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Construction Techniques for Segmental Concrete

•2•.i.

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.i. James M. Barker .i. Associate •Z• H. W. Lochner, Inc. Consulting Engineers Chicago, Illinois

ne of the primary advantages of struction or in design in the United O segmental concrete con- States, few have been or will be con- struction is the economics. In a large structed in exactly the same manner. majority of cases, segmental construc- The multitude of choices available to tion has been the winner where alternate contractors allows them to tailor each construction methods have been avail- project to their manpower and equip- able to contractors at the time of bidding. ment in the interest of maximizing effi- There are many reasons for this rela- ciency and optimizing cost. tively new method of bridge construction Segmental bridge construction is also in the United States competing so well in revising the basic thinking of design en- the 10 years since the European transfer gineers. Until recently, designers have technology was started. I believe, how- concerned themselves mainly on how to ever, that the principle reason for the build the project after preparing compu- success of segmental concrete con- tations and plans. Segmental construc- struction is the number of construction tion has revised this thinking. The first techniques available to build these question asked about a project now is bridges. Of approximately 35 to 40 such "What is the best and most economical bridges either completed, under con- way to build this project?" Once this question has been satisfactorily an- swered, the designer can proceed with a NOTE: This article is based on a presentation given design based on the most efficient con- at the Long Span Concrete Bridge Conference in Hartford, Connecticut, March 18-19, 1980. The struction method. conference was sponsored by the Federal Highway In order to make intelligent decisions, Administration, Portland Cement Association, Pre- stressed Concrete Institute, Post-Tensioning Insti- both designers and contractors need to tute, and Concrete Reinforcing Steel Institute. become familiar with the available

66 0FFIT

ARRIAGE

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

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

methods of segmental construction. Not methods to be discussed utilize the con- all methods are applicable to all projects cept of match casting. The basic prem- but all should be considered for each ise of match casting is to cast the seg- job. There are several fundamental con- ments so their relative erected position cepts relating to casting and erection of is identical to their relative casting posi- segments which could be considered for tion. This requires a perfect fit between practically every bridge project. By no the ends of the segments and is accom- means should these basic concepts be plished by casting each segment directly the only ones considered. Segmental against the face of the preceding one bridge construction was born out of in- using a debonder to prevent bonding of novation and will continue to grow the concrete. The segments are then through more innovation by both con- erected in the same sequence they were tractors and design engineers. cast. The most common method for match Precasting Techniques casting segments is called the "short line" method. Fig. 1 shows a schematic Short line system—All the casting of a short line match casting system.

PCI JOURNAUJuly-August 1980 67 Fig. 2. Casting machines (forms) fold back hydraulically or with screw jacks to permit moving of segments.

With this system, the rate of segment When the workers arrive at the pre- production will approach one segment casting yard on Wednesday morning, per line of forms per day. A good aver- the side forms are closed, the inside age to use for a project is four segments form is rolled forward, Tuesdays seg- per line every 5 days. ment is in the new segment position and For the sake of explaining the casting Mondays segment is in the old segment procedure, assume today is Wednes- position. The first operation is to deter- day. The older segment was cast on mine the relative position as the seg- Monday and is now cured and ready for ments actually were cast. This is done the storage yard. The old segment was by shooting elevations and centerline cast yesterday or Tuesday and was with an accurate survey instrument. match cast against Mondays segment. These shots are called "early morning Today a new segment will be cast shots." (Geometry control will be dis- against Tuesdays segment. cussed in greater detail later.) Once the Fig. 2 shows the form arrangement early morning shots are taken, the forms for short line match casting. The French are released and Mondays segment is call these forms casting machines. taken to the storage yard. Tuesdays The appropriateness of this name will segment is then moved from the new soon become apparent for they really segment position to the old segment po- are machines. The length of the side sition. forms is equal to the length of the seg- All of the geometry of the bridge (hori- ment being cast plus 1 or 2 in. (25 or 51 zontal or vertical curves and super- mm) to seal around the match cast elevation or transitions) is cast in by ad- joint. The side forms have the capability justing the old segment. The forms are of being folded back away from the never adjusted for geometry. Therefore, segment to permit removal of the seg- once Tuesdays segment is in the old ment. This is done either with screw segment position, its attitude is adjusted jacks or hydraulic rams. The collapsible by screw jacks between the carriage and inside formwork which forms the void of the soffit to provide the proper bridge the box girder rolls on rails to allow re- geometry. A prefabricated reinforcing moval of the form, enabling the segment bar cage is then set in the new segment to be lifted vertically. position. Once the side forms are closed

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

and the inside form is rolled forward, the screw jacks for adjusting the attitude of casting machine is ready for casting to- the old segment. days (Wednesdays) segment. Fig. 4 is a schematic of the geometry Fig. 3 shows a casting machine for a control layout for a short line casting short line match cast system. Note the method. The survey instrument is

PERMANENT PERMANENT TARGET INSTRUMENT

^ I

STEEL BULKHEAD I H NEW SEGMENT

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

PERMANENT SOFFIT

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

PCI JOURNAUJuly-August 1980 69 nent survey control points. its are located over the ninate any influence of top rents. The center points 9oretical centerline.

ally a theodilite capable of measuring Round-headed bolts placed 2 or 3 in. accurately to 1/32 of an inch (nearly 1 (51 or 76 mm) from the edge of segment mm). The permanent target is generally are used for the elevation control points. a concrete pile driven into the ground They are assumed to be at the edge of and insulated to prevent bending due to the segment for computation purposes the sun shining on one side. Elevations and should always be placed over the are shot using a survey rod. A metric rod webs to eliminate any influence of top may be more practical because the slab deflections. Since only relative po- smallest graduations are approximately sitions of segments are of concern, 1/32 of an inch which eliminates some of these bolts do not have to be placed at the guesswork involved when using rods any specific elevation but may be placed marked in feet and inches. The data in the wet concrete so the bottom of the measured are elevation differences so head is approximately at the concrete the metric rod does not result in exten- level. The early morning shots on these sive unit conversion. points establish the basis of relative po- The segment survey control point po- sitions. sitions are shown in Fig. 5. Each seg- The two centerline control points usu- ment has six control points—four over ally consist of U-shaped wires placed in the two webs and two on the centerline. the wet concrete after the top slab has

ERECTED CANTILEVER FINAL PROFILE

\ a THEORETICAL CASTING CURVE

PIER LOCATION

A = DEFLECTION DUE TO PRESTRESS Fig. 6. Theoretical casting curve is drawn from data provided by the design engineer.

70 been finished. The theoretical centerline by the erected cantilever curve in Fig. 6. is established by notching these wires Therefore, it is obvious the segments with a hammer and chisel during the must be cast with -a downward deflection early morning shots. of A so when the camber occurs the The designer of the bridge will provide proper alignment will be achieved. A information to develop a theoretical curve depicting this downward deflection casting curve. The theoretical casting is the theoretical casting curve. In reality, curve is a curve along which the seg- when all the deformations are consid- ments should be cast so the final de- ered, the theoretical casting curve usu- sired alignment will be achieved after all ally bends upward rather than down- deformations. The computation of these ward. deformations is quite intricate since most Fig. 7 (top) shows the theoretical are time dependent and interdependent. casting curve developed previously. Thus, a good computer program is Since segments cannot be cast curved, needed for maximum accuracy. Among the curve is approximated by casting the causes of deformations are self segments on the chords. This is the pro- weight of the structure, camber due to cedure followed whether the curve is prestress, prestress losses, creep and horizontal or vertical. Therefore, chords shrinkage of the concrete and tempera- equal to the length of the segments are ture variations. laid out on the theoretical casting curve For the sake of simplifying this discus- so a tangent to the curve can be drawn sion, let us consider the deformation of at the points of intersection of the camber due to prestressing. Fig. 6 chords. Angles B, and B 2 can then be shows a crest vertical curve as a final measured from the local tangent defin- desired alignment. Assuming balanced ing the desired relative position of the cantilever erection, the erected can- segments as they are match cast and tilever would deflect upward an amount erected. This must then be related to the A due to the prestressing as represented position of the casting machine.

B, B2

THEORETICAL CASTING CURVE

CASTING 8 E RECTI ON DIRECTION

^HORIZONT^^ R +R

VERTICAL Z i NEW SEGMENT0 tnmininrnm

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

PCI JOURNAUJuly-August 1980 71 AS-CAST CURVE

JB,+B2+C

THEORETICAL Lt®vi CASTING CURVE CA ST ING a ERECTION _ \ \ J DIRECTION

HORIZONTAL B1+BZ+C I-- —--

VERTICAL NEW SEGMENT OLD SEGMENT 2 O

177/77/77 ,77,7 rirrim

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

Two assumptions relative to the cast- is always vertical and the soffit is always ing machines must be made—the first horizontal, one must adjust the attitude can be controlled, the second cannot. of Segment 1 to duplicate the segment The first assumption is that the steel relationship found in Fig. 7 (top). This is bulkhead at the opposite end of the new done simply by rotating Segment 1 by segment from the cast-against segment an angle equal to the summation of B, is established and maintained absolutely and B2. vertical with the top being completely The procedure just described is horizontal. In addition, the bottom soffit theoretical and idealized. Now lets get is established and remains absolutely practical. When we remember the seg- horizontal. The second assumption is ments weigh 40 to 50 tons or more and that the segment being cast is perfect. the concrete is steam cured, raising the While this second assumption is not too temperature of the concrete and steel important to this explanation, it is very casting machine to 150 F (65 C), things significant when performing actual are likely to move. In fact, they always geometry control procedures. do! To transfer the segment relationship The purpose of the early morning from Fig. 7 (top) to the casting machine, shots is to determine the magnitude and one has to examine the direction of direction of movement or casting error. casting and erection. In this case Seg- These data are plotted directly on the ment 1 is cast and erected before Seg- theoretical casting curve as shown by ment 2. Therefore, on the casting Point B on Fig. 8 (top). In this case, the machine Segment 1 is in the old position actual relationship between the two and Segment 2 is in the new segment segments previously cast results in the position as shown by Fig. 7 (bottom). end of Segment 1 being above the Remembering the steel bulkhead lo- theoretical casting curve. However, it cated on the left side of Fig. 7 (bottom) could just as well have been below it.

72 Therefore, to get back to the theoretical centralized operation. Any geometry de- casting curve when casting Segment 2, sired can be obtained by twisting the po- assumed to be perfect, a correction sition of the cast-against segment. The must be included in the attitude of Seg- primary disadvantage of the method is ment 1 as it is placed in the old or cast- the accuracy at which the cast-against against position. Therefore, as shown by segment must be set. Also, the casting Fig. 8 (top and bottom) the proper angle machine must be flexible enough to of rotation of the segment is B, plus B2 conform to the twisted cast-against plus a correction C. segment but rigid enough to adequately The curve generated by plotting all the support the loads. This is particularly so early morning shot data is a curve which when casting segments for a superele- wiggles on either side of the theoretical vation transition. casting curve. This curve is known as Long line system—An alternative to the "as-cast curve." If the as-cast curve the previously discussed short line sys- starts deviating away from the theoreti- tem is the long line system. The system cal casting curve, the engineer knows he is similar except that a continuous soffit has serious problems and can take cor- the length of a cantilever is built. Figs. 9 rective steps before the situation gets and 10 show such an example. All the out of hand. The as-cast curve is also segments are cast in their correct rela- valuable information for the field en- tive position with the side forms moving gineer because it shows the actual re- down the line as each segment is cast. lationship between the segments as they Geometry control is established by ad- were cast. This relationship must be du- justing the side forms and soff it. Variable plicated again when the segments are depth structures may be cast by varying erected. the elevation of the soffit, i.e., curves are It is strongly recommended that all of cast by curving the soffit. the casting geometry control be set up A long line is easy to set up and to graphically and drawn to the largest maintain control of the segments as they possible scale. This not only includes are cast. Also, the strength of the con- the two previously mentioned curves but crete is not as critical since the seg- the determination of rod readings to set ments do not have to be moved im- the proper attitude of the cast-against mediately. segment. Also, a separate set of curves When considering a long line system should be used for each line of control several things must be taken into ac- points even though two of them may be count. First of all, substantial space is theoretically symmetrical. Frequently, required. The minimum length of soffit the early morning data will not be sym- required is generally a little more than metrical. one-half the longest span of the struc- Of course, mathematical equations ture. The foundation must be strong and can be set up to calculate settings for all relatively settlement free because the the points since all have a straight line segment weight to be supported can be geometrical relationship. However, these 5 tons per lineal foot or more. Any curing equations should only be used as an in- and handling equipment must be mobile dependent check of the graphics. It is since the side forms travel along the much more difficult to determine tenden- soffit. The contractor must set up a cies and directions by examining sets of monitoring system and adjust the soffits numbers than by examining graphical periodically to correct for any settlement. plots. Fig. 11 shows the long line casting The short line system does offer some system used to cast the segments of the advantages. For example, the space re- Kentucky River Bridge near Frankfort, quired for set up is minimal resulting in a 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. Side forms move along a permanent soffit to cast individual segments.

ELEVATION

PLAN

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

74 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- Cast-in-place construction proves to tion Products, Inc., believes that this be very advantageous when large, very system proved easier and was more ef- heavy segments are encountered. In- ficient for the variable depth segments. stead of handling the segments, only materials have to be transported thus in- Cast-in-Place Segments fluencing the type and size of required Another alternative is to cast the seg- equipment. ments in their final position on the The commonly used method for cast- structure. Numerous projects have been ing segments in place is with the use of constructed in this manner across North form travelers such as that shown in Fig. America. The bridge located near Vail, 12. Form travelers are moveable forms Colorado (see Fig. 12) is one such supported by steel cantilever trusses example. attached to previously completed seg-

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

PCI JOURNAL/July-August 1980 75 8 7 I 6 5 4 3 2 I 2 3 4 5 6 171819 IL POSSIBLE TEMPORARY II STRUT

Fig. 13. Balanced cantilever erection will probably be the most commonly used method for constructing segmental bridges. It solves many problems such as environmental or existing traffic constraints.

ments. The forms themselves may be in a significant difference in the creep constructed of either wood or steel. rate when an entire cantilever is When balanced cantilever erection is analyzed. (This complexity is a further used, a minimum of two form travelers is reason for using a computer in any required. bridge job.) The segment production rate for form Erection Methods travelers is usually one segment every 5 days per traveler. Therefore, to ap- Balanced cantilever—Balanced can- proach the common precast segment tilever erection, as shown in Fig. 13, is production rate previously discussed, at quickly becoming the "classic" technique least four travelers are required. The when considering segmental construc- 5-day cycle time may be reduced with tion. This method solves a multitude of concrete admixtures to increase the problems such as environmental restric- early strength gain of concrete and by tions, existing traffic problems, inacces- the application of partial post-tensioning. sible terrain and many others. It can also However, a practical minimum cycle be used readily with either precast or time is around 3 days per traveler. cast-in-place segments. However, the Alignment variations and corrections following discussion only relates to pre- are more easily accommodated in cast segments. cast-in-place construction; but more cor- The general concept is to attach the rections will probably be necessary. The segments in an alternate manner at op- increase in alignment corrections for posite ends of cantilevers supported by cast-in-place construction compared to piers. As the segments are attached the precast construction relates directly to moment to be carried at the pier in- the age of the concrete when loaded. creases in a manner shown by Fig. 14. Generally, the concrete is much younger The hatched area represents the change when loaded in cast-in-place construc- in moment when attaching Segment 8. tion. The compressive stresses in the bot- The deformations due to creep and tom of the concrete section at the pier shrinkage vary logarithmically with the build up similar to the moment variation. age of the concrete, with the values of However, the theoretical tensile stresses the deformations decreasing as the age occurring at the top of the same section at loading increases. For instance, the are offset by the post-tensioning forces ultimate creep deformation of concrete applied at a rate similar to the moment loaded at 7 days after casting will ap- increase. It is important to remember proach 1.5 times that for the same con- that the top of the concrete section is crete when loaded 28 days after casting. essentially operating at capacity during Also, one has to remember the 5-day the entire erection sequence. Therefore, age difference for each segment results the construction loads must not increase

76 Q

Fig. 14. As the length of the cantilever grows, the magnitude of moment at the pier increases. Since the post-tensioning tendons are also installed and stressed in increments as segments are attached, the top concrete stresses 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 by post-tensioning the pier segment down to the pier stem, providing temporary supports on either side of the pier or stabilizing the cantilevers with the erec- tion equipment. The final choice belongs to the contractor but the designer must assume and detail a method for a stress evaluation and parameters for the con- tractor. The segments may be delivered to the ends of the cantilevers by many means. The most economical and probably most commonly used method in the United States is lifting the segments with cranes. Crane erection will probably be more common in the United States than has been experienced in Europe be- cause of the greater available capacity. Only restrictions which limit crane mobil- Fig. 15. Crane erection is probably most ity make other methods more attractive. economical in the United States due to Fig. 15 shows segments being lifted by a crane availability. Access to the area barge crane. under the bridge must be available.

PCI JOURNAL/July-August 1980 77 Fig. 16. The launching gantry eliminates the need for construction access beneath the structure. The gantry shown in this photo is in the process of moving to the next span to start a new cantilever.

Fig. 16 shows a launching gantry placing segments for balanced can- tilever erection. The launching gantry, developed by Jean Muller in , is a machine capable of transporting a seg- ment from a completed portion of the bridge or from below the bridge to either end of the cantilevers being erected. The first project in the United States to be erected with a launching gantry is the Kishwaukee River Bridge near Rockford, Illinois (see Fig. 17). Launching gantries come in all sizes and shapes. They can vary from the large one shown in Fig. 16 to the small simple one shown in Fig. 18. Both serve identical functions; only the size of the spans and segments change. Most launching gantries have the capability to move themselves once a cantilever is completed and another is ready to

Fig. 17. This launching gantry is being used to erect the Kishwaukee River Bridge near Rockford, Illinois. This is the first such use of a launching gantry in the United States.

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

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

PCI JOURNAUJuly-August 1980 79 ING PMENT

CART FOR SEGMENT TR4

(0)

Fig. 20. A schematic of the progressive placing erection system. The system may prove valuable when the area is restricted for substructure construction such as the Linn Cove in (see Fig. 21 on opposite page).

begin. They may be equipped with two to the area beneath is not required ex- lifting devices enabling simultaneous cept to build the substructure. New attachment of segments minimizing re- bridges can be erected over existing quired overturning moment provisions. traffic and/or buildings with minimal dis- The various details of the launching turbance. This is a tremendous con- gantry depend on the size of structure struction advantage in urban areas. and the economics involved. Launching gantries can be used to erect Launching gantries are particularly curved bridges as well as straight ones. advantageous when accessibility to the Accessibility to the structure is generally area beneath the structure is restricted the overriding consideration for balanced by environmental consideration. By de- cantilever erection with a launching livering segments across previously gantry. completed portions of the bridge, access Gantry cranes similar to the one

80 Fig. 21. A computer generated photo of the Linn Cove Viaduct. The bridge is being built 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 lift of the bridge and swinging around and the segments. The gantry crane, lowering the segment to be attached to whether mounted on rubber tires or rails, the end of the cantilever. The crane travels between the ends of the can- shown in Fig. 20 (top) is a swivel crane tilevers to lift the segments. Obviously, available in Europe. A stiff leg derrick the practicality of this type of equipment may also be used. is limited to low level structures over As the cantilever extenas in one di- land such as viaducts. But gantry cranes rection, the capacity of the section lo- possess faster movement than track cated at the pier is soon exceeded. mounted cranes and may eliminate the Therefore, a temporary support must be need for a second large capacity crane provided to prevent overstress. The on a project. method shown in Fig. 20 is a system of Progressive placing—Progressive temporary cable stays which are moved placing is a modification of the balanced from pier to pier as construction pro- cantilever concept. Fig. 20 shows the ceeds. basic concepts of progressive placing. As shown in Fig. 20, hydraulic jacks Instead of starting erection at a pier and can be attached to the stays to control proceeding in two directions, progres- the stay stresses and orientation of the sive placing erects cantilevers in only cantilever. An alternate and maybe sim- one direction. pler method is to provide jacks beneath The equipment required is a crane the legs of the vertical steel tower. Thus, capable of lifting a segment delivered the stress in the stays can be varied by along the previously completed portion raising or lowering the steel tower. The

PCI JOURNAL/July-August 1980 81 Fig. 22. Steel launching nose used to control erection stresses for the incremental launching concept. As the bridge moves across the supports, the concrete is subjected to both maximum positive and negative dead load moments.

primary advantage here is having only at midspan between the permanent two jacks to control the operation. piers. Both the bents and piers are being Instead of the temporary cable stay constructed from the top with the stiff leg system, a system of temporary bents derrick used to place the precast seg- may be provided beneath the structure. ments. As the end of the cantilever ap- If permitted by the terrain, temporary proaches a pier location, the derrick bents may be a more economical and lowers men and equipment to drill and faster solution. Of course, each project cast 9-in. microshaft piles. The elliptical must be evaluated separately. shaped footing is then cast with concrete Progressive placing not only provides delivered over the completed portion of an opportunity to construct the the bridge and lowered with the derrick. superstructure unhindered by obstacles The pier stems consist of precast seg- but provides the ability to also build the ments delivered and placed in a similar piers from the top. A case in point is the manner. Once the vertical pier post-ten- Linn Cove Viaduct located near Grand- sioning tendons are installed and father Mountain in North Carolina. This stressed, the superstructure segment project, representing the final link of the placing resumes until the next pier loca- Blue Ridge Parkway, will take the park- tion is reached. Then the process is re- way around a mountain in a scenic and peated. environmentally sensitive area. In fact, Incremental launching—Incremental the terrain is so rough, it is impossible to launching is a technique where seg- get heavy construction equipment to the ments are cast at the end of the crossing pier sites without extensive damage. and pushed across by large hydraulic The National Park Service, owner of the rams. This method is most useful when bridge, stipulated a construction road the piers can be easily located at regular could only extend from the abutment to intervals. the first pier. Fig. 21 shows a computer Temporary support bents may or may generated image of the completed proj- not be required at midspans depending ect. Construction was started in 1979. on the span length. A steel launching The Linn Cove viaduct is being nose is generally attached to the end of erected by the progressive placing the segments, as shown by Fig. 22, to technique with temporary bents located control erection stresses. The segments

82 are usually 50 to 100 ft (15 to 30 m) in precast segmental construction erected length. by balanced cantilevers with cranes. The Incremental launching is best adopted contractor exercised an option to alter to bridge lengths of 1000 to 2000 ft (305 the construction method with a reported to 610 m) unless other considerations savings to the State of Indiana. are involved. For instance, when the It is believed precast segments may working area is severely restricted, also be launched although the author is bridges up to 4000 ft (1220 m) in length not presently aware of any projects may be achieved by launching from both using this method. Identical restrictions ends. would probably apply but the basic prin- When considering incremental ciples could be used to launch precast launching, one must consider some re- segments to achieve a desired result. strictions. The horizontal and vertical Span by Span—Span by span erec- alignment must either be straight or of a tion may be the most economical constant radius of curvature; preferably technique for erecting segmental bridges 250 ft (76 m) or greater. In addition, the in the medium span range [less than top slab must have a constant crown or 250 ft (76 m)]. This method utilizes an constant superelevation without any assembly truss spanning between per- transitions. manent piers to support precast seg- The Wabash River Bridge near ments prior to installation and stressing Covington, Indiana, was the first incre- of post-tensioning tendons. Segments mentally launched bridge in the United are placed on the assembly truss by a States. This project (see Fig. 23) was crane in approximately their final posi- completed in 1977. The original design tion. After all segments comprising a plans and specifications were based on span are assembled, the post-tensioning

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

PCI JOURNAUJuly-August 1980 83 Fig. 24. The span by span erection scheme has proven to be very economical for shorter span segmental bridges. The method is applicable to both precast and cast-in-place segmental construction. (Courtesy: Figg Muller Engineers, Inc.)

tendons are installed and stressed. Fig. length. Fig. 25 shows the assembling of 24 shows the basic system. segments for one of the spans. The Jean Muller developed the span by contractor has readily achieved an erec- span concept in an effort to Americanize tion rate of three spans per week result- segmental construction. The basic ob- ing in the essentially complete construc- jectives were to simplify the system tion of 354 ft (108 m) of superstructure thereby reducing the number of opera- per week. Only the casting of the barrier tions required. Reduced labor require- curbs remains to complete the structure. ments are not a significant factor be- The span by span erection technique cause segmental construction in general allowed two other modifications of nor- is not labor intensive. mal segmental construction procedures. Span by span techniques allow addi- The Long Key Bridge is the first precast tional modifications to the components segmental bridge to be constructed with of the structure. Primarily, the post-ten- dry joints. Normal practice is to seal the sioning tendons may all be continuous joints with epoxy. However, dry joints for the total span length and may be lo- are not recommended for bridges which cated in a draped manner providing may be subject to freeze-thaw condi- most efficient use of post-tensioning tions and deicing chemicals. Also the forces. Also, only one operation of in- post-tensioning tendons are located in stalling and stressing tendons is re- the void of the box girder as opposed to quired per span. locating the tendons in the concrete The Long Key Bridge in Florida is the walls of the sections. The tendons are first structure to be erected in this man- protected with plastic conduits and ner. The spans are 118 ft (33 m) in grout. This tendon location simplifies the

84 ERECTED CLOSURE SEGMENT /_ J01NT y_SEGMENT ASSEMBLY

I

PIER ASSEMBLY TRUSS

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

casting of the segments and eliminates provide an even faster construction rate any problems of tendon alignment at the than achieved at Long Key. segment joints. The located near the Long Key Bridge in the Florida Keys Concluding Remarks (see Fig. 26) will also be erected span The bottom line when choosing a con- by span but the contractor has elected to struction technique for a particular seg- assemble the segments on a barge and mental bridge is economics—and cor- lift the entire 135 ft (41 m) span at one rectly so. The prospective contractor time. A temporary post-tensioning sys- should thoroughly investigate all sys- tem and support frame will hold the tems complying with specified parame- segments together during the lift. The ters and choose the one which provides contractor hopes this modification will the least cost. The casting method,

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

PCI JOURNAL/July-August 1980 85 post-tensioning system or erection pro- which design engineers could tell con- cedure cannot be evaluated separately tractors exactly what to do are quickly for each is only a part of the entire con- coming to an end. To create the most struction technique to be used. All as- economical projects, competition be- pects of the project have to be consid- tween materials and the methods of ered together. using the materials must be encouraged. The design engineer should evaluate We in North America are not going to the parameters of the project and base the design-build concepts prevalent in the design on the most economical Europe, but are going to approach that method. However, all other methods idea and probably be somewhere in complying with the parameters should between. This is also a tendency in be allowed as contractors options. Europe. There is essentially no difference in the In the future, projects and construction final quality of the project. methods are going to be more en- Many different construction tech- gineering oriented requiring a coopera- niques have been discussed in this tive effort between designers and con- paper. However, it should not be inferred tractors with a required increase in con- that these are the only techniques avail- tractor engineering staffs. Also, there will able. Time and space permitted the in- be more competition between construc- clusion of only some of the most com- tion materials including the availability of mon construction methods. Engineers, two complete sets of plans based on whether contractors or designers, should different materials. The final result use their ingenuity to use the common should be a much more economical use techniques and develop new techniques of construction resources. to reduce construction costs. Geometry control techniques were SELECTED REFERENCES discussed more extensively for short line Muller, Jean, "Ten Years of Experience in match casting than for the other 1. Precast Segmental Construction," PCI methods. This is because this geometry JOURNAL, V. 20, No. 1, January-February control is probably the most difficult to 1975, pp. 28-61. understand. With proper understanding 2. CEB/FIP Recommendations for the De- and attention to details, the results will sign and Construction of Concrete Struc- be excellent. For instance, the Long Key tures, Third Edition, Cement and Concrete Bridge is being cast by the short line Association, London, England, 1978. method and will not receive an additional 3. PCI Committee on Segmental Construc- wearing surface. The author has ridden tion, "Recommended Practice for Seg- on the completed portion of the bridge in mental Construction in Prestressed Con- a vehicle traveling at various highway crete," PCI JOURNAL, V. 20, No. 2, March-April 1975, pp. 22-41. speeds. The segment joints could 4. Precast Segmental neither be felt nor heard. This is a tribute Manual, Joint Venture: Prestressed Con- to what can be accomplished. crete Institute, Chicago, Illinois; Post-Ten- Some contractors have expressed sioning Institute, Phoenix, Arizona, 1979. concern over the additional time and 5. Post-Tensioned Box Girder Bridge Man- money required to evaluate the options ual, Post-Tensioning Institute, Phoenix, before bidding projects. The days in Arizona, 1978.

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