section undergoes complete stress reversals as it progresses from a cantilever to the first support and thereafter over the follo(ing spans to its final position.: Clearl'/ this erection se#uence generates a bending moment envelope in the structure depending on the span lengths that needs to be accounted for in designing the cross$section properties and the amount of reinforcement and prestressing tendons. The stresses due to the aforementioned high bending moments re#uire much longitudinal prestressing both at top and bottom of the cross$section. Another disadvantage is the large (or)space that is needed for the casting bed at the abutment and the ad-acent storage areas 3Podoln' and Muller 14=06. The Aichtal ridge in &erman' that (as built mainl' bet(een 14=1 and 14=? serves as a good e>ample of the incremental launching method. According to
asse et al. 314=6 this bridge (ith its total length of 1/121 m is the longest one ever built (ith incremental launching. It crosses t(o valle's at a ma>imum of ,= m and 5 m above ground/ respectivel'. A fi>ed bearing is located at the pier bet(een the t(o valle's. At the same location a second -ac)ing s'stem (as installed for use in later construction stages. The normal pier spacing for the 01 spans of the Aichtal ridge is 1 m/ reaching a ma>imum of =5 m and =, m respectivel' at the deepest parts of the valle's. These (ide spans re#uired use of temporar' to(ers that (ere braced (ith sta' cables from the ground to resist the horiBontal forces from launching. The (hole bridge superstructure consists of t(o parallel single cell bo> girders that are ?.5 m deep/ m (ide at the soffit and carr' 1?.5$m (ide dec)s. After completion of one girder all construction e#uipment (as relocated for the second bo> girder. An enclosed 0.5$m long casting bed (ith ad-acent assembl' 'ard for the reinforcement cages (as erected behind the abutment (ith the launching -ac)s. For (inter construction (or) another 5$m long tent (ith large heaters (as set up for proper curing and the piers (ere built (ith thermall' insulated climbing form(or). Casting of segments (as done in a (ee)l' c'cle. oth longitudinal and transverse limited prestressing (as implemented. asse et al. 314=/ p0?6 give information on the se#uence of casting steps that is compiled in Table ,$?7 Table 40" Se7#en$e of Casting Ste%s for Ai$+tal Bri,ge Da/ Monda' Tuesda'
A$ti*ities Post$tensioning/ stripping of form(or)/ incremental Placement of reinforcement cages for bottom slab and (ebs/ Installation of ducts and tendons/ installation of Completion of form(or) installation/ concreting of emoval of (eb form(or)/ installation of interior top slab form(or)/ Placement of top slab reinforcement/ Concreting of top slab Curing of concrete
Launching of the cured and post$tensioned segments re#uired man' personnel
for supervision of all sliding bearings. An overall longitudinal slope of the bridge reduced the -ac)ing forces necessar' for launching. An overall curvature (ith radius 1/55 m in the horiBontal plane and a constant ?. cross slope of the bridge superstructure induced stresses due to restrained deformations during launching. Furthermore/ the changing span lengths had to be considered in coming up (ith the prestressing program to optimiBe the tendon profile and prestressing forces. A surve'ing program had been prepared to control geometr' during all construction steps. Several fi>ed surve'ing points (ere located along the site. Overall/ tolerances for deviations from the planned bridge geometr' during casting and launching (ere less than 1. mm 3asse et al. 14=6.
False8or3
False(or) has been used in construction since ancient times/ (hen oman bridge builders erected their semicircular stone arches for bridges/ a#ueducts/ and vaults on (ooden centering. False(or) provides continuous support for the form(or) that gives shape to the superstructure. In most cases false(or) is used for cast$in$place concrete structures. It re#uires firm/ relativel' even ground on (hich it can be erected. Apart from custom$built timber structures a (ide range of steel elements and modules for false(or) is available in the construction industr'. Liebenberg 314406 gives a range of up to ?55 m in length and about 15 m in height for bridges to be built (ith this method. *e also specificall' points at the necessit' for stable foundations of the false(or)/ sufficient bracing of the false(or) structure/ and consideration of the deflection of the false(or) in the overall superstructure camber.
False(or)/ either stationar' or traveling/ can also be configured as casting girders that hold the form(or) into (hich the concrete is placed. In that/ these girders resemble the erection girders of the span$b'$span method/ (hich are used to assemble precast segments. Liebenberg 314406 further distinguishes the girders depending on their location to the bridge superstructure as overhead or supporting it from belo(/ or combination of both. *e also reminds that use of ma-or pieces of e#uipment/ such as casting girders needs to be considered carefull' because of the high capital investment that is necessar'.
Stationary Falsewor
Stationar' false(or) is the
simplest method of erecting the
bridge
superstructure. Advantages mentioned b' Liebenberg 314406 are that stationar' false(or) can be erected b' less specialiBed (or)ers to an' desired shape. Dse of modern standard elements of (hich the false(or) is put together allo(s uncomplicated erection. There are/ ho(ever/ several disadvantages related to stationar' false(or). A lot of material is re#uired for stationar' false(or)/ (hich re#uires much time and manual labor to be spent on its erection/ in addition to the cost of purchasing or renting the materials themselves. Therefore/ Liebenberg 314406 concludes that it needs to be erected some spans in advance to )eep up (ith rates of placement of concrete that can be achieved. Toda'/ false(or) is competitivel' used e.g. for the comple> alignments of high(a' interchanges/ as Cassano 314=@6 reports for the e>ample California. *e states that apart from the feasibilit' for the highl' curved superstructures/ use of ma-or false(or) s'stems also allo(s placement of ver' large volumes of
concrete at the same time and thus speeds up construction. Cassano 314=@6 mentions that all false(or) in California has to be designed and built according to the False(or) Manual of the California !epartment of Transportation and is revie(ed and inspected b' #ualified engineering personnel.
Traveling Falsewor
Traveling false(or) alleviates some of the problems associated (ith stationar' form(or). This t'pe of false(or)/ including form(or)/ is assembled to larger units that can be moved to the ne>t span to be cast. As (ith stationar' false(or)/ this method re#uires the site to have relativel' level and firm ground to allo( movement of the false(or) on a (heel assembl'. In case the ground conditions are less favorable/ using the span$b'$span method might be advisable/ (here the superstructure segments are assembled on erection girders. Span$b'$span erection is introduced in the follo(ing Section ,.0.,. Traveling false(or) is sho(n in Figure ,$=.
Tem!orary Towers
A )ind of false(or) used fre#uentl' in construction is the use of temporar' to(ers to support the superstructure that is under construction at intermediate positions. These to(ers can e.g. be used in the balanced cantilevering method to stabiliBe the structure against tipping over from construction loads. The' can be additionall' strengthened (ith prestressing rods to (ithstand the forces that are imposed on them during construction. Other applications are found during incremental launching/ (here together (ith the launching nose the' serve to )eep the range of bending moments small. Instead of using temporar' to(ers as supports Liebenberg 31440/ p126 also mentions stiffening girders 9b' means of prestraining (ith a )ing post and ad-ustable inclined ties.:
S%anB/S%an Ere$tion
Levintov 3144/ p,@6 la's out the characteristics of the span$b'$span erection
method as assembling all segments for a span in a set/ (hich is 9then aligned/ -ointed/ and longitudinall' post$tensioned together to ma)e a complete span.: The principle of span$b'$span erection is sho(n in Figure ,$4. Span$b'$span erection is t'picall' limited to bridges that consist of bo> girders (ith constant depth. The actual construction can have several variants/ the segments can be assembled on the ground and lifted in place as a group b' a heav'$dut' crane or the' can all be put into their final position on erection girders along the spans to be completed. The second method (as e.g. used for constructing the ilo>i Interstate I$115 %iaduct. In this pro-ect different t'pes of erection girders (ere used. The authors report that on some spans triangular trusses (ere implemented/ and on the other hand steel bo> girders came into use/ (hich left more clearance for traffic underneath 3Phipps and Spruill 14456. "rection girders (ere supported at their ends 9b' steel false(or) resting on the footings at each pier: 3Phipps and Spruill 1445/ p1?56. After completion of a span the erection girders (ere set for(ard to the ne>t span and ad-usted. ' then/ the precast segments for this span had alread' been supplied and (ould be lifted in place b' crane. Fine ad-ustments of the segments on the erection girders (ere possible b' means of variable individual supports. Finall'/ post$tensioning (ould be performed to lin) all the segments together to form a complete span. The structure became self$supporting after casting of the closure -oints (ith the pier table segments had been done and the longitudinal prestressing force (as induced.
"rection girder need not rest on the ground/ but can also be supported b' alread' e>isting substructure or superstructure/ e.g. the piers of a span that is to be
constructed. Pro-ect specific design of substructure and superstructure and considerations as e.g. for traffic clearances set the boundaries for erection (ith erection girders.
CONSIDERATION STRESSES
OF
CONSTR)CTION
LOADS
AND
The follo(ing sections deal (ith the relationship bet(een construction loads and the stresses that these induce in structures and the structures themselves (hile the' are under construction and still a(aiting completion. The central issue for all considerations is structural safet'/ meaning failure against structural failure. The generic concept of resistance R that is greater than the most unfavorable combination of load S that induces stresses has been introduced in Section ?..0. Codes re#uire that all construction influences (ill be properl' ta)en into account during design. "ven a professional code applicable for bridges 3ACI 144/ p16 in its Section .? onl' points out that 9Consideration should be given to temporar' loads caused b' the se#uence of construction stages/ forming/ false(or)/ or construction e#uipment and the stresses created b' lifting and placing precast members.: It assigns the responsibilit' for the construction scheme/ (hich imposes stresses on the structural members/ to the contractor. It is further pointed out that the stabilit' of precast members and prestressing should be ta)en into account. 9"nvironmental loads should be considered during construction using an appropriate return period or reduced severit'. Lo(er load factors ma' be used to account for the acceptabilit' of higher temporar' stress levels: 3ACI 144/ p16. The aforementioned load factors are provided in tables in the same code. *o( the actual process has to be accomplished in detail remains the structural engineers tas). Close cooperation of designer and contractor in development of the construction se#uence contributes to #ualit'. For better understanding of the relationship bet(een structures under construction and actions influencing them it is useful to get a clear definition of the technical terms related to this topic. The nature of actions has been e>plained
in Section ?..? as an' loads or restrained deformations that can cause stresses (ithin the structural s'stem. The term construction loads should in this conte>t be understood as the broader sense of an' actions that occur during construction of the structure prior to normal service conditions. Then again/ the concept of construction loads needs to be e>tended b' consideration of the uncompleted structure during construction/ (hich ma' not have reached full resistance to imposed actions. The term erection method denotes the ph'sical means of putting the foundations/ bridge substructure/ and especiall' its superstructure into place. "ver' t'pe of erection method re#uires certain e#uipment and site installations to carr' out the (or) tas)s. The erection methods goes along (ith specific limitations imposed on the flo( of (or) tas)s to be scheduled/ e.g. that bridge piers have to be
finished
prior
to
begin
of
incremental
launching
of
a
bridge
superstructure. The construction sequence is the specific succession of (or) tas)s for one particular pro-ect developed under consideration of the erection method chosen for its conditions and restraints for economic construction. Construction stages are notable steps (ithin the progression of (or) tas)s from the initial site operations startup to the finished structure. These steps can be distinguished b' the appearance of the structural s'stem/ loading conditions/ or other factors. Load steps can be defined as specific sets of loads on a structure in its current construction stages. These load steps or load cases are combinations of actions that are anticipated to occur at the same time (ith a certain probabilit'/ and are incorporated into anal'tical calculations (ith partial factors of safet'.
T/%es of Constr#$tion Loa,s an, Infl#en$es
&enerall'/ construction loads are b' nature of relativel' short duration in comparison (ith the overall planned service life of a structure. Construction loads ma' influence a structure over a brief time onl'/ e.g. from e#uipment or
material for a ne( segment that is temporaril' stored on an alread' completed part of the superstructure of a bridge. Construction loads can affect the structure in ver' unfavorable conditions/ e.g. (hen a crane is located at the tip of a cantilever to place segments. Thus resulting stresses in the structure can even e>ceed stresses due to permanent and d'namic loads under service. eal loads are generall' distinguished into t(o classes/ dead loads and live loads. !uring construction the structure has to carr' its o(n (eight/ its dead load/ and the superimposed dead loads of bridge parts that are not structurall' important but necessar' for service/ as e.g. the bridge furniture/ the so$called accessories. Man' different live loads influence the structure. In most of all cases/ live loads are idealiBed either as uniforml' loaded areas on the superstructure or as singular loads from larger pieces of e#uipment. Live loads can result from erection e#uipment/ e.g. the launching nose in incremental launching and lifting devices placed on the structure such as cranes and launching girders. Forces are also imposed on the structure through restraints from fi>ed bearings during construction/ e.g. on piers for cantilevering. Along (ith these structural details/ the boundar' conditions can still change8 e.g. considerable settlements can occur (hen the soil is initiall' loaded. Temporar' supports/ e.g. additional temporar' to(ers for long spans or sta' cables also generate stresses in the superstructure. Another factor to be considered is the prestressing tendons that are installed in the concrete members to (ithstand stresses during construction and under service. More longitudinal forces can be caused b' the erection itself/ e.g. horiBontal -ac)ing forces during incremental launching. Form(or) and supporting installations/ e.g. the forms and frames in a launching girder also impose live loads/ as (ell as the fresh concrete that it carries. Finall'/ environmental influences also create changing loads on the structure/ e.g. (ind/ sno(/ and temperature gradients. ">treme events/ such as floods/ storms/ and earth#ua)es can also hit a structure during construction and ma' need to be considered in the calculations. Apart from these Acts of &od/ accidents ma' happen. Podoln' and Muller 314=06 note that to prevent a scenario such as falling of a form traveler during cantilevering/ inspections are
necessar'. The' also note that critical fi>tures/ e.g. suspension rods that reach through the superstructure and anchor bars that hold the traveler/ need to have a large safet' margin and ma' be provided in double numbers. In general/ 9cast$ in$place cantilever construction has established an e>tremel' good safet' record: 3Podoln' and Muller 14=0/ p,=06. The aforementioned e>treme load cases are mostl' considered (ith a lo(er factor of safet' than for service conditions 3ACI 1446. The reason for this approach lies in the reduced probabilit' of occurrence during the relativel' short construction period in comparison (ith the total duration of service for (hich the bridge structure is designed. More e>planation for this rationale lies in the fact that although the resistance is reduced during construction the structure produces less danger for the general public/ as it has not been opened for traffic b' then. Furthermore/ the bridge under construction is under direct control
of
engineering
personnel
on
site
that
can immediatel' ta)e
appropriate measures if necessar' to ensure safet' of further construction (or)s.
Apart from that structural resistance the material resistance ma' also be (ea)er than in the final state. "speciall' for cast$in$place segmental bridges the still 'oung concrete usuall' has not developed its full specified strength (hen it is being prestressed and loaded (ith more segments for #uic) erection. The structural resistance and material resistance can also be understood as the t(o components of structural safet'/ namel' strength and stabilit'/ as outlined in Section ?.1.?. SummariBing/ stresses induced b' construction loads ma' be higher than those from service loads as the incomplete structural s'stem is mostl' different and (ea)er than finished structures/ concrete has not gained full strength/ and the boundar' conditions ma' be different from the service state. In other (ords/ the great importance of construction stages lies in the criticalit' that results from the still lo( structural and material resistance/ (hile loads ma' be actuall' more adverse and boundar' conditions different.
SPAN CONFI#URATION AND T&PICAL SECTIONS +he structure is a five-span bridge with span configuration of %('H, K''H, K''H, K''H, %('H, producing a total length of &'' feet. +he bridge carries two %KH-'G lanes of traffic in one direction
with a left shoulder width of H-'G and a right shoulder width of %'H-'G. $xpansion bearings are placed at all piers except Pier O which is fixed. +he typical section selected is the AA2+<-P7I-A2*I 2egmental *ox @irder 2tandard +ype K''-K, a single-cell concrete box girder with O0H-'G wide deck and &H-'G in depth. 7antilevered overhangs are %'H-O.(G each. inimum top slab thickness is &G. +he thickness of the bottom slab is %QG for three segments on both sides of each pier and &G thick elsewhere. +he thickness of the webs is %G, which are sloped at K.(=%. +he top slab can accommodate %K tendons in each half of the box girder, for a total of KO tendons in the top slab. +he bottom slab can accommodate tendons in each half of the box, for a total of %K tendons in the bottom slab. Additional tendons may still be accommodated either in the top or bottom slab. hen dealing with development of a cross-section, it is important to investigate the efficiency of the proposed cross-section. +he section efficiency of the AA2+<-P7I-A2*I K''-K section can be computed using @uyonHs formula= ρ
=
Ic A c y t yb
where, Ic
R oment of inertia of the section
Ac R Area of the section yt
R ;istance from the top fiber to the center of gravity of the section
yb R ;istance from the bottom fiber to the center of gravity of the section +he efficiency of the cross-section, ρ, is '. which is considered to be high. comparison, the flat slab is the most inefficient section with a ρ value of '.00. +his design example utili6es a %KH-'G typical segment length, resulting in a maximum segment weight of K.( tons for the thin bottom slab segment and Q' tons for the thick bottom slab segment.
Safet$ an 'ealt" at a t Pre(cast &ars Concrete Pre(cast &ars S
anu anufa fact ctur ure e
prepre-ca cast st
concr oncret ete e
compo ompone nent nts s
that that
are are
tran trans sport ported ed
to
construction sites for installation in buildings and other structures S +he work processes involved in precast concrete components manufacture are construction-like processes S Production Processes=> 7asting 9inishing 2torage +ransporting Common 'a)ars T a6ard a6ards s associ associate ated d with with materi material al handli handling ng of pre-ca pre-cast st 7ompo 7omponen nents, ts, form form panels, reinforcement wire U rebar T a6ards associated with storage of pre-cast components T ?ehicular ha6ards T achinery U $lectrical a6ards T 9alling ha6ards T :oise T Piercing ha6ards due to protruding rebar T 9ire ha6ards T 7hemical ha6ards Pre(cast Com!onent 'anling
Pre-cast concrete products need to be lifted several times during production using overhead cranes, cranes, mobile cranes cranes U tower tower cranes for=emoving from the casting bed or form +ransporting from the casting bed to storage yards #oad #oadin ing g from from stor storag age e to trai traile lers rs for for tran transp spor orti ting ng to works orksit ites es for for installation. 'a)ars from Material 'anling T 2truck by dropped pre-cast components => 9ailure of #ifting machines. T ;efective lifting gears 3$xample >chain slings with defective safety catches4. T 9ailure of lifting lug of the pre-cast pre-cast components 3inade!uate design design or lack of control of fabrication process4 T $ntanglement $ntanglement of slings with with other materials T
Safe Lifting O!eration *#eneral+ S ;aily operator visual inspection on crane components - hoist rope, hook block, slings U lifting gears U operational checks on limit switches and various brakes. S Periodic examination of the cranes by authorised examiners S
CAS" STD!+
Construction of the Precast Segmental Structures for Sutong ridge Abstract T+e %re$ast seg&ental a%%roa$+ str#$t#re of t+e S#tong Bri,ge 8it+ 59& s%an lengt+ 8as $onstr#$te, #sing balan$e, $antile*er &et+o,- T+is %re$ast seg&ental *ia,#$t 8as $+ara$teri:e, b/ ,ee% fo#n,ation in t+e ri*erbe,1 ;<& +ig+ $ol#&ns $onne$ting t+e &ain bri,ge an, relati*el/ long en, s%ans in t+e balan$e, $antile*er ,e$3T+is %a%er $o*ers t+e $onstr#$tion &et+o,s an, logisti$s a,o%te, to s#it t+e %arti$#lar site $on,itions- It ,is$#sses t+e $asting /ar, sele$tion an, set #% in t+e *i$init/ of t+e ri*er ban31 t+e ,esign of t+e s%e$ial e7#i%&ent s#$+ as t+e s+ort line &at$+ $asting &o#l,s an, t8o !;<& long la#n$+ing gir,ers 8+i$+ 8ere tailor&a,e for t+is %ro=e$t- T+e *al#e engineering e>er$ise in t+e %ost Contra$t a8ar, stage for fa$ilitating $onstr#$tion is ,etaile,T+e geo&etr/ $ontrol &et+o, #se, in bot+ t+e $asting /ar, an, ere$tion front1 t+e te&%orar/ 8or3s an, stabili:ing s/ste& are ,elineate,T+is %a%er also +ig+lig+ts t+e $onstr#$tion ,iffi$#lties en$o#ntere, an, sol#tions to t+e %roble&s-
Introuction The Sutong ridge is part of a traffic truc) connecting cities ;antong and SuBhou 3Changshu6 across the +angtBe iver in the south of Hiangsu Province 3Figure 16. The total length of the high(a' is ?0.,)m. It consists of three parts7 north ban) lin)/ central crossing and south ban) lin). The central crossing consists of a total length of =.0)m bridge structures (ith 2$lane dual carriage(a'. The main navigation channel is a cable$ sta'ed bridge that has a central span of 15==m. oth the north and south approach structures consist of ?5m/ 5m or @m long span prestressed concrete continuous single$cell bo> girder structure to be constructed b' either cast in$situ or precast segmental method. The construction of the precast segmental approach structures commenced in April 055, and completed in earl' 055@. 9igure %= #ocation Plan
The Sutong ridge pro-ect is located at the do(nstream of the +angtBe iver close to the estuar' (ith strong tidal effects at about 115 )m from the river mouth. The site conditions (ere characteriBed b' sno(/ storm/ t'phoon/ strong tide/ variable riverbed and rapid flo( rate. The observed ?5$'ear returned (ind speed (as ?.mNs. The ma>imum difference bet(een high tide and lo( tide observed (as more than m. The ma>imum (ater depth (as about 10m and the section flo( rate (as over ,mNs. These environmental factors posed constraints and challenges to man' construction activities in this pro-ect.
Brige C"aracteristics eing the first precast segmental bridge built using short$line match$casting method in China/ the approach structure consisted of 0 spans of single cell bo> girders. The t'pical bridge units (ere formed b' $span or 15$span continuous dec) supported on the pot bearings 3Figure 06. The total length of the bo> girder measured along its centerline (as appro>imate ?.@)m. The dec) (idth of a single carriage(a' (as 1@.m 3Figure ?6. Other than one end span that (as ad-acent to the in$situ bo> girder at ;atong side (as a 5m unit/ all other spans had an e#ual span length of @m. The reinforced concrete columns (ith ,.m > 2.m rectangular hollo( section (ere adopted for the ma>imum height of 25m and supported on pile caps (ith bored pile foundation.
9igure K= *ridge $levation
9igure 0= *ridge 2ectional ?iew
,alue Engineering The "ngineers original design of the precast segmental bridge dec) (as based on the balanced cantilever construction techni#ue using long$line casting method (ith epo>' -oints. A mi>ed prestressing s'stem (as emplo'ed. The cantilever and span tendons (ere internall' prestressed (hile the continuit' tendons (ere e>ternal. !uring the Tender stage/ +ercise focusing on improving constructabilit'. The detailed design (as later e>ecuted b' the original designer after a(ard of the Contract. Some )e' aspects of the value engineering included the follo(ing7 i6
Setting out concept of the bo> girder the bo> girder soffit (as al(a's horiBontal in the "ngineers design8 thus the cross section of the bo> (as not s'mmetric about its centerline. The depth of t(o (ebs varied (ith respect to changes in super$elevation. The alternative setting out concept emplo'ed a constant depth section b' allo(ing rotation in the bo> a>is 3efer to Figure ,6. This arrangement (ould simplif' the mould design and casting operation in the production.
9igure O= *ox @irder 2etting-
9igure (= *ox @irder eb ?ariation
iii6
Second stage casting of segment there (as a significant difference in self$(eight bet(een the t'pical and diaphragm segments 30= 15 1?2 ton6. The heavier diaphragm (ould demand e>tra capacit' for all construction plants and e#uipment e.g. launching girder. The alternative construction concept involved re$design of the pier segment so that onl' a light (eight shell segment (as formed in the 'ard and the solid core (as cast b' insitu mean at a later stage after it (as erected.
iv6
StandardiBation of structural elements in the alternative proposal/ man' t'pical structural elements/ such as/ shear )e's/ blisters for cantilever tendons and diaphragm segments at end spans/ (ere re$detailed and standardiBed in order to facilitate constructabilit'.
v6
Closure segment at mid span the "ngineers design at the mid span location consisted of a ?m long insitu cast closure segment. In the value engineering e>ercise/ a precast option (as proposed and adopted. The closure segment (as 0.@m long (ith t(o 15mm insitu stitches.
Founation The pro-ect site is located in the alluvial plain of the +angtBe !elta/ (hich is characteriBed b' a thic) la'er of #uaternar' deposits. The bedroc) is ver' deep ranging bet(een 0@5m to 0=5m. In general/ the upper la'er of soil 3$m to $2m6 consists of a loose to medium dense silt' sand underlain b' a la'er of soft and compressible silt' cla'. elo( the soft la'er is the dense sand' deposits/ (hich constitutes mainl' the load bearing la'ers for the pile foundation. The foundation of the precast segmental approach bridge (as composed of ??5 bored piles of diameter 1.=m. Piers (ere supported b' either = or 4 numbers piles (ith 11m > 10m > ?m or 10m > 10m > ?.0m pilecaps respectivel'. All the piles (ere designed as friction piles (ith the founding levels ranging from $=@m to $4m. The temporar' steel casings used for bored pile construction (ere formed b' 10mm thic) tubular section/ and provided at the top portion from cap to $1=m belo( seabed. The length varied from 01m to ??m depending on the geological condition. The casings (ere driven b' a vibration hammer of @= ton. The drilling of the bored pile (as carried out using reverse$circulation drilling rig mounted on the temporar' staging. &lobal position s'stem 3&PS6 (as emplo'ed in the setting out of the piles. The foundation of the pile staging consisted of 10 numbers of 5.=m > 2mm or 1.5m > 15mm tubular piles of about ?5m long that (ere tied together on top in 0 la'ers (ith I sections and baile' trusses to form the staging platform of ?5m > 1m for one pile group construction. entonite slurr' (as emplo'ed to stabiliBe the bored holes. The characteristics for entonite suspensions are given in Table 1.
+able %. 7haracteristics of the Proposed *entonite 2uspensions Propert' Fresh ead' for re$use efore concreting P* 15Q10 =Q15 @Q4 !ensit' 3&Nml6 R1.5, R1.5= 1.52Q1.15 Marsh viscosit' 3Sec6 02 Q ? 0Q02 05Q0, Fluid loss 3mlN?5min6 R15 R1 R15 Sand content 36 R5.? 5.Q1.5 R5.
The bored hole (as filled (ith slurr' before drilling of the portion belo( the casing commenced. The head of the drilling fluid (as )ept constant at 1.m above the (ater level so as to ensure sufficient stabiliBing pressure in the uncased shaft area. After completion of drilling and initial base cleaning/ the prefabricated reinforcement cages 3= numbers of 10m long per unit/ 0@ ton each6 (ere installed b' a 055 ton crane. Placement of concrete after final cleaning of the pile base (as carried out using a 0@?mm diameter tremie pipe. For 4m long piles/ it re#uired a ?$@ da' construction c'cle from commencement of drilling to completion of concreting (or)s. See Figure 2 for the temporar' platform and e#uipment for the piling (or)s.
9igure = 2taging Platform for Piling
9igure = Pilecap 7onstruction
Pilecap construction (as e>ecuted using heav' dut' cofferdams (hich consisted of , side forms and a soffit (ith a total (eight of @ ton 3See Figure @6. This false(or) s'stem (as designed for constructabilit'. The ma>imum (eight of each module (as limited to 1 ton to facilitate eas' handling. Initiall'/ the cofferdams (ere overhang (ith temporar' steel(or)s supported on the steel casing of the piles. After fine ad-ustment of cofferdam using h'draulic -ac)s/ one meter thic) la'er of non$structural tremie concrete (as poured in order to seal the base and stabiliBe the s'stem prior to de(atering. The rest of the operations (ere carried out in dr'. The t'pical c'cle time for constructing a pile cap (as ?5$,5 da's.
Column The reinforced concrete hollo( columns (ere constructed using a self$climbing form (ith an integration of a (or)ing platform 3See Figure =6. *'draulic rams (ere emplo'ed for the climbing mechanism. A @5m high to(er crane (ith a capacit' of 105 ton$m (as fi>ed on the pilecap for material handling. Concrete (ere supplied b' a mi>er barge and pumped via a pipe mounted on a scaffold s'stem tied to the column. The slump of concrete (as 15mm and each t'pical pour (as ,m. The climbing form (ould be self$launched (hen the concrete reached a strength of 15 MPa. In general/ a ,$da' c'cle time for a ,m pour height could be achieved.
9igure Q= 7olumn 7onstruction
Segment Casting The segments (ere fabricated in a casting 'ard located at the north ban) of the pro-ect site 3;atong side6. The 'ard (as chosen at a favorable location to facilitate eas' transportation of the heav' segments b' barge. The loading point (as connected b' a 0,5m long temporar' bridge and a -ett'. See Figure 4 for the la'out of the 'ard.
9igure &= 7asting Card #ayout Plan In this pro-ect/ a total of 15=2 segments (ere fabricated in a period of 1@ months. !ue to the importance of the pro-ect/ 2 sets of short$line casting cells (ere invested under a conservative assumption of 0.$da' casting c'cle. The actual production characteristics/ ho(ever/ reached a t'pical c'cle of 0 da's for the standard segments after a brief initial learning period of 0 months. In the summer time/ a 1$da' c'cle (as achieved. The casting 'ard (as about 0 ,5/555m and e#uipped (ith 0 numbers of 125 ton gantr' cranes for segment handling and 0 numbers of 12 ton gantr' cranes for light dut' tas)s/ such as/ manipulating the reinforcement cage. The casting 'ard had a storage capacit' of ?, segments based on t(o$la'er stac)ing 3Figure 156. Concrete (as mi>ed in the casting 'ard b' using 0 numbers of batching plants of ? 5m Nh capacit' each.
9igure %'= 2egment 2torage
9igure %%= 2egment 7uring 7hamber
The bridge segments (ere produced b' short$line match$casting method. It (as a ne( technolog' used in China as earlier concrete segmental bridges had al(a's been built (ith multi$insitu -oints in order to deal (ith the geometr' variations from the bridge alignment. In this pro-ect/ the classical precast segmental method (as emplo'ed in (hich the overall
geometr' of a bridge unit (as captured in a casting cell in stages based on a given segmentation scheme. !uring each casting operation/ the spatial relationship of a pair of con-ugate segments in global coordinates (as transformed to the local reference frame of the casting cell. &eometric errors that occurred during a casting operation (ere controlled and ad-usted in the subse#uent casting operations. Control points (ere fi>ed to the (et$cast segment before hardening of concrete for geometr' controlling purposes. Surve' (as carried before and after casting a pair of con-ugate segments. A proprietar' computer soft(are/ &eomPro/ developed b' +imatel' 45 ton each. In the design/ the stiffness and fle>ibilit' of the mould (ere properl' balanced (ith due consideration of the geometr' of the viaduct. In o (inter seasons/ the ambient temperature in the +angtBe iver area (ould drop belo( $15 C. In order to achieve the target production rate/ the air$conditioned bric) houses (ere constructed for all casting cells to accelerate the gain of concrete strength of segments. In o side these curing chambers/ the temperature (as maintained at above 15 C during (inter. See Figure 11.
Segment Erection The total erection period (as about 1 months. T(o overhead launching girders 3Figure 106 (ere emplo'ed. Since the Sutong ridge (as a signature structure over the +angtBe iver/ the aesthetics of both the main bridge and approach structures (as an important aspect of the design. The pier spacing and the proportion of dec) and column (ere carefull' considered and approved b' a national bridge committee. ceeding ?5m 3efer to Figure 1?6. This imposed a ta>ing condition to the design of the launching girder. The 125m long overhead girders (eighted about 1555 ton each and functioned both some(hat as a cantilever and span$b'$span 3end span6 girder at the same time. "ach girder had t(o (inches (ith lifting capacities of 1=5 ton and 15 ton respectivel'. The lifting height of the (inches (as allo(ed for @5m so that segments could be pic)ed up at sea level. The launching girder had been designed to resist a ma>imum (ind speed of = mNs. *o(ever/ the ma>imum (ind speed (as limited to12mNs during self$launching/ and 00 mNs during segment erection. At out of service condition/ no special tie do(n s'stem (as needed (hen the (ind speed (as belo( ?5 mNs/ other(ise the t'phoon tie$do(n device (as to be engaged.
9igure %K= #@ Assembling
9igure %0= $nd 2pan $rection
The diaphragm segments of the initial t(o spans (ere erected using a barge crane of =55 ton in advance in order to facilitate the launching girder assembling and testing 3Figure 106. The remaining diaphragms (ere erected either b' the barge crane or launching girder. All other segments in the balanced cantilevers and end spans (ere constructed using the launching girders. !uring erection of the end span/ a careful investigation of the dec)$girder interaction (as conducted in order to avoid overstressing of the hangers in the load transfer process. The stabiliBation of the partiall' completed balanced cantilever (as achieved b' the use of vertical nailing consisted of 2 numbers of D shaped tendons 3101.0mm6 embedded in the column and preloaded to 5 ton each. The diaphragm segment (as supported on , numbers of 155 ton temporar' -ac)s 3Figure 1,6. After geometr' ad-ustment/ the pier segment (as fi>ed in position b' concrete pac)ers sand(iched (ith a la'er 305mm6 of sulphurous mortar that (as later molten b' the embedded electric arc for removal of the pac)ers. 9igure %O= $rected ;iaphragm 2egment After completion of the cantilever arms/ the closure segment (as lifted b' the girder (inch to the final position and supported b' a pair of clamping beams mounted onto the erected cantilever tips. The launching girder could then be launched to the ne>t span for continuing the erection (or)s (ithout completion of the stitches as all supports of the girder (ere rested on the pier segments. The insitu stitches (ere cast at the lo(est temperature of the da'. See The cantilever and span tendons (ere internall' prestressed s'stem consisted of 10/ 1/ 1@ or 14 strands 31.0mm6. The continuit' tendons (ere e>ternall' prestressed s'stem consisted of 0 strands and anchored at the diaphragm of ever' t(o spans. Figure 1a and 1b for the launching girder in action. The geometr' control during erection (as monitored at various stages. The theoretical profile of the dec)/ ta)ing into consideration of the camber and stage effects/ (as compared (ith the observed results. Initiall'/ some discrepancies (ere observed. After a thorough investigation/ it (as discovered that the problem (as due to improper application of the temporar' stressing s'stem. &reat improvement on the geometr' accurac' (as achieved for the remaining (or)s.
9igure %(a, %(b= Partially $rected *ridge ;eck
