A special report Ten years of experience in precast segmental construction Jean Muller Chief Design and Research Engineer Campenon Bernard Design and Construction Engineers Paris, France This state-of-the art report is drawn from the practical job experience of an engineer who became intimately involved in the design and construction intricacies of precast prestressed segmental construction from its very earliest beginning. A s bridge spans increased in length, it became apparent that conven- tional precast prestressed concrete gird- ers would soon be limited by their maximum transportable weights and/or lengths. For example, when delivered over highways, the length of precast girders is usually limited to 100 ft (30 m) maximum. The answer to this problem lay in the development of precast prestressed seg- mental construction in which the bene- fits of both precasting and post-tension- ing could be combined advantageously. In segmental construction, large quantities of high quality precast units (or segments) are manufactured under controlled factory conditions and then transported with conventional carriers to the job site. There, the segments are assembled easily and lifted into place on the superstructure. The segments themselves are then tied together using post-tensioning.
Transcript
A special report
Ten years ofexperience in precastsegmental constructionJean MullerChief Design and Research EngineerCampenon BernardDesign and Construction EngineersParis, France
This state-of-the art report isdrawn from the practical jobexperience of an engineerwho became intimatelyinvolved in the design andconstruction intricacies ofprecast prestressedsegmental construction fromits very earliest beginning.
A s bridge spans increased in length,it became apparent that conven-
tional precast prestressed concrete gird-ers would soon be limited by theirmaximum transportable weights and/orlengths. For example, when deliveredover highways, the length of precastgirders is usually limited to 100 ft (30m) maximum.
The answer to this problem lay in thedevelopment of precast prestressed seg-mental construction in which the bene-
fits of both precasting and post-tension-ing could be combined advantageously.
In segmental construction, largequantities of high quality precast units(or segments) are manufactured undercontrolled factory conditions and thentransported with conventional carriersto the job site. There, the segments areassembled easily and lifted into placeon the superstructure. The segmentsthemselves are then tied together usingpost-tensioning.
Saint-Cloud
Segmental construction is very effi-cient in congested urban areas (causingonly a minimum of traffic disruption)and also is very adaptable over longstretches of water or rugged terrain.Since expensive falsework is eliminated,there is little interference with the en-vironment.
Another major advantage of segment-al construction is that by a small varia-tion in the precasting operation (or ofthe jointing material between the seg-ments), a structure can be produced totake up any required horizontal or ver-tical curvature as well as the requiredsuperelevation.
Scope of Survey
There are two broad systems estab-lished using segmental construction.The difference between these two sys-tems lies in the formation of the jointbetween the adjacent segments.
On the one hand, there is the systemwhere a cast-in-place concrete joint
(mortar or grout joint) is used betweenthe precast elements. This joint is usu-ally of the order of 3 to 4 in. (75 to 100mm). This type of joint will not be con-s_dered in this report.
In the other and more prevailing sys-tem the jointing material is an epoxyresin which is of only very nominalthickness (not greater than 1722 in. or0.8 mm). Because of these very tighttolerances, special techniques must bedeveloped to fabricate segments withends suitable for epoxy resin jointing.
To minimize the joint thickness it isnecessary to obtain a perfect fit be-tween the mating ends of adjacent seg-ments. This is achieved by castingeach segment against the end face ofthe preceding one (so-called match cast-ing), and later erecting the segments inthe same order they were cast.
This state of-the-art report will onlyconsider segmental construction withmatch-cast segments in which the joint-ing material between adjacent segmentsis an epoxy resin.
PCI JOURNAL/January-February 1975ҟ
29
The author, whose companypioneered in the develop-ment of precast prestressedsegmental construction, re-ports on the state-of-the-artof this versatile and fast-growing construction tech-nique.The report confines itself tothe discussion of segmentalconstruction with match-castsegments in which the joint-ing material between adja-cent segments is an epoxyresin.Following a historical reviewof the development of seg-mental construction, designconsiderations are given fora typical segment and gluedjoint, choice of structuralsystem and erection tech-nique, selection of transversecross section, design of lon-gitudinal members, design ofpiers and stability duringconstruction, control of de-flections, precasting meth-ods, and hoisting equipmentneeded for erecting the seg-ments.The last part of the reportdescribes the application ofsegmental construction tofreeway overpasses and thelatest technique of progres-sive segment placing.It is concluded that precastsegmental construction isan efficient and economicalconstruction method for me-dium to long-span structures.
Historical Review
Although the principle of segmentalconstruction has been known by designengineers for at least a few decades, itwas not until 1952 that a single spancounty bridge in New York State, de-signed by the Freyssinet Co., was builtusing that concept.
The bridge girders were divided lon-gitudinally into three precast segmentswhich were cast end to end. After cur-ing, the segments were transported tothe job site where they were reassem-bled and post-tensioned with coldjoints.
This project was the first use, al-though admittedly on a small scale, of atechnique called match-making seg-mental construction. Since then thetechnique has been refined and has de-veloped into an important world wideconstruction method suited to a broadrange of applications and in particularbridges.
The first major commercial applica-tion of segmental construction was donein France 10 years later (1962). Thislandmark was the Choisy-le-Roi bridgeover the Seine River, south of Paris, de-signed and built by Entreprises Campe-non Bernard. Several other structures ofthe same type were built in due course.At the same time, the techniques ofprecasting segments and placing themin the structure were continually re-fined.
For example, the 10,000-ft longOleron Viaduct, built between 1964and 1966, made use of the launchinggantry for the first time. With thisequipment the segments could bemoved over the completed part of thestructure and placed in cantilever sec-tions over successive piers. Using thismethod an average rate of 900 ft (270m) of finished deck per month was at-tained.
As time went on, improvements weremade in precasting methods and in gan-
30
try design to allow for larger segmentsand longer spans which curved in plan.
The technique of segmental con-struction not only gained rapid accep-tance in France but spread to othercountries. For example, the Nether-lands, Switzerland and later Brazil andNew Zealand adopted the method.Many other countries are today usingthe segmental technique for various ap-plications. Currently, several bridges ofsegmental construction have been builtor are under contract in North America.
Meanwhile, the advantages of pre-cast segmental design have been ex-tended to short-span structures. Thisfield was previously reserved to otherconstruction methods such as cast inplace or precast girders.
Recent projects have shown that anurban elevated viaduct can be erectedat the rate of two to three completespans of 120 to 150 ft (35 to 45 m)per week. Also, a deck for freewayoverpasses could be constructed in lessthan one week at a cost substantiallylower than any other constructionmethod.
Figs. 1.1 through 1.20 present apanorama of some precast segmentalstructures built between 1963 and1973. The examples further show thatwith segmental construction it is pos-sible to attain many variations ofbridge shapes which are both function-al and esthetic.
Typical Segmentand Glued Joint
Although the concept of match-castsegments can be applied to many dif-ferent types and shapes of structuralmembers, special consideration shouldbe given to box sections since theseare the most commonly used units.
As can be seen from Fig. 2, there is adistinct difference between a precastsegmental deck and a precast girder
deck. For example, in the typical girderstructure, long slender members are as-sembled transversely to form a typicalspan with a conventional cast-in-placedeck. However, in segmental construc-tion, the method is based on the assem-bly by post-tensioning of segments laidlongitudinally end to end, thus compris-ing the total deck width.
Hence, precast segmental construc-tion can be considered to be an alter-native method to cast-in-place box gird-er construction. Very significantly,though, the use of segments eliminatesthe need for falsework but at the sametime retains the advantages of optimumdepth and structural rigidity.
Fig. 3 shows typical sections of asimple box segment. This type of seg-ment would be appropriate for a canti-levered roadway slab such as wouldaccommodate a two-lane bridge.
The segments are cast in seriesagainst each other preferably in thesame order as they will be assembledsubsequently in the structure. Thejoints are perfectly matched and conse-quently there is no need for convention-al mortar joints except at certain spe-cific locations.
End sections such as (1) are providedwith keys in the webs (2), to insure tem-porarily the transfer of shear stressesduring construction while an additionalkey (3) allows transverse alignment dur-ing erection.
Ribs and/or anchor blocks are nowcommonly used inside the box girderfor anchoring the permanent longitudi-nal prestressing units and/or for tem-porary assembly of the segments.
Fig. 4 shows how a typical segmentis assembled to the rest of the structureby temporary devices placed, for exam-ple, at the top and bottom slab levels.The two loads, F1 and F2, can be re-solved together with the segmentweight, W, into a resultant, R, inclinedwith respect to the joint between seg-ment (S) and the previous one.
Fig. 2. Comparison between a precast girder deck system and a segmentaldesign.
LONGITUDINAL SECTION A-A
(I) JOINT
(2) WEB KEY(3) SLAB KEY FOR ALIGNMENT(4) POSSIBLE WEB STIFFENER FOR
TENDON ANCHORAGE
(5) HOLES OR INSERTS FOR HANDLINGAND PROVISIONAL ASSEMBLY
(6) LONGITUDINAL DUCTS FORPRESTRESSING TENDONS
TRANSVERSE SECTION B-B
HORIZONTAL SECTION C-C
Fig. 3. Typical precast segment (contrast with Fig. 5).
40
F1
F F1+F2R2 R2 G
R
^ ^5)
F2
W
(4a) PROVISIONAL ASSEMBLY OF SEGMENTS
DETAIL (A
(4b) SEGMENTS IN FINISHED STRUCTURE
Fig. 4. Typical segment in relation to finished structure and force system.
Epoxy JointAt the time of assembly the thin
epoxy glue used in the joint acts onlyas a lubricant. Thus, the epoxy jointhas little shear or friction capabilityand the shear force through the jointcan be carried only by the web keys.Together, the keys contribute, with re-action Rl (perpendicular to the lowerface of the interlock), to the stability ofthe unit while the balance R2 of the re-sultant force induces compressivestresses in the fresh joint.
It is considered prudent to select thelocation and magnitude of the tempo-rary prestress forces, Fl and F2, in sucha way so as to induce compressivestresses in the total height of the sec-tion even to the point of obtaining auniform stress distribution (R2 then be-ing at the level of the center of gravity).
In general, the fresh glue in the jointhas a dual purpose during construc-tion, i.e., as a lubricant and as a meansof achieving perfect matching.
In the completed structure the hard-ened glue will further provide the jointwater tightness and contribute also tothe structural rigidity of the members.In cold climates where deicing materi-als are used on bridge pavements, prop-er treatment of the joint at the roadwayslab level is essential. Tightness of thejoint is also essential for efficient cementgrouting of the prestressing units acrossthe various joints where ducts are ne-cessarily interrupted.
It should be noted that in the earlydevelopment of segmental constructionthe main purpose of the glue was totransfer across the joints both bendingand shear stresses between adjacentsegments. Epoxy glues properly select-ed, mixed, and placed show strengthswell above those of concrete, althoughit is difficult to restore the tensilestrength of concrete across a joint.
This attribute of epoxies was neverconsidered mandatory because segmen-tal structures are usually fully pre-
PCI JOURNAL/January-February 1975 41
KEY
(1) Castellated web key. (4) Tendon duct and anchoragefor final assembly.
(2) Slab key for (5) lnsert for handling andalignment, temporary assembly.
(6) Tendon ducts for temporary(3) Web stiffener. assembly.
Fig. 5. Precast segment with multiple keys and internal stiffeners (contrastwith Fig. 3).
stressed and thus no appreciable ten-sile stresses develop at any point acrossthe joints. It is more important for theglue to have the capability of transfer-ring shear stresses which may reach amagnitude of 600 psi (42.3 kg/cm2).
Improper choice or use of the epoxyglue may be critical with respect toshear strength. However, quality con-trol is about the same for epoxy glues asfor any other structural material such asconcrete (including grouts and admix-tures).
The next refinement in the evolutionof epoxy joints was the development ofa method whereby the epoxy glue couldhe relieved of any structural function.This improvement was thought to havethe advantage of simplicity, safety, andcost. The new multiple key designs em-body this concept (see Fig. 5).
Webs and chords of the section are
provided with a large number of smallinterlocking keys designed to carry allstresses across the joint with no struc-tural assistance from the glue. Today,the major purpose of epoxies used insegment joints is three-fold:
1. To lubricate the adjoining sur-faces of the segments.
2. To perfectly match the adjoiningsegments.
3. To provide water tightness anddurability at the joints.
A further improvement to the origi-nal concept of the glued joint was re-cently brought about with regard to thetensile strength of the joints and thecontinuity of the non-prestressed lon-gitudinal reinforcement. There aretoday satisfactory ways of placingdowels or tensioned bolts across thejoints.
42
Choice of StructuralSystem and Principle
of Construction
In general, the precast segmentaltechnique is closely related to the meth-od of construction and the structuralsystem employed.
This is the reason why precast seg-mental design has often been identifiedwith cantilever construction which wasused in most applications. However,other construction methods are avail-able which will be discussed below.
Cantilever constructionIn this construction method the seg-
ments are placed in balanced cantileverstarting generally from a pier in a sym-metrical operation.
The designer should always remem-ber that construction proceeds withsymmetrical cantilever deck sectionscentered above the piers and not withcomplete spans between successivepiers. For a typical three-span struc-ture, the side spans should preferablybe only 65 percent of the main centerspan (instead of 80 percent in conven-tional cast-in-place structures). This isdone to reduce to a minimum thelength of the deck portion next to theabutment which cannot be convenient-ly built in balanced cantilever (see Fig.6).
(6a)
LI 112 (LI+L2) L2
(6b)
(6c)Fig. 6. Cantilever construction showing choice of span lengths and location
of expansion joints.
PCI JOURNAL/January-February 1975ҟ 43
(7a ) END RESTRAINT IN ABUTMENT
(i+)ҟ(.J2)ҟM
(7b) CONVENTIONAL BEARING ON ABUTMENT
aҟM
(7c) ANCHOR FOR UPLIFT IN ABUTMENT
Fig. 7. Construction of end spans.
Where two different spans Li andL2 (for example at the transition be-tween approaches and main spans in aviaduct), an intermediate span of aver-age length will optimize the use of thecantilever concept.
Individual cantilever sections aresubsequently assembled into a continu-ous deck in most cases and no perma-nent hinges are maintained near thecenter of the various spans. Continuousdecks up to more than 2000 ft (600 m)long have been built in this manner andhave been satisfactory to both the own-er (because of low maintenance cost)and the user (because of the good rid-ing surface).
On very long structures, however,intermediate expansion joints are neces-sary because of volume changes. Thesejoints should preferably be located nearthe contraflexure point to avoid the ob-jectionable angle break which often de-
velops when the joint is located at mid-span.
Construction of end spansBecause of unavoidable requirements
in the layout of piers, it may not alwaysbe possible to select the optimum spanarrangement. Thus, the end spans mayvery well be much above or below theoptimum length (see Fig. 7).
(a) Long end spans—In this case, abridge deck may be extended over theabutment wall to provide a short addi-tional span.
With reference to Fig. 7 a conven-tional bearing (1) is provided over thefront abutment wall, while a rear pre-stressed tie (2) will oppose uplift andpermit cantilever construction to pro-ceed outwards from the abutment up tothe joint section (JI) where a connec-tion is made with the cantilever con-struction starting from the first inter-
44
mediate pier.(b) Normal end spans—Here, a spe-
cial segment is placed over the abut-ment and one or two segments are tem-porarily cantilevered out so as to reachthe first balanced cantilever constructedfrom the next pier.
(c) Short end spans—In this situation,cantilever construction starts from thefirst pier and reaches the abutment onone side well before the midspan sec-tion of the following span. An uplift re-
action has to be transferred to the abut-ment during construction and in thecompleted structure.
Consequently, the webs of the mainbox girder deck are cantilevered overthe expansion joint into slots providedin the main abutment wall (see Fig. 8).The neoprene bearings are placedabove the web cantilever rather thanbelow to transfer the uplift forcewhile allowing the deck to expand free-ly.
LONGITUDINAL SECTION
SECTION C-C
Fig. 8. End span anchor in abutment for uplift.
PCI JOURNAL/January-February 1975ҟ 45
(DIMENSIONS IN METERS)
LWLIit3.66
13.144 3.664
1 4.00
0NIV
LI I4.114.8110.60
6.0077816.60
9.50 815.25
iM
20.40
BRIDGE(AND MAX. SPAN)
CHOISY-LE-ROI(55M)
SEUDRE(79 M)
BLOIS(91 M)
CHILLON
(104 M)
SAINT ANDREDE CUBZAC
(95M)
B3 SOUTH
(50 M)
SAINT-CLOUD(106 M)
SEGMENT MAXIMUMLENGTH SEGMENT WT.
(TONS)
2.50M 258.20 FT
3.30Mҟ7510.80 FT
3.50Mҟ75I1.50FT
3.20 M 8010.50 FT
3.40M 8011.20 FT
2.50M-3.40M 508.20FT-II.2OFT
2.25M 1307.40 FT
Fig. 9. Evolution of segment shape and weight.
Selection of TransverseCross Section
In general, box girders have beenshown to be best suited to most de-sign and construction requirements. Inaddition, the torsional rigidity of thesection provides excellent stability dur-
ing construction and later during thelife of the structure.
Because cantilever construction in-duces high dead load moments overthe supports, the large bottom chordrequired to carry the compressivestresses is conveniently obtained in thebox design. At ultimate, the strength ofthe compression flange is such that the
46
limiting capacity of the prestressing ten-dons is exhausted thus avoiding anearly brittle concrete failure.
Depending upon the deck width, thedesign of the transverse cross sectionwill vary. For example:
1. Up to 40 ft (approx. 12 m), a con-ventional single box with two webs maybe used.
2. For wider decks, several individu-al boxes are assembled transversely byprestressing. However, for single boxgirders, a more refined design is used:three or four webs (with either verticalor tapered facia) for widths up to 70 ft(approx. 21 m).
3. For intermediate widths, singlebox girders may be used in conjunctionwith a ribbed roadway slab (e.g., St-Andre de Cubzac Viaducts) or boxedcantilevers (e.g., Chillon Viaducts).
Fig. 9 summarizes some pertinent de-sign features of various cross sections.The bridge sections further show thetrend towards an increase in size andunit weight of the segments. Note, forexample, the difference between:
(a) Choisy-le-Roi (1962) where twoparallel box girders with 25-ton seg-ments were used for a 46-ft (14 m) widedeck, and
(b) Saint-Cloud (1972) where a sin-gle box girder with four webs and 130-ton segments were used for a 67-ft (20m) wide deck.
Design ofLongitudinal Members
Shape of deck in elevationA constant depth design has the ad-
vantage of simplicity for constructionand is sometimes preferred for estheticreasons. However, the constant depthsection is not economical in the presentstate of the art for spans much longerthan 200 ft (60 m).
For longer spans, a variable depthwith circular soffits or straight haunchesshould be used because the deeper see-
tions are better able to resist the canti-lever moments.
Design of longitudinal prestressTo resist the increasing dead load
moments over the piers during the can-tilever operation, prestressing tendonsare placed at each step of the erection.Running in the deck slab through pre-formed holes, tendons are anchoredsymmetrically in each pair of segments.
It might be noted that in earlierstructures, anchors were placed in theouter face of the segments. However,there is now a tendency to have the an-chors in the haunches inside the boxsection because it is preferable toplace the segments and stress the ten-dons in two independent operations.
Continuity of individual cantileversmeeting at the center of the successivespans is achieved by another series ofprestressing tendons installed afterplacing the drop-in segment and castingthe joint between adjacent deck sec-tions. Most of the tendons run at thesoffit level to resist the positive bend-ing moments along the spans whilethe cantilever prestress is designed toresist all dead and live moments overthe supports.
Overlapping the two series of pre-stressing tendons is often achieved bydraping most of the tendons in thewebs. If straight tendons are used, avertical web prestressing is generallyrequired for that purpose. This verticalprestress also takes care of shear stress-es, particularly in constant depth decks.
Redistribution of momentsdue to concrete creepand prestress losses
In continuous structures the finalstresses in the completed structure aresubstantially different from what theywere initially during construction.However, subsequent volume changesin the materials will induce deforma-tions which will tend to make the ini-tial and final stresses get closer to each
PCI JOURNAL/January-February 1975 47
other. This means that there will be aredistribution in the moments of thestructure.
In general, the negative momentsover the piers will decrease while thepositive moments at midspan will in-crease by a corresponding amount.
This redistribution of momentsshould be allowed for in the design.
Ultimate capacity of structureBecause the moments near the cen-
ter of the spans are relatively small inthe elastic stage, the amount of continu-ity prestress is usually substantially low-er than that of cantilever prestress.Thus, there is little need for tendons atthe deck slab level.
The capacity of the structure to with-stand reversals (for example, in an un-loaded span when a live load is ap-plied in the adjacent spans) is conse-quently very limited. Thus, the ultimatestrength should be carefully checkedsince it may be necessary to place ad-ditional tendons at midspan to insurecontinuity between the longer cantile-ver tendons at deck slab level. Thischeck is important because a negativeplastic hinge must not be allowed to
appear before the other sections havereached their ultimate capacity.
Design of Piersand Stability
During Construction
Piers with many different shapeshave been used in conjunction withcantilever construction. For example,single piers, double piers, and moment-resisting piers have all been used.
Single slender piersIf in the finished structure the piers
are designed solely to transfer the deckloads to the foundations (includinghorizontal loads), there is the likelihoodthat the piers will be unable to resistthe unsymmetrical moments due to thecantilever construction (i.e., with onesegment plus the equipment load).
Thus, temporary shoring is often re-quired (see Fig. 10) at considerablecost.
More recently, the stability of thecantilever under construction has beenprovided by the equipment used forplacing the segments.
Fig. 11. Concrete strains versus age and duration of loading. Note that strainis given as a dimensionless ratio between the actual strain and the reference
strain of a 28-day old concrete subjected to short-term load.
Twin piers
With double piers, two parallel con-verging walls make up the pier struc-ture, which usually rests on a singlefoundation.
Such a configuration was successfullyused at Choisy-le-Roi, Courbevoie, andJuvisy, and later for the Chillon Via-ducts. Stability during construction isexcellent and requires little temporaryequipment (except for some bracing be-
tween the slender walls to preventelastic instability).
Moment resisting piers
These piers are designed to with-stand the unbalanced moments duringconstruction while temporary verticalprestress rods make a rigid connectionbetween the deck and the pier cap.
When the ratio between span lengthsand pier height allows it, the above
PCI JOURNAL/January-February 1975 49
SEGMENTS N°ҟz
02ҟ3ҟ4 5– chi
oҟ 2 --^ 3ҟENVELOPE OFaҟ̂/ҟ DEFLECTION CURVES
1N
ec1 4
a3 ^^^^ \\
oC4 ^
a5
Fig. 12. Deflections of a typical cantilever.5
rigid connection and the correspondingframe action may be maintained per-manently between the deck and piers.
Another method is to use twin neo-prene bearings which allow for deck ex-pansion. With this device full advan-tage is taken of the elastic restraint be-tween the piers and the deck and thecapacity to transfer live load momentsat midspan.
Flat jacks are usually placed betweenthe pier top and the deck soffits to per-mit the substitution of temporary bear-ings for the permanent neoprene pads.
Calculation and
Control of Deflections
The key to understanding deflectioncontrol in segmental structures is to beable to accurately predict the long-term
behavior of concrete under sustainedloading applied at different ages. Fig-ure 11 shows the variation of concretestrains in relation to age and duration ofload. Note that in Fig. 11 the strain isgiven in terms of a dimensionless ra-tio between the actual strain and thereference strain of a 28-day old con-crete subjected to a short-term load.
Short-term strains vary little with theage of concrete at the time of first load-ing (except at a very early age).
However, long-term strains are sig-nificantly affected by the age of con-crete. For example, a 3-day old con-crete will show a final strain 2% timesgreater than a 3-month old concrete.
Fortunately, in precast constructionthese deformations are significantly re-duced because the segments are usuallystored a few weeks before final erec-tion. Thus, precasting the segments is a
50
distinct advantage over casting them inplace because the deflections of a pre-cast cantilever are usually less than one-half those of an equivalent cast-in-placecantilever.
A step-by-step computation of thedeflection curves of a cantilever is madeto follow the erection sequence. A typi-cal plot of this calculation is shown inFig. 12. An accurate determination ofthe deflection will also be needed tocompute the necessary camber usedduring segment casting in order to ob-tain the required profile of the finisheddeck.
Precasting Methods
There are basically two methods inuse for precasting segments, namely,casting beds and casting machines.
1. Casting beds—In this method, anentire or one-half a cantilever scan is
cast on a bed which reproduces exactlythe profile of the deck soffit with dueallowance for camber.
2. Casting machines—In this secondmethod, single casting units are de-signed to have a variable geometry cor-responding to the bridge profile.
Fig. 13 shows the principle of thecasting bed system. The position ofeach segment is fixed and the formworktravels along the bed. When many setsof formwork are available, several dif-ferent segments may be cast at thesame time.
For a small structure, it may be suffi-cient to use a bed for only one-half of acantilever.
Casting beds have been used for sev-eral structures such as Choisy-le-Roi(see Fig. 14), Courbevoie, and Oleron.
In the casting machine method, theformwork remains fixed while the seg-ments progress from the casting posi-tion to the match-cast position as shown
_SEGMENTS BEING CAST
SEGMENTS COMPLETED
Fig. 13. Typical precasting bed.
PCI JOURNAL/January-February 1975ҟ 51
Fig. 14. Choisy-Ie-Roi showing seg-ments on precasting bed.
in Figs. 15a and 15b.The mold soffit remains with each
segment until removed for storage. Thissoffit is equipped for longitudinal trans-fer and position adjustment. The ex-ternal forms are usually hinged for easystripping.
One end of the machine is blankedoff by a fixed bulkhead while the otherend matches the preceding segment.The internal form is of the collapsibletype (with removable lower panels forheight variation as required). It may beretracted through the bulkhead to leavethe space entirely free for placing theprefabricated cage of reinforcing steeland prestressing ducts. An overall viewof some typical casting machines isshown in Fig. 16.
One segment can be easily cast perday if adequate curing is provided andif preheated concrete is used in coldweather.
Vertical casting of the segments(they are cast on edge) is an improve-ment on the horizontal casting tech-nique because it allows easier concreteplacing and vibration.
Horizontal casting was first used atthe Pierre-Benite bridges for a straightstructure. The concept was extended tocurved bridges with variable heightgirders such as the Chillon Viaducts.However, on the B3 South Viaducts thesheer magnitude of the project and thetime constraint on construction made
mass production necessary. A centralplant produced two units per day forseveral months on each of the five cast-ing machines.
Placing Segments
Several methods have been devel-oped for placing the precast segmentson the superstructure. They can beclassified as follows:
CranesMobile cranes moving on land or
floating on pontoons are commonlyused where access is available. For ex-ample, Choisy-le-Roi, Courbevoie, andthe Belt bridges were constructed usingmoving cranes.
Occasionally, a portal crane strad-dling the deck has been used withtracks installed on temporary trestles oneither side of the bridge.
Winch and beamIn this method a lifting device, at-
tached to an already completed part ofthe deck, raises the segments whichhave been brought to the bridge site byland carrier or barge. The segments arelifted into place by winches carried atdeck level on a short cantilever mecha-nism anchored on the bridge.
In the first applications (for example,Pierre-Benite and part of the Beltbridges) the segment over the pier hadto be placed independently (either castin place or handled by a separate mo-bile crane).
Recently, this drawback has beenovercome (for example, the Saint-Andrede Cubzac Viaducts). Now the precastpier segment may be placed on the pierwith the same basic equipment canti-levered temporarily out of a tower at-tached to the pier.
Launching gantryIn this method a special mechanism
travels along the completed spans and
52
TO STORAGE
CONJUGATE UNIT
BLANK END
INTERNAL FORMWORKMOLD BOTTOM
Fig. 15a. Formwork used in casting segments.
BLANK ENDҟ TO STORAGE
Fig. 15b. Steps used in precasting operation.
Fig. 16. Precasting machine for Paris Belt bridges.
maintains the work flow at the decklevel.
The crane gantry, which was firstused for the Oleron Viaduct, contribut-ed significantly to the development ofprecast segmental construction.
The principle behind segmental erec-tion using the crane gantry system isshown in Fig. 17. An essential compo-nent in the system is a truss girderwhich has a length somewhat greaterthan the maximum bridge span.
The system consists essentially of:1. A main truss where the bottom
chords act as rolling tracks.2. Three leg frames which may or
may not be fixed to the main truss.Note that the rear and center framesallow the segments to go through theirend ways.
3. A trolley which can travel alongthe girder and is capable of longitudi-nal, transverse, and vertical movementas well as horizontal rotations.
To complete a full construction cyclefor a typical span, the gantry assumesthree successive positions:
1. For placing typical segments incantilever, the center leg rests directlyover a pier while the rear leg is seatedtowards the end of the previously com-pleted deck cantilever.
2. For placing the segment over theadjacent pier, the girder is moved alongthe completed deck until the center legreaches the end of the cantilever. Thefront leg rests on a temporary corbelfixed to the pier while the pier segmentis placed and adjusted into position.
3. Finally, the segment placing trol-ley is used as a launching cradle withthe help of an auxiliary tower bearingon the newly placed pier segments. Thegantry is then transferred to its initialposition one span further thus allowingthe segment placing cycle to repeat it-self.
For structures combining vertical andhorizontal curvatures, including vari-able superelevation, the launching gan-try can be designed to follow the geo-metry of the bridge while maintainingoperational stability and segment plac-ing capability.
54
52.00 54.0010 6. 00
106.00
Fig. 17. Operational stages of a launching gantry (first type).
In the last few years several impor-tant technical improvements have beenmade in gantry design. These advance-ments are exemplified starting at theChillon Viaducts in Switzerland and lat-er at the Saint-Cloud bridge where130-ton segments were easily placed ina 337-ft (102 m) span with a 1090-ft(330 m) radius of curvature in place(see Fig. 1-20).
It should be noted that on certainstructures, a somewhat different ap-proach is used in designing the launch-ing gantry system (see Fig. 20).
The total length of the truss girder isnow slightly greater than twice themaximum span length. In this system,all three gantry supports rest directlyover a pier. Although the investmentcost is higher in this system than in theoriginal concept, this type of gantry hasseveral advantages. For example:
1. The completed deck carries nogantry reactions.
2. Stability against unsymmetricalloading due to unbalanced cantilevererection may be provided by thegantry.
3. The pier segment may be placed
PCI JOURNAL/January-February 1975ҟ 55
Fig. 18. Oleron gantry.
and adjusted during the normal placingcycle for the preceding cantileverspans.
4. Construction time may be furtherreduced if two placing trolleys arcused.
In the advanced system, segmentsmay be moved in place over the com-pleted bridge (or beneath the bridge).This procedure was used on the largeRio-Niteroi bridge where all segmentswere floated on pontoons and lifted intoplace by four 540-ft (164 m) longlaunching gantries weighing 400 tonseach (see Fig. 1-17). A similar ap-proach was also used for the B3 SouthViaducts near Paris (see Fig. 1-18).
Segmental Design forFreeway Overpasses
A recent example of the wide ap-plicability of precast segmental con-
struction is demonstrated in the Rhone-Alpes motorway. This project willinvolve the construction of 150 over-passes to be built over a 5-year period.The bridges are three-span structureswith main spans ranging from 60 to 100ft (18 to 30 m).
Some significant features includevertical precasting of segments, thecomplete elimination of the conven-tional closure joint, and the use of con-ventional prestressing cable profiles in-stead of the cantilever-type cable ar-rangement.
Stability during construction is pro-vided by secondary supports close tothe piers and temporary prestress us'ngbars anchored along the deck surface(see the diagram in Fig. 21). The totalconstruction time for a single overpass(foundations plus piers and deck) isless than 2 weeks (see Fig. 22).
Fig. 23. Construction sequence (elevation) using progressive segment placing.
TOWER
IDERM
Fig. 24. Construction sequence (isometric view) using progressive segmentplacing.
PCI JOURNAL/January-February 1975ҟ 59
Fig. 25. Saint-Cloud (completed structure).
Progressive Placing
The latest development of precastsegmental construction embodies theconcept of progressive placing. Thisapproach actually comes directly fromcantilever design. Here, segments areplaced continuously from one end ofthe deck to the other in successivecantilevers on the same side of the vari-ous piers rather than in balanced canti-lever at each pier.
When the deck reaches one pier,permanent bearings are installed andconstruction proceeds to the next span.
Some noteworthy advantages of themethod are:
1. The operations are continuousand are performed at the deck level.
2. The method seems to be of inter-est primarily in the 100 to 160-ft (30 to50 m) span range where conventionalcantilever construction is not always
economical.3. During construction the piers are
not subjected to unbalanced momentsalthough the vertical reaction is sub-stantially increased.
One disadvantage of the method isthat construction of the first span mustbe carried out with a special system.
It should also be noted that thestresses in the deck are completely re-versed during construction and aftercompletion.
Consequently, special stabilizationdevices must be used temporarily tokeep the concrete stresses within safelimits and to minimize the amount oftemporary prestress.
A tower and guy cable system hasbeen used effectively to control the un-desirable temporary stresses.
Figs. 23 and 24 show schematicallythe principle of progressive segmentplacing together with some of the con-struction details.
Conclusions
Following this brief survey of precastsegmental construction with match-castglued joints, we may now summarizethe principal conclusions of the study.
1. Precast segmental constructionhas been shown to be an efficient andeconomical construction method for me-dium to long-span structures.
2. The method is applicable to mosttypes of structures. Even structures thatare curved in plan with variable super-elevation may be easily accommodated(see Fig. 25).
3. By precasting the segments undercontrolled factory conditions, high qual-ity units with precise dimensioned tol-erances are obtained.
4. With precasting, the concreteshrinkage and creep will have a great-ly reduced effect both during erectionand in the completed structure becausethe segments will already have attainedmost of their desired strength.
5. Falsework and temporary supportsare usually completely eliminated aswell as the related hazards due to un-known soil conditions.
6. Where clearance requirementsduring construction are critical, theremay be further savings on the approachstructures because no head room isneeded for falsework.
7. Interference with existing trafficduring construction is significantly re-duced and expensive detours can beeliminated.
8. The speed of erection may bechosen to suit the job requirements.Segmental construction is much faster
This report is based on a talk presentedat the FIP/PCI Congress, New York City,May 28, 1974.
Discussion of this report is invited. Pleaseforward your discussion to PCI Headquar-ters by June 1, 1975.
than any other currently used erectionmethod.
9. Construction is not greatly in-fluenced by weather conditions. Theprecast segments can be manufacturedwith mass production techniques in en-closed factories and brought to theproject site at any desired time.
10. The overall labor requirementis less than for conventional construc-tion methods while a major part of thework force on site is replaced by plantlabor.
11. The cost savings in comparisonto conventional construction methodshave reached 20 percent on severalprojects. For example, in France, oneprecast segmental bridge design hasbeen shown to be one-half the cost ofa similar steel bridge design.
12. Last but not least, the techniqueshows an exceptionally high safety rec-ord. If all the structures designed andbuilt or supervised by the author's com-pany are combined (representing morethan 6,000,000 sq ft (560,000 m 2) ofdeck and 17,000 segments), only oneincident is found where one segmentwas dropped and lost (with no injuryto a workman). Furthermore, no acci-dent has been reported on projects us-ing launching gantries.
With the experience and successgained during the last decade it may beconcluded confidently that precast seg-mental construction is today competitivein a very wide range of applicationswith other materials and constructionsystems while adding a further refine-ment to the intrinsic advantages of pre-stressed concrete.
