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Construction Techniques forSegmental Concrete Bridges.i.•2•

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.i.•Z•James M. BarkerAssociateH. W. Lochner, Inc.Consulting EngineersChicago, Illinois

One of the primary advantages ofsegmental concrete bridge con-

struction is the economics. In a largemajority of cases, segmental construc-tion has been the winner where alternateconstruction methods have been avail-able to contractors at the time of bidding.

There are many reasons for this rela-tively new method of bridge constructionin the United States competing so well inthe 10 years since the European transfertechnology was started. I believe, how-ever, that the principle reason for thesuccess of segmental concrete con-struction is the number of constructiontechniques available to build thesebridges. Of approximately 35 to 40 suchbridges either completed, under con-

NOTE: This article is based on a presentation givenat the Long Span Concrete Bridge Conference inHartford, Connecticut, March 18-19, 1980. Theconference was sponsored by the Federal HighwayAdministration, Portland Cement Association, Pre-stressed Concrete Institute, Post-Tensioning Insti-tute, and Concrete Reinforcing Steel Institute.

struction or in design in the UnitedStates, few have been or will be con-structed in exactly the same manner.The multitude of choices available tocontractors allows them to tailor eachproject to their manpower and equip-ment in the interest of maximizing effi-ciency and optimizing cost.

Segmental bridge construction is alsorevising the basic thinking of design en-gineers. Until recently, designers haveconcerned themselves mainly on how tobuild the project after preparing compu-tations and plans. Segmental construc-tion has revised this thinking. The firstquestion asked about a project now is"What is the best and most economicalway to build this project?" Once thisquestion has been satisfactorily an-swered, the designer can proceed with adesign based on the most efficient con-struction method.

In order to make intelligent decisions,both designers and contractors need tobecome familiar with the available

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0FFIT

ARRIAGE

CCC e ^ ^^^1^i1i1^ooo oo o O

Fig. 1. Schematic of short line match casting system. The usual rate ofproduction is four segments per week per set of forms.

methods of segmental construction. Notall methods are applicable to all projectsbut all should be considered for eachjob. There are several fundamental con-cepts relating to casting and erection ofsegments which could be considered forpractically every bridge project. By nomeans should these basic concepts bethe only ones considered. Segmentalbridge construction was born out of in-novation and will continue to growthrough more innovation by both con-tractors and design engineers.

Precasting TechniquesShort line system—All the casting

methods to be discussed utilize the con-cept of match casting. The basic prem-ise of match casting is to cast the seg-ments so their relative erected positionis identical to their relative casting posi-tion. This requires a perfect fit betweenthe ends of the segments and is accom-plished by casting each segment directlyagainst the face of the preceding oneusing a debonder to prevent bonding ofthe concrete. The segments are thenerected in the same sequence they werecast.

The most common method for matchcasting segments is called the "shortline" method. Fig. 1 shows a schematicof a short line match casting system.

PCI JOURNAUJuly-August 1980 67

Fig. 2. Casting machines (forms) fold back hydraulically or withscrew jacks to permit moving of segments.

With this system, the rate of segmentproduction will approach one segmentper line of forms per day. A good aver-age to use for a project is four segmentsper line every 5 days.

For the sake of explaining the castingprocedure, assume today is Wednes-day. The older segment was cast onMonday and is now cured and ready forthe storage yard. The old segment wascast yesterday or Tuesday and wasmatch cast against Monday's segment.Today a new segment will be castagainst Tuesday's segment.

Fig. 2 shows the form arrangementfor short line match casting. The Frenchcall these forms casting machines.The appropriateness of this name willsoon become apparent for they reallyare machines. The length of the sideforms is equal to the length of the seg-ment being cast plus 1 or 2 in. (25 or 51mm) to seal around the match castjoint. The side forms have the capabilityof being folded back away from thesegment to permit removal of the seg-ment. This is done either with screwjacks or hydraulic rams. The collapsibleinside formwork which forms the void ofthe box girder rolls on rails to allow re-moval of the form, enabling the segmentto be lifted vertically.

When the workers arrive at the pre-casting yard on Wednesday morning,the side forms are closed, the insideform is rolled forward, Tuesday's seg-ment is in the new segment position andMonday's segment is in the old segmentposition. The first operation is to deter-mine the relative position as the seg-ments actually were cast. This is doneby shooting elevations and centerlinewith an accurate survey instrument.These shots are called "early morningshots." (Geometry control will be dis-cussed in greater detail later.) Once theearly morning shots are taken, the formsare released and Monday's segment istaken to the storage yard. Tuesday'ssegment is then moved from the newsegment position to the old segment po-sition.

All of the geometry of the bridge (hori-zontal or vertical curves and super-elevation or transitions) is cast in by ad-justing the old segment. The forms arenever adjusted for geometry. Therefore,once Tuesday's segment is in the oldsegment position, its attitude is adjustedby screw jacks between the carriage andthe soffit to provide the proper bridgegeometry. A prefabricated reinforcingbar cage is then set in the new segmentposition. Once the side forms are closed

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Fig. 3. Casting machine. Proper geometry is obtained by adjusting the alignment ofthe cast-against (concrete) segment with screw jacks under the supporting soffit.

and the inside form is rolled forward, thecasting machine is ready for casting to-day's (Wednesday's) segment.

Fig. 3 shows a casting machine for ashort line match cast system. Note the

screw jacks for adjusting the attitude ofthe old segment.

Fig. 4 is a schematic of the geometrycontrol layout for a short line castingmethod. The survey instrument is

PERMANENTPERMANENT TARGETINSTRUMENT

^ I

STEELBULKHEAD IH NEW

SEGMENT

7/'7',177/77/77/ 77777/77?/

PERMANENTSOFFIT

Fig. 4. Geometry control layout for short line system. The instrument support andtarget should be stationary and permanent.


nent survey control points.its are located over theninate any influence of toprents. The center points9oretical centerline.

ally a theodilite capable of measuringaccurately to 1/32 of an inch (nearly 1mm). The permanent target is generallya concrete pile driven into the groundand insulated to prevent bending due tothe sun shining on one side. Elevationsare shot using a survey rod. A metric rodmay be more practical because thesmallest graduations are approximately1/32 of an inch which eliminates some ofthe guesswork involved when using rodsmarked in feet and inches. The datameasured are elevation differences sothe metric rod does not result in exten-sive unit conversion.

The segment survey control point po-sitions are shown in Fig. 5. Each seg-ment has six control points—four overthe two webs and two on the centerline.

Round-headed bolts placed 2 or 3 in.(51 or 76 mm) from the edge of segmentare used for the elevation control points.They are assumed to be at the edge ofthe segment for computation purposesand should always be placed over thewebs to eliminate any influence of topslab deflections. Since only relative po-sitions of segments are of concern,these bolts do not have to be placed atany specific elevation but may be placedin the wet concrete so the bottom of thehead is approximately at the concretelevel. The early morning shots on thesepoints establish the basis of relative po-sitions.

The two centerline control points usu-ally consist of U-shaped wires placed inthe wet concrete after the top slab has

ERECTED CANTILEVER

FINAL PROFILE

THEORETICAL \ a

CASTINGCURVE

PIERLOCATION

A = DEFLECTION DUE TO PRESTRESS

Fig. 6. Theoretical casting curve is drawn from data provided by the design engineer.

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been finished. The theoretical centerlineis established by notching these wireswith a hammer and chisel during theearly morning shots.

The designer of the bridge will provideinformation to develop a theoreticalcasting curve. The theoretical castingcurve is a curve along which the seg-ments should be cast so the final de-sired alignment will be achieved after alldeformations. The computation of thesedeformations is quite intricate since mostare time dependent and interdependent.Thus, a good computer program isneeded for maximum accuracy. Amongthe causes of deformations are selfweight of the structure, camber due toprestress, prestress losses, creep andshrinkage of the concrete and tempera-ture variations.

For the sake of simplifying this discus-sion, let us consider the deformation ofcamber due to prestressing. Fig. 6shows a crest vertical curve as a finaldesired alignment. Assuming balancedcantilever erection, the erected can-tilever would deflect upward an amountA due to the prestressing as represented

by the erected cantilever curve in Fig. 6.Therefore, it is obvious the segmentsmust be cast with -a downward deflectionof A so when the camber occurs theproper alignment will be achieved. Acurve depicting this downward deflectionis the theoretical casting curve. In reality,when all the deformations are consid-ered, the theoretical casting curve usu-ally bends upward rather than down-ward.

Fig. 7 (top) shows the theoreticalcasting curve developed previously.Since segments cannot be cast curved,the curve is approximated by castingsegments on the chords. This is the pro-cedure followed whether the curve ishorizontal or vertical. Therefore, chordsequal to the length of the segments arelaid out on the theoretical casting curveso a tangent to the curve can be drawnat the points of intersection of thechords. Angles B, and B 2 can then bemeasured from the local tangent defin-ing the desired relative position of thesegments as they are match cast anderected. This must then be related to theposition of the casting machine.

B,

THEORETICAL CASTINGCURVE

CASTING 8 E RECTI ONDIRECTION

^HORIZONT^^

VERTICAL Z NEW SEGMENTi 0tnmininrnm

Fig. 7. General method for determining relative position of segments to obtain thedesired geometry.


B2

R +R

AS-CAST CURVE

JB,+B2+C

Lt®viCA ST ING a ERECTION _ \ \ J

DIRECTION

HORIZONTALB1+BZ+C

I-- -- —

VERTICAL NEW SEGMENT OLD SEGMENT2 O

177/77/77 ,77,7 rirrim

THEORETICALCASTING CURVE

Fig. 8. Segments must be adjusted to compensate for casting errors.

Two assumptions relative to the cast-ing machines must be made—the firstcan be controlled, the second cannot.The first assumption is that the steelbulkhead at the opposite end of the newsegment from the cast-against segmentis established and maintained absolutelyvertical with the top being completelyhorizontal. In addition, the bottom soffitis established and remains absolutelyhorizontal. The second assumption isthat the segment being cast is perfect.While this second assumption is not tooimportant to this explanation, it is verysignificant when performing actualgeometry control procedures.

To transfer the segment relationshipfrom Fig. 7 (top) to the casting machine,one has to examine the direction ofcasting and erection. In this case Seg-ment 1 is cast and erected before Seg-ment 2. Therefore, on the castingmachine Segment 1 is in the old positionand Segment 2 is in the new segmentposition as shown by Fig. 7 (bottom).Remembering the steel bulkhead lo-cated on the left side of Fig. 7 (bottom)

is always vertical and the soffit is alwayshorizontal, one must adjust the attitudeof Segment 1 to duplicate the segmentrelationship found in Fig. 7 (top). This isdone simply by rotating Segment 1 byan angle equal to the summation of B,and B2.

The procedure just described istheoretical and idealized. Now let's getpractical. When we remember the seg-ments weigh 40 to 50 tons or more andthe concrete is steam cured, raising thetemperature of the concrete and steelcasting machine to 150 F (65 C), thingsare likely to move. In fact, they alwaysdo!

The purpose of the early morningshots is to determine the magnitude anddirection of movement or casting error.These data are plotted directly on thetheoretical casting curve as shown byPoint B on Fig. 8 (top). In this case, theactual relationship between the twosegments previously cast results in theend of Segment 1 being above thetheoretical casting curve. However, itcould just as well have been below it.

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Therefore, to get back to the theoreticalcasting curve when casting Segment 2,assumed to be perfect, a correctionmust be included in the attitude of Seg-ment 1 as it is placed in the old or cast-against position. Therefore, as shown byFig. 8 (top and bottom) the proper angleof rotation of the segment is B, plus B2plus a correction C.

The curve generated by plotting all theearly morning shot data is a curve whichwiggles on either side of the theoreticalcasting curve. This curve is known asthe "as-cast curve." If the as-cast curvestarts deviating away from the theoreti-cal casting curve, the engineer knows hehas serious problems and can take cor-rective steps before the situation getsout of hand. The as-cast curve is alsovaluable information for the field en-gineer because it shows the actual re-lationship between the segments as theywere cast. This relationship must be du-plicated again when the segments areerected.

It is strongly recommended that all ofthe casting geometry control be set upgraphically and drawn to the largestpossible scale. This not only includesthe two previously mentioned curves butthe determination of rod readings to setthe proper attitude of the cast-againstsegment. Also, a separate set of curvesshould be used for each line of controlpoints even though two of them may betheoretically symmetrical. Frequently,the early morning data will not be sym-metrical.

Of course, mathematical equationscan be set up to calculate settings for allthe points since all have a straight linegeometrical relationship. However, theseequations should only be used as an in-dependent check of the graphics. It ismuch more difficult to determine tenden-cies and directions by examining sets ofnumbers than by examining graphicalplots.

The short line system does offer someadvantages. For example, the space re-quired for set up is minimal resulting in a

centralized operation. Any geometry de-sired can be obtained by twisting the po-sition of the cast-against segment. Theprimary disadvantage of the method isthe accuracy at which the cast-againstsegment must be set. Also, the castingmachine must be flexible enough toconform to the twisted cast-againstsegment but rigid enough to adequatelysupport the loads. This is particularly sowhen casting segments for a superele-vation transition.

Long line system—An alternative tothe previously discussed short line sys-tem is the long line system. The systemis similar except that a continuous soffitthe length of a cantilever is built. Figs. 9and 10 show such an example. All thesegments are cast in their correct rela-tive position with the side forms movingdown the line as each segment is cast.Geometry control is established by ad-justing the side forms and soff it. Variabledepth structures may be cast by varyingthe elevation of the soffit, i.e., curves arecast by curving the soffit.

A long line is easy to set up and tomaintain control of the segments as theyare cast. Also, the strength of the con-crete is not as critical since the seg-ments do not have to be moved im-mediately.

When considering a long line systemseveral things must be taken into ac-count. First of all, substantial space isrequired. The minimum length of soffitrequired is generally a little more thanone-half the longest span of the struc-ture. The foundation must be strong andrelatively settlement free because thesegment weight to be supported can be5 tons per lineal foot or more. Any curingand handling equipment must be mobilesince the side forms travel along thesoffit. The contractor must set up amonitoring system and adjust the soffitsperiodically to correct for any settlement.

Fig. 11 shows the long line castingsystem used to cast the segments of theKentucky River Bridge near Frankfort,Kentucky. This bridge was completed in

PCI JOURNAL/July-August 1980 73

OUTSIDE FORMWORK

INSIDE FORMWORK

ELEVATION

PLAN

Fig. 9. Schematic of long line casting system. Sideforms move along a permanent soffit to cast individualsegments.

ELEVATION

PLAN

Fig. 10. As segments reach desired concrete strength,they can be removed to the storage yard. A secondcantilever may be started with the addition of a secondset of side forms.

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Fig. 11. Long line casting system used for Kentucky River Bridge, Frankfort,Kentucky. (Photo courtesy: Construction Products, Inc., Lafayette, Indiana.)

1979. Jack Kelly, manager of Construc-tion Products, Inc., believes that thissystem proved easier and was more ef-ficient for the variable depth segments.

Cast-in-Place SegmentsAnother alternative is to cast the seg-

ments in their final position on thestructure. Numerous projects have beenconstructed in this manner across NorthAmerica. The bridge located near Vail,Colorado (see Fig. 12) is one suchexample.

Cast-in-place construction proves tobe very advantageous when large, veryheavy segments are encountered. In-stead of handling the segments, onlymaterials have to be transported thus in-fluencing the type and size of requiredequipment.

The commonly used method for cast-ing segments in place is with the use ofform travelers such as that shown in Fig.12. Form travelers are moveable formssupported by steel cantilever trussesattached to previously completed seg-

Fig. 12. Form traveler used on a segmental project near Vail, Colorado. The usualproduction rate for a form traveler is one segment every 3 to 5 days.


8 7 I 6 5 4 3 2 I 2 3 4 5 6 171819

IL POSSIBLE TEMPORARYII STRUT

Fig. 13. Balanced cantilever erection will probably be the mostcommonly used method for constructing segmental bridges. Itsolves many problems such as environmental or existing trafficconstraints.

ments. The forms themselves may beconstructed of either wood or steel.When balanced cantilever erection isused, a minimum of two form travelers isrequired.

The segment production rate for formtravelers is usually one segment every 5days per traveler. Therefore, to ap-proach the common precast segmentproduction rate previously discussed, atleast four travelers are required. The5-day cycle time may be reduced withconcrete admixtures to increase theearly strength gain of concrete and bythe application of partial post-tensioning.However, a practical minimum cycletime is around 3 days per traveler.

Alignment variations and correctionsare more easily accommodated incast-in-place construction; but more cor-rections will probably be necessary. Theincrease in alignment corrections forcast-in-place construction compared toprecast construction relates directly tothe age of the concrete when loaded.Generally, the concrete is much youngerwhen loaded in cast-in-place construc-tion.

The deformations due to creep andshrinkage vary logarithmically with theage of the concrete, with the values ofthe deformations decreasing as the ageat loading increases. For instance, theultimate creep deformation of concreteloaded at 7 days after casting will ap-proach 1.5 times that for the same con-crete when loaded 28 days after casting.Also, one has to remember the 5-dayage difference for each segment results

in a significant difference in the creeprate when an entire cantilever isanalyzed. (This complexity is a furtherreason for using a computer in anybridge job.)

Erection MethodsBalanced cantilever—Balanced can-

tilever erection, as shown in Fig. 13, isquickly becoming the "classic" techniquewhen considering segmental construc-tion. This method solves a multitude ofproblems such as environmental restric-tions, existing traffic problems, inacces-sible terrain and many others. It can alsobe used readily with either precast orcast-in-place segments. However, thefollowing discussion only relates to pre-cast segments.

The general concept is to attach thesegments in an alternate manner at op-posite ends of cantilevers supported bypiers. As the segments are attached themoment to be carried at the pier in-creases in a manner shown by Fig. 14.The hatched area represents the changein moment when attaching Segment 8.

The compressive stresses in the bot-tom of the concrete section at the pierbuild up similar to the moment variation.However, the theoretical tensile stressesoccurring at the top of the same sectionare offset by the post-tensioning forcesapplied at a rate similar to the momentincrease. It is important to rememberthat the top of the concrete section isessentially operating at capacity duringthe entire erection sequence. Therefore,the construction loads must not increase

76

Q

Fig. 14. As the length of the cantilever grows, the magnitude of moment atthe pier increases. Since the post-tensioning tendons are also installedand stressed in increments as segments are attached, the top concretestresses are close to the design limits at all times.

significantly over what has been as-sumed in the design.

As segments are attached to the can-tilever ends one at a time, an overturn-ing moment is created and must be re-sisted. This moment may be resisted bypost-tensioning the pier segment downto the pier stem, providing temporarysupports on either side of the pier orstabilizing the cantilevers with the erec-tion equipment. The final choice belongsto the contractor but the designer mustassume and detail a method for a stressevaluation and parameters for the con-tractor.

The segments may be delivered to theends of the cantilevers by many means.The most economical and probably mostcommonly used method in the UnitedStates is lifting the segments withcranes. Crane erection will probably bemore common in the United States thanhas been experienced in Europe be-cause of the greater available capacity.Only restrictions which limit crane mobil-ity make other methods more attractive.Fig. 15 shows segments being lifted by abarge crane.

Fig. 15. Crane erection is probably mosteconomical in the United States due tocrane availability. Access to the areaunder the bridge must be available.


Fig. 16. The launching gantry eliminates the need for construction access beneaththe structure. The gantry shown in this photo is in the process of moving to the nextspan to start a new cantilever.

Fig. 16 shows a launching gantryplacing segments for balanced can-tilever erection. The launching gantry,developed by Jean Muller in France, is amachine capable of transporting a seg-ment from a completed portion of thebridge or from below the bridge to eitherend of the cantilevers being erected.The first project in the United States tobe erected with a launching gantry is theKishwaukee River Bridge near Rockford,Illinois (see Fig. 17).

Launching gantries come in all sizesand shapes. They can vary from thelarge one shown in Fig. 16 to the smallsimple one shown in Fig. 18. Both serveidentical functions; only the size of thespans and segments change. Mostlaunching gantries have the capability tomove themselves once a cantilever iscompleted and another is ready to

Fig. 17. This launching gantry is beingused to erect the Kishwaukee RiverBridge near Rockford, Illinois. This isthe first such use of a launching gantryin the United States.

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Fig. 18. Launching gantries may be very simple depending on projectrequirements. This gantry was used to erect high level viaducts in France.

Fig. 19. Low level segmental bridges maybe erected with gantry cranes similar tothe one shown by this photo. The crane may be either rail mounted or be on rubbertires.


CART FORSEGMENT TR4

INGPMENT

(0)

Fig. 20. A schematic of the progressive placing erection system. The system mayprove valuable when the area is restricted for substructure construction such as theLinn Cove Viaduct in North Carolina (see Fig. 21 on opposite page).

begin. They may be equipped with twolifting devices enabling simultaneousattachment of segments minimizing re-quired overturning moment provisions.The various details of the launchinggantry depend on the size of structureand the economics involved.

Launching gantries are particularlyadvantageous when accessibility to thearea beneath the structure is restrictedby environmental consideration. By de-livering segments across previouslycompleted portions of the bridge, access

to the area beneath is not required ex-cept to build the substructure. Newbridges can be erected over existingtraffic and/or buildings with minimal dis-turbance. This is a tremendous con-struction advantage in urban areas.Launching gantries can be used to erectcurved bridges as well as straight ones.Accessibility to the structure is generallythe overriding consideration for balancedcantilever erection with a launchinggantry.

Gantry cranes similar to the one

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Fig. 21. A computer generated photo of the Linn Cove Viaduct. The bridge is beingbuilt from the top (including foundations) by the progressive placing concept.(Photo courtesy: Figg and Muller Engineers, Inc.)

shown in Fig. 19 may also be used to liftthe segments. The gantry crane,whether mounted on rubber tires or rails,travels between the ends of the can-tilevers to lift the segments. Obviously,the practicality of this type of equipmentis limited to low level structures overland such as viaducts. But gantry cranespossess faster movement than trackmounted cranes and may eliminate theneed for a second large capacity craneon a project.

Progressive placing—Progressiveplacing is a modification of the balancedcantilever concept. Fig. 20 shows thebasic concepts of progressive placing.Instead of starting erection at a pier andproceeding in two directions, progres-sive placing erects cantilevers in onlyone direction.

The equipment required is a cranecapable of lifting a segment deliveredalong the previously completed portion

of the bridge and swinging around andlowering the segment to be attached tothe end of the cantilever. The craneshown in Fig. 20 (top) is a swivel craneavailable in Europe. A stiff leg derrickmay also be used.

As the cantilever extenas in one di-rection, the capacity of the section lo-cated at the pier is soon exceeded.Therefore, a temporary support must beprovided to prevent overstress. Themethod shown in Fig. 20 is a system oftemporary cable stays which are movedfrom pier to pier as construction pro-ceeds.

As shown in Fig. 20, hydraulic jackscan be attached to the stays to controlthe stay stresses and orientation of thecantilever. An alternate and maybe sim-pler method is to provide jacks beneaththe legs of the vertical steel tower. Thus,the stress in the stays can be varied byraising or lowering the steel tower. The


Fig. 22. Steel launching nose used to control erection stresses for the incrementallaunching concept. As the bridge moves across the supports, the concrete issubjected to both maximum positive and negative dead load moments.

primary advantage here is having onlytwo jacks to control the operation.

Instead of the temporary cable staysystem, a system of temporary bentsmay be provided beneath the structure.If permitted by the terrain, temporarybents may be a more economical andfaster solution. Of course, each projectmust be evaluated separately.

Progressive placing not only providesan opportunity to construct thesuperstructure unhindered by obstaclesbut provides the ability to also build thepiers from the top. A case in point is theLinn Cove Viaduct located near Grand-father Mountain in North Carolina. Thisproject, representing the final link of theBlue Ridge Parkway, will take the park-way around a mountain in a scenic andenvironmentally sensitive area. In fact,the terrain is so rough, it is impossible toget heavy construction equipment to thepier sites without extensive damage.The National Park Service, owner of thebridge, stipulated a construction roadcould only extend from the abutment tothe first pier. Fig. 21 shows a computergenerated image of the completed proj-ect. Construction was started in 1979.

The Linn Cove viaduct is beingerected by the progressive placingtechnique with temporary bents located

at midspan between the permanentpiers. Both the bents and piers are beingconstructed from the top with the stiff legderrick used to place the precast seg-ments. As the end of the cantilever ap-proaches a pier location, the derricklowers men and equipment to drill andcast 9-in. microshaft piles. The ellipticalshaped footing is then cast with concretedelivered over the completed portion ofthe bridge and lowered with the derrick.The pier stems consist of precast seg-ments delivered and placed in a similarmanner. Once the vertical pier post-ten-sioning tendons are installed andstressed, the superstructure segmentplacing resumes until the next pier loca-tion is reached. Then the process is re-peated.

Incremental launching—Incrementallaunching is a technique where seg-ments are cast at the end of the crossingand pushed across by large hydraulicrams. This method is most useful whenthe piers can be easily located at regularintervals.

Temporary support bents may or maynot be required at midspans dependingon the span length. A steel launchingnose is generally attached to the end ofthe segments, as shown by Fig. 22, tocontrol erection stresses. The segments

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are usually 50 to 100 ft (15 to 30 m) inlength.

Incremental launching is best adoptedto bridge lengths of 1000 to 2000 ft (305to 610 m) unless other considerationsare involved. For instance, when theworking area is severely restricted,bridges up to 4000 ft (1220 m) in lengthmay be achieved by launching from bothends.

When considering incrementallaunching, one must consider some re-strictions. The horizontal and verticalalignment must either be straight or of aconstant radius of curvature; preferably250 ft (76 m) or greater. In addition, thetop slab must have a constant crown orconstant superelevation without anytransitions.

The Wabash River Bridge nearCovington, Indiana, was the first incre-mentally launched bridge in the UnitedStates. This project (see Fig. 23) wascompleted in 1977. The original designplans and specifications were based on

precast segmental construction erectedby balanced cantilevers with cranes. Thecontractor exercised an option to alterthe construction method with a reportedsavings to the State of Indiana.

It is believed precast segments mayalso be launched although the author isnot presently aware of any projectsusing this method. Identical restrictionswould probably apply but the basic prin-ciples could be used to launch precastsegments to achieve a desired result.

Span by Span—Span by span erec-tion may be the most economicaltechnique for erecting segmental bridgesin the medium span range [less than250 ft (76 m)]. This method utilizes anassembly truss spanning between per-manent piers to support precast seg-ments prior to installation and stressingof post-tensioning tendons. Segmentsare placed on the assembly truss by acrane in approximately their final posi-tion. After all segments comprising aspan are assembled, the post-tensioning

Fig. 23. The Wabash River Bridge near Covington, Indiana, was the firstincrementally launched bridge in the United States. This photo shows the bridgeapproaching the opposite side of the river.


Fig. 24. The span by span erection scheme has proven to be very economical forshorter span segmental bridges. The method is applicable to both precast andcast-in-place segmental construction. (Courtesy: Figg & Muller Engineers, Inc.)

tendons are installed and stressed. Fig.24 shows the basic system.

Jean Muller developed the span byspan concept in an effort to Americanizesegmental construction. The basic ob-jectives were to simplify the systemthereby reducing the number of opera-tions required. Reduced labor require-ments are not a significant factor be-cause segmental construction in generalis not labor intensive.

Span by span techniques allow addi-tional modifications to the componentsof the structure. Primarily, the post-ten-sioning tendons may all be continuousfor the total span length and may be lo-cated in a draped manner providingmost efficient use of post-tensioningforces. Also, only one operation of in-stalling and stressing tendons is re-quired per span.

The Long Key Bridge in Florida is thefirst structure to be erected in this man-ner. The spans are 118 ft (33 m) in

length. Fig. 25 shows the assembling ofsegments for one of the spans. Thecontractor has readily achieved an erec-tion rate of three spans per week result-ing in the essentially complete construc-tion of 354 ft (108 m) of superstructureper week. Only the casting of the barriercurbs remains to complete the structure.

The span by span erection techniqueallowed two other modifications of nor-mal segmental construction procedures.The Long Key Bridge is the first precastsegmental bridge to be constructed withdry joints. Normal practice is to seal thejoints with epoxy. However, dry jointsare not recommended for bridges whichmay be subject to freeze-thaw condi-tions and deicing chemicals. Also thepost-tensioning tendons are located inthe void of the box girder as opposed tolocating the tendons in the concretewalls of the sections. The tendons areprotected with plastic conduits andgrout. This tendon location simplifies the

84

ERECTED CLOSURESEGMENT /_ J01NT y_SEGMENT ASSEMBLY

I

PIER ASSEMBLY TRUSS

Fig. 25. Assembly truss being used to erect the Long Key Bridge in Florida. Thisphoto shows the truss being installed between two piers. The contractor erectedthree 118-ft spans per week. (Courtesy: Figg and Muller Engineers, Inc.)

casting of the segments and eliminatesany problems of tendon alignment at thesegment joints.

The Seven Mile Bridge located nearthe Long Key Bridge in the Florida Keys(see Fig. 26) will also be erected spanby span but the contractor has elected toassemble the segments on a barge andlift the entire 135 ft (41 m) span at onetime. A temporary post-tensioning sys-tem and support frame will hold thesegments together during the lift. Thecontractor hopes this modification will

provide an even faster construction ratethan achieved at Long Key.

Concluding RemarksThe bottom line when choosing a con-

struction technique for a particular seg-mental bridge is economics—and cor-rectly so. The prospective contractorshould thoroughly investigate all sys-tems complying with specified parame-ters and choose the one which providesthe least cost. The casting method,

Fig. 26. Rendering of Seven Mile Bridge. (Courtesy: Figg & Muller Engineers, Inc.)


post-tensioning system or erection pro-cedure cannot be evaluated separatelyfor each is only a part of the entire con-struction technique to be used. All as-pects of the project have to be consid-ered together.

The design engineer should evaluatethe parameters of the project and basethe design on the most economicalmethod. However, all other methodscomplying with the parameters shouldbe allowed as contractor's options.There is essentially no difference in thefinal quality of the project.

Many different construction tech-niques have been discussed in thispaper. However, it should not be inferredthat these are the only techniques avail-able. Time and space permitted the in-clusion of only some of the most com-mon construction methods. Engineers,whether contractors or designers, shoulduse their ingenuity to use the commontechniques and develop new techniquesto reduce construction costs.

Geometry control techniques werediscussed more extensively for short linematch casting than for the othermethods. This is because this geometrycontrol is probably the most difficult tounderstand. With proper understandingand attention to details, the results willbe excellent. For instance, the Long KeyBridge is being cast by the short linemethod and will not receive an additionalwearing surface. The author has riddenon the completed portion of the bridge ina vehicle traveling at various highwayspeeds. The segment joints couldneither be felt nor heard. This is a tributeto what can be accomplished.

Some contractors have expressedconcern over the additional time andmoney required to evaluate the optionsbefore bidding projects. The days in

which design engineers could tell con-tractors exactly what to do are quicklycoming to an end. To create the mosteconomical projects, competition be-tween materials and the methods ofusing the materials must be encouraged.We in North America are not going tothe design-build concepts prevalent inEurope, but are going to approach thatidea and probably be somewhere inbetween. This is also a tendency inEurope.

In the future, projects and constructionmethods are going to be more en-gineering oriented requiring a coopera-tive effort between designers and con-tractors with a required increase in con-tractor engineering staffs. Also, there willbe more competition between construc-tion materials including the availability oftwo complete sets of plans based ondifferent materials. The final resultshould be a much more economical useof construction resources.

SELECTED REFERENCES1. Muller, Jean, "Ten Years of Experience in

Precast Segmental Construction," PCIJOURNAL, V. 20, No. 1, January-February1975, pp. 28-61.

2. CEB/FIP Recommendations for the De-sign and Construction of Concrete Struc-tures, Third Edition, Cement and ConcreteAssociation, London, England, 1978.

3. PCI Committee on Segmental Construc-tion, "Recommended Practice for Seg-mental Construction in Prestressed Con-crete," PCI JOURNAL, V. 20, No. 2,March-April 1975, pp. 22-41.

4. Precast Segmental Box Girder BridgeManual, Joint Venture: Prestressed Con-crete Institute, Chicago, Illinois; Post-Ten-sioning Institute, Phoenix, Arizona, 1979.

5. Post-Tensioned Box Girder Bridge Man-ual, Post-Tensioning Institute, Phoenix,Arizona, 1978.

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