design and construction of the kishwaukee river bridges · 2018. 11. 1. · james k. iverson...

Special Report

Design and Constructionof the

Kishwaukee River Bridges

R. Shankar NairVice President and Chief

Structural EngineerAlfred Benesch & CompanyChicago, Illinois

James K. IversonConsultant

Wiss, Janney, Elstner & AssociatesNorthbrook, Illinois

(Formerly, Project Managerwith J. W. Peters & Sons

segment fabricators forthe Kishwaukee Bridges.)

T he Kishwaukee River Bridges aretwin prestressed concrete box gir-

der structures, located in WinnebagoCounty, Illinois. The bridges are part ofthe Supplemental Federal Aid Free-way, FA412, which is being built toInterstate standards and financedjointly by the Federal Highway Ad-ministration and the State of Illinois.This Supplemental Freeway will be-come the main North-South highwayserving the mid-section of Illinois. Thesite of the Kishwaukee River Bridges isabout 4 miles (6'/2 km) south ofRockford.

The box girders were constructedfrom precast segments which wereerected by means of a launching truss.

This project represented the first use ofa launching truss for segmental bridgeerection in the United States. Anothersignificant feature of the project is thatcontractors were given an unusuallyhigh (for 1976) degree of freedom tochoose their own construction methodsand details.

Each of the two bridges supports twolanes of traffic and has a total length ofapproximately 1170 ft (357 m). The boxgirders are five-span continuous struc-tures with three interior spans of 250 ft(76.2 m) and end spans of 170 ft (51.8m). The bridges cross a deep, woodedriver gorge. The roadway surface ismore than 120 ft (36.6 m) above theriver.

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SELECTION OFTYPE OF BRIDGE

The bridges are in an environmentallysensitive area. Early in the develop-ment of the project, the State of Illinoisdecided on various restrictions to beimposed on construction operations toprotect the river and the adjacent land-scape. For example, removal ofhardwood trees larger than 6 in. (152mm) in diameter was to be permittedonly in certain specified areas aroundeach pier. In other areas, only smallertrees could be removed and even thosewould have to be replaced by the con-tractor under certain conditions. Theseand other environmental protection re-quirements limited the constructionprocedures and bridge types that couldbe considered for this site.

Comparative designs and cost esti-mates were prepared for several differ-ent steel and concrete structural sys-tems, subject to the environmental re-strictions. The bridge types studied in-cluded an orthotropic deck steel girderstructure, a concrete deck on steelstringers, a steel arch structure, and apost-tensioned segmental concrete boxgirder.

The evaluation of alternative bridgetypes indicated that the post-tensioned

segmental concrete bridge was at leastas economical as any other type ofbridge, in absolute terms, and offeredclearly the best combination of econ-omy, aesthetic value, and compatibilitywith environmental requirements.Further analysis showed a constant-depth design to be more appropri-ate — for this location and the antici-pated spans — than a variable-depthdesign with haunches at the piers.

For environmental and economic rea-sons, it was considered desirable thatall piers be out of the water at normalriver level. The roadway is not perpen-dicular to the river. The skewed cross-ing, together with the requirement thatthe piers be out of the water, requiredeither lengthening of the span orskewing of the piers with respect to thesuperstructure (which is not appropri-ate for the type of structure selected) orthe• use of twin bridges with staggeredpiers. Twin bridges were judged to beclearly the best of these alternatives.

On the basis of the preliminarystudies and the analysis and evaluationof alternative bridge types, it was de-cided that the Kishwaukee Rivercrossing for FA412 should be. a pair ofpost-tensioned segmental concrete boxgirder bridges. A five-span configura-tion was selected for each structure,

PCI JOURNAL/November-December 1982 23

250' 250'

ELEVATION

PLANFig. 1. General layout of Kishwaukee River Bridges.

with interior spans of 250 ft (76.2 m)and outer spans of 170 ft (51.8 m). Eachbridge was required to support two12-ft (3.66 m) traffic lanes; shoulders,curbs, and parapets brought the overallwidth of each structure to 42 ft (12.8 m).The configuration of the bridges isshown in Fig. 1.

DESIGN APPROACHAND CRITERIA

The Illinois Department of Trans-portation (IDOT) and its consultantswere well aware that many alternativemethods of casting, erection, andpost-tensioning were available for thetype of structure selected for thiscrossing. It was decided that contractorsshould be given the greatest possibledegree of freedom to, choose fromamong the available methods and also toinnovate and develop new techniques.This approach was expected to result inthe most efficient utilization of avail-

able resources of material, equipmentand expertise and, consequently, toprovide the State of Illinois with themost economical structure.

The major areas in which contractorswere to be given the freedom to selecttheir own methods and techniqueswere (1) method of casting segments,which could be either precast or cast inplace, (2) post-tensioning system anddetails, and (3) method of erection. Itwas recognized that the contractor'schoices in these areas would have asignificant effect on the design of thebridge. Nonetheless, it was decidedthat IDOT should provide bidders witha complete design and a complete set ofplans.

The set of plans provided to biddersby IDOT was based on the assumptionthat the box girder segments would beprecast, post tensioning would be bydraped strand tendons with detailssimilar to those of the Freyssinet sys-tem, and erection would be accom-

24

pushed by the balanced-cantilevermethod using a launching truss. Theseand other assumptions (including de-tails such as the assumed weight andmanner of use of the launching truss)were clearly described in the plans andoutlined in the specifications. Thespecifications then gave the contractorthe option of using different erectionand post-tensioning methods andcast-in-place instead of precast con-struction.

With respect to erection and post-tensioning, the specifications statedthat "Alternate methods of erection andalternate post-tensioning systems maybe acceptable. If the Contractor choosesan alternate method of erection, heshall submit all necessary computa-tions, drawings and specifications forapproval by the Engineer prior to anywork on the project. ... Calculationsshall also be submitted to substantiatethe system and method of stressingproposed by the Contractor."

It may be noted that contractors werenot required to provide calculations orother information on proposed alternateerection or post-tensioning methods atthe time of bidding; this informationwas required only during the shopdrawing phase of the project.

On alternates to precast construction,the specifications stated that "Bids willbe accepted on cast-in-place post-tensioned segmental box girder con-struction. If the option of cast-in-placeconstruction is exercised, the contractorshall specify that his bid is based on thecast-in-place option and, with his bidproposal, shall submit a set of prelimi-nary plans and computations preparedby a Structural Engineer registered inIllinois relative to h) design andmethod of erection of sufficient detailto allow the State to evaluate his propo-sal for structural adequacy. ... Theaforementioned submittals will beevaluated by the State only if the bidfor cast-in-place construction is thelowest.bid received. This evaluation

will be done prior to notification ofaward. ... If the Contractor is awardedthe contract based upon the cast-in-place option, he shall submit a com-plete set of design plans and computa-tions ... for review and approval by theState."

The basic design criteria for the"original" design (which is the designrepresented by the plans supplied tobidders by IDOT) included the fol-lowing:

1. Design to be in conformance withthe 1973 AASHTO StandardSpecifications for HighwayBridges.

2. There are to be no tensile stressesin the concrete due to longitudinalflexure of the box girder or trans-verse flexure of the top slab at anytime during construction or underdesign loadings on the completedbridge.

3. Design live loading to be HS-20.4. Design dead load to include pos-

sible future wearing surfaceweighing 25 psf (1.20 kPa).

5. Design to include the effect of thetemperature of the top slab being18 deg F (10 deg C) higher or 9deg F (5 deg C) lower than thetemperature of the remainder ofthe box section.

These basic design criteria were alsoapplicable to alternate designs pro-posed by contractors. The specificationsalso required that the alternates complywith the following restrictions:

1. The exterior dimensions of the boxsection could not be altered.

2. The location and general shape ofpiers and abutments could not bealtered.

3. All environmental requirements inthe specifications were fully ap-plicable to the alternate designs.(Note that these requirements ef-fectively, eliminated the possibilityof building the superstructure onfalsework off the floor of thegorge.)


41'-0"

:

Fig. 2. Girder cross section.

BIDDING ANDCONTRACT AWARD

The Kishwaukee River Bridges wereexpected to be the first precast seg-mental bridge structures constructed inIllinois and the first segmental bridgesanywhere in the United States to beerected with a launching truss. IDOTfelt that because of the new design anderection concepts involved in this proj-ect, a pre-bid orientation conferencewould be useful. At the pre-bid confer-ence, engineers from IDOT and theconsulting firms made presentations todescribe segmental bridges in generaland the Kishwaukee project in particu-lar, with the main objective of makingpotential bidders aware of the com-plexities of the type of construction an-ticipated for these bridges. The confer-ence was attended by about 100 peoplerepresenting general contractors, pre-casters, post-tensioning contractors andconsultants.

Seven general contract bids were re-ceived. Two of the bids were based oncast-in-place construction of thesuperstructure and were submittedwith packages of design information asrequired by the specifications. Sinceneither of these bids was the low bid,the design packages were returned

without review or evaluation. The lowbid of about $5.3 million was receivedfrom Edward Kraemer & Sons, Inc. ofPlain, Wisconsin, and was based on theuse of precast segments. This bidamounted to about $53 per sq ft($570/m2 ) of deck and was well belowIDOT's estimate. The second lowestbid was less than 3 percent higher thanthe low bid. The contract was awardedto Kraemer in November 1976.

Edward Kraemer & Sons, Inc. put to-gether a team that included J. W. Peters& Sons as precasters to fabricate box-girder segments, Heavy ConstructionServices as erectors of the superstruc-ture and Dywidag Systems Interna-tional USA, Inc. as post-tensioners anddesigners of alterations to the originalsuperstructure design. Foundation, pierand abutment construction, sitework,and supplemental deck work was doneby Kraemer's own forces.

The contractors decided on certainchanges to the original superstructuredesign, as permitted by the specifica-tions. The most significant change wasthe use of straight threaded bars insteadof draped tendons for longitudinalpost-tensioning. Other changes in-cluded an increase in the size of seg-ments and minor changes in erectionprocedure. These alterations required

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CANTILEVER CANTILEVE R CANTILEVE CANTILEVEUNIT I UNIT 2 UNIT 3 UNIT 4

PIER \ PIER PIER

FALSEWORK

FALSEWORKCAST- IN- PLACECLOSURE (TYP.)

Fig. 3. Division of bridge into balanced cantilevers.

complete re-analysis and redesign ofthe bridge superstructure. As called forin the specifications, calculations anddesign drawings were developed by thecontractor and submitted to IDOT forreview by IDOT's consultants.

In the following discussion of the de-sign of the Kishwaukee bridges, the de-sign represented by the plans suppliedto bidders by IDOT (which was de-veloped by IDOT's consultants) will bereferred to as the "original" design.The design that resulted from thechanges made by the contractor will beidentified as the "contractor's redesign"or by other appropriate means.

DETAILS OF DESIGNThe basic bridge configuration is

shown in Fig. 1. The superstructure is afive-span continuous post-tensionedconcrete box girder made up of precastsegments. The three inner spans are250 ft (76.2 m) long; the outer spansmeasure 170 ft (51.8 m). The bearings atthe two innermost piers are fixedagainst longitudinal movement; expan-sion bearings are used at the other piersand the abutments. All bearings are"pot" bearings that allow free rotationof the superstructure relative to thesubstructure components.

Each of the piers is a single non-prestressed reinforced concrete col-umn. The tallest pier measures ap-proximately 110 ft (33.5 m) from bear-ing seat to the ground surface and isabout 25 ft (7.62 m) wide by 10 ft (3.05m) thick at the base. All pier and abut-ment foundations are either footings onrock or piles bearing on rock. The rockoccurs close to the ground surface overmost of the site and is exposed in someareas. Piers were designed for all thenormal AASHTO loadings plus anearthquake-induced horizontal load of 6percent of structure weight. The designstrength of the concrete in the piers is3500 psi (24.1 MPa).

The cross section of the superstruc-ture is shown in Fig. 2. The box girdersection is essentially constant through-out the five-span structure except for anincrease in the thickness of the bottomslab [from 8 to 18 in. (203 to 457 mm)]near each pier. The width of the girderis 41 ft (12.5 m) and the overall depth isabout 11 ft 8 in. (3.56 m) (excluding thecast-in-place parapets). The webs are 14in. (356 mm) thick. The thickness of thetop slab is 10 in. (254 mm) at the endsof the wings, 9 in. (229 mm) at the mid-dle of the box, and 18 in. (457 mm) atthe webs. The top slab is post-tensioned in the transverse direction by

PCI JOURNAUNovember-December 1982 27

8'- O° TYPICAL

5 @7'-8" I7^7'-O5"

IIIIIHHHHHLC.I.P. CLOSURE

170- 0"END SPAN

Fig. 4. Division of bridge into segments.

17@7'-O5 "

IIIIIIIIIIIIII9IIC.I.P. CLOSURE

250'- 0"INTERIOR SPAN

means of high-strength bars. Thesecross-sectional dimensions are as actu-ally used in the bridge, following thecontractor's redesign. The dimensionsin the original design were not signifi-cantly different. The design strength ofthe concrete in the precast box girdersegments is 5500 psi (37.9 MPa).The division of the bridge

superstructure into balanced cantilev-ers is indicated in Fig. 3. Thesuperstructure design is based on con-struction of a pair of balanced cantilev-ers at each of the four piers. The systemof cantilevers is complemented by ashort length of bridge erected onfalsework adjacent to each abutment.The cantilevers and the structureerected on falsework are constructedout of precast segments. The girders arecompleted by short sections cast inplace between the portions of bridgeassembled from precast segments.

Segment lengths are indicated in Fig.4. The dimensions shown reflect theincrease in segment size that was partof the contractor's redesign. In theoriginal design, segment weight waslimited to 40 tons to facilitate highwaytransportation; maximum segmentlength was 5 ft 6 in. (1.68 m). The con-tractor's redesign reduced the numberof segments by about 20 percent. Theheaviest segments weigh about 50 tons.

At each interface between segments,there is a large key in each web andsmaller alignment keys in the top and

bottom slab. The dimensions of theweb key are shown in Fig. 5. Thesedimensions were not altered in thecontractor's redesign.

The web keys were not designed tofunction as the primary shear-transferdevice between segments. Epoxy isused in all joints. The keys are only in-tended to support the weight of one or afew segments before the epoxy hascured. After proper curing of the epoxy,the jointed girder was expected to sup-port shear in a manner similar to amonolithic structure, with the epoxysustaining a combination of shear andnormal stress over the entire height ofthe joint.

The girder webs were designed ac-cordingly. Mild steel (Grade 60) shearreinforcement was provided as re-quired. In the original design, whichincluded draped tendons for flexure, aportion of the shear force was sup-ported by the vertical component of thetension in the tendons. The contractor'sredesign eliminated the draped tendonsand required an increase in shear rein-forcement. [The maximum shear rein-forcement consists of #7 bars at 10-in.(254 mm) centers on each face of eachweb.]

The original design for longitudinalflexure was based on the use of post-tensioning tendons located in the websand in the top and bottom slabs close tothe webs. The details of the designwere based largely on the details and

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TOWARD PIER TOWARD MIDSPAN

Fig. 5. Typical web key.

dimensions of the Freyssinet system. Inthe contractor's redesign, longitudinalpost-tensioning is achieved entirely bymeans of Dywidag high-strength [155ksi (1069 MPa)] threaded bars locatedin the top and bottom slabs. The use ofthese threaded bars, which can be usedin small increments of length and cou-pled very conveniently, offered certainadvantages during erection. All theDywidag bars are 1 1/4 in. (32 mm) indiameter. The number of bars in the topslab varies from one hundred over thepiers to two at the cast-in-place closuresections; the number of bottom barsvaries from forty at typical midspan clo-sures to two at the piers.

The design of the box girder for lon-gitudinal flexure was determined, to alarge degree, by the erection procedureand construction loads. (Constructionprocedures are described elsewhere inthis article.) Both the original designand the contractor's redesign werebased on erection using a launchingtruss. The erection sequence and man-ner of operation of the launching trusswere similar in both designs. The con-tractor's method of moving the trussfrom one pier to another was different.from that assumed in the original de-sign, but this difference did not affect

the maximum loading on the bridge.The weight of a particular Europeanlaunching truss was used in the originaldesign computations. The contractor'sredesign took advantage of a lightertruss that was designed and built spe-cifically for the Kishwaukee project bythe erection subcontractor.

FABRICATIONOF SEGMENTS

FormThe precaster chose the short line

method of precasting and picked theU.S. Division of Stelmo, Limited, a.British company, to accomplish bothdesign and manufacture of the segmentform. Completion required about 6months, although a portion of the formwas shipped early and used to cast anumber of special diaphragmed seg-ments over the piers.

The short form casting methodutilized for this project features twoadjustable soffits or carriages support-ing the match-cast and casting segment.The cantilevers or individual portionsof the bridge from each pier to centerspan are cast segment-by-segment. Afixed bulkhead forms the leading face


Fig. 6. Form under wings.

of each new segment and the match-cast segment forms the rear face. Sideforms that seal at the match-cast seg-ment and the bulkhead form the out-side of the segment and support thesegment wings. Fig. 6 shows the formsunder the wings.

To aid the access to the form and easelifting equipment requirements, thecarriages were placed in a sunken pit,as shown in Fig. 7.

An expanding inner form or mandrelsupports the center portion of the topslab and inside vertical faces of the twowebs. The mandrel is also movable andcantilevers through the bulkhead froman exterior support frame upon which itcan be rolled in or out of the segmentfor cleaning. This is shown in Fig. 8.The fixed bulkhead can also be seenhere with the void boxes for the post-tensioning anchorage pockets alreadymounted. The threaded bar system ledto a number of anchor locations.

The design and detailing of this type

of form follow European developmentsand have been described in the litera-ture on segmental bridges. However,the precaster must follow the develop-ment of each particular design, relate tothe details of the post-tensioning andalways strive to minimize setup andstripping effort. For example, theKishwaukee form utilizes tie boltsthrough the webs. These tie bolts con-trol the local spreading of the form andwere found to be required if the seg-ments were to meet specified toler-ances. However, they are a problemduring setup when they must bethreaded through the reinforcing cageand their removal is time-consumingand an easily missed step during strip-ping. The installation and removal ofthese items occur on every casting forover 300 segments, and even a minordifficulty can lead to large total projectcosts.

This form utilizes a mix of hydraulicjacks and large screw jacks to provide

30

Fig. 7. End view of soffit form on carriage in pit.

Fig. 8. Fixed bulkhead with post-tensioning void boxes and mandrel extracted. Themandrel cantilevers through the bulkhead from an exterior support frame upon which itcan be rolled in or out of the segment for cleaning.


Fig. 9. Modified travel-lift.

form movement and adjustment. Thelarge screw jacks utilized under thewing are visible in Fig. 6. These areprimarily used for stripping and sealingmovements. A critical spot for formmovement is adjustment of the bottomforms or carriages. The carriages mustbe capable of vertical movement (forelevation control) and horizontal ad-justment. Vertical adjustment is madeby 6-in. (152 mm) hydraulic jacks withlocking collars as can be seen in Fig. 7.The horizontal adjustment is handledby two turn bolts on the front and rearof the carriage and a set of teflon slideplates between two lower "layers" ofthe carriage. Straightforward adjust-ment of the form is a necessity if speedand efficiency in production are to berealized. Adjustment tolerances on thematch-cast segments were normally setat 11i6 in. (1.6 mm).

The bulkhead, shown in Fig. 8, isfixed perpendicular to the longitudinalcontrol line and is not moved or ad-justed in normal casting sequences. It

can be moved longitudinally on its bedto permit casting different length seg-ments, but this was done only a fewtimes on the project, and was a time-consuming adjustment.

Improvements in Casting YardThe casting on this project was done

in an existing J. W. Peters precastingyard in Burlington, Wisconsin. Fortu-nately, considerable unused land wasavailable, but it did require some re-grading and stabilization for storageareas. This allowed several desirablefeatures in the casting site:

1. Adjacent segment storage, materialstorage and concrete hatchingfacility.

2. Geometry control transit sightingto north to minimize sun interfer-ence.

3. Good soil conditions.4. Existing electric service and steam

lines.5. Space available for , an adjacent

rebar cage assembly jig.

32

Fig. 10. Winter form cover.

The handling of the segments wasaccomplished by modifying an oldtravel-lift or straddle crane to lift the50-ton segments. The span of themachine was reduced to obtain the re-quired capacity and the lifting systemwas totally rebuilt. Retractable jackswere added at the wheels to increasethe static lifting capacity. The machineis shown in Fig. 9. A similar standardlarge span machine was used for lightersegments, cage-handling and otherlifting.

The scheduling of this project re-quired casting through the Wisconsinwinter. Another older travel-lift wasmodified to span the entire casting op-eration and a canvas hood was placedover this machine. This provided a roll-ing cover for the form. This machine isshown in Fig. 10. Even with this pro-tection and additional heat, continuingone-a-day production was neverrealized in the coldest winter months.

To allow one day turnaround, con-ventional high-early-strength cement

and steam heat were used and weregenerally satisfactory. Slumps over 3 in.(76 mm) were required in casting thelower portions of the segment. Externalvibrators were used, but could only beused for short intervals during criticalportions of the casting or seal difficul-ties appeared. Hand-held internal vi-brators were utilized throughout thecasting.

The existing 4 cu yd (3.06 m3) turbinemixer which was used for all plant con-crete was used to mix the concrete forthe segments. The use of superplas-ticizers was considered but was initiallydiscarded based on high cost and con-cern about reliability under widely vary-ing temperatures. However, later inthe job this additive was used withgood success. The specified concretestrength of 5500 psi (37.9 MPa) wasobtained reliably except in the coldestwinter months.

This type of construction demandsvery close geometry control to assurethat the completed bridge alignment is


Fig. 12. Reinforcing cage in jig.

Fig. 13. A separate form for diaphragmed segments, shown here with insulating tarps,was assembled to speed casting operations.


and four at 10 ft (3.05 m) on each side ofthis centerline, near the joints, for ele-vation control. The bridges atKishwaukee are straight horizontally,but each has a partial curve and alsostraight tangent in the vertical direc-tion.

Geometry ControlThe general procedure for geometry

control consists of making the necessarycomputations, adjustment of thematch-cast segment for casting, and fi-nally taking record readings betweenthe two hardened segments while theyare still locked in the forms beforestripping.

The computations can be accom-plished graphically or utilizing formsdeveloped for the purpose. They areaccomplished in two steps. First, therecord readings taken in the casting bedare utilized to calculate where thenewly-cast segment will lie in the com-pleted bridge. Then the attitude of thenewly-cast segment as a match-castelement is computed.

A self-leveling level and surveyor'stransit were used to make mea-surements. The readings were taken to1/64 in. (0.4 mm) to assure that 1/32-in. (0.8mm) accuracy was attained. This highdegree of precision is necessary onsegments near the piers since any errorwill be magnified up to 20 times nearthe end of the cantilever. The setup ofthe segments over the piers in thematch-cast position is particularly sen-sitive and had to be very accuratelymeasured and carefully checked. Asystem of cross-slope checks wasutilized to monitor elevation accuracyand a secondary triangular measure-ment was made at the pier segments tocheck horizontal accuracy.

Shop Drawings and ProductionDrawings

A set of shop drawings showing seg-

ment locations and designations, detailsof concrete post-tensioning and rein-forcing steel, overall location of post-tensioning pockets and casting profilecurves was prepared.

A production drawing was made foreach segment. This drawing was usedat the casting bed and reinforcing cagejig and shows -the details appropriate tothe segment being produced.

A number of submittals to IDOT wasrequired. All production and shopdrawings had to be submitted for ap-proval in several copies. Well over 5000sheets of drawings were submitted.

Transverse Post-TensioningThe Kishwaukee project features the

use of high-strength Dywidag bars forall post-tensioning. The bridge utilizedtransverse post-tensioning and this wasinstalled prior to casting and wasstressed in the yard. A typical installa-tion is shown in Fig. 14.

Hauling of SegmentsThe 11 ft 8 in. (3.56 m) high, 50-ton

segment required special low-boy trail-ers. These were obtained from a priorproject of this type. The haul of over 50miles (80 km) through Wisconsin andIllinois led to a number of permits andthe requirement for escorts on theturnpike. Standard hauling trucks wereused, but special care was required atthe project site to ensure that all haulroads were well graded and main-tained.

ERECTION OF BRIDGESThe box girder segments were

erected using a launching truss. Themanner of use of the launching trussand the procedure for moving the trussalong the bridge are shown schemat-ically in Fig. 15. The sequence of oper-ations is as follows:

1. A few segments adjacent to the

36

Hg. 14. Transverse post-tensioning anchors in form.

abutment are erected on falsework.Epoxy is used in the joints betweensegments; longitudinal prestress barsare installed and tensioned as erectionproceeds. A steel tower is erected onthe first pier. Rollers are installed onthe legs of the launching truss and thetruss is moved forward until its frontend is over the pier tower.

2. With the truss supported on its rearleg and on the pier tower in front, therollers are removed from the middleleg. The truss is moved forward untilthe middle leg is over the pier. Therollers on the rear legs are removed.The pier tower is partially removed andreplaced with a strut that allows roomon the pier top for installation of thepier segments. (The "pier segments"are the two diaphragmed box-girdersegments located directly above eachpier.) The launching truss is now usedto erect the pier segments, which areinstalled on temporary bearings and

rigidly tied down to the pier by meansof temporary vertical post-tensioning.The middle leg of the launching truss islowered onto the pier segments. Thelaunching truss is now in its workingposition over the first pier.

3. The segments in the first balancedcantilever unit are erected using thelaunching truss. Epoxy is used in thejoints between segments. Longitudinalprestressing bars are installed and ten-sioned as erection proceeds.

4. The cast-in-situ "closure" isformed and poured between the end ofthe cantilever and the short length ofbridge previously erected on falsework.Longitudinal post-tensioning is appliedacross the closure. The temporary verti-cal "tie-down" post-tensioning at thepier is released and the permanentbearings are installed. The steel piertower is erected on the second pier.Rollers are installed on the legs of theIaunching truss and the truss is moved


Fig. 15. Sequence of launching truss operations.

38

Fig. 16. Segment near abutment being erected on falsework.

forward until its front end is over thepier tower. (The moment on the can-tilever will reach its maximum value atthis time, before the front end of thelaunching truss has been picked up bythe pier tower.)

5. The launching truss is moved for-ward and installed in its working posi-tion over the second pier, using a pro-cedure similar to Step 2. The secondbalanced cantilever unit is then erectedand the midspan closure between thefirst and second units is formed andpoured. The launching truss will thenbe ready to be moved to the next pier.This sequence of operations continuesuntil the entire superstructure has beenerected.

The erection of the cantilever seg-ments for the west bridge (for thesouthbound traffic lanes) began in thesummer of 1978. By this time, a consid-erable stockpile of segments had beenbuilt up in the precaster's yard; the fab-

rication of segments had begun about ayear earlier. All four balanced cantilev-ers for the west bridge were completedby November 1978 and the entire boxgirder from abutment to abutment wasin place by late April 1979. The erec-tion procedure closely followed the se-quence of operations listed above andillustrated in Fig. 15. Various stages oferection are shown in Figs. 16 through20.

The superstructure of the secondbridge was erected in 1980. Thoughboth bridges were now structurallycomplete, they were not opened topublic use until August 1982. In theinterim, they were used to transportmore than 1,000,000 cu yds (764,100m3 ) of earth fill for the highway ap-proaches. The wearing surface on thebridge decks was installed just prior toopening to avoid damage by the earth-hauling equipment. The completedbridges are shown in Fig. 21.


Fig. 17. Launching truss about to be moved to new location. Note temporary steel toweron pier in foreground.

Fig. 18. Launching truss at completion of movement to new location.

40

Fig. 19. Cantilever nearing completion.

Fig. 20. Early stage of cantilever erection.


Fig. 21. Completed bridges.

P

JOINT PROBLEMSAND REPAIRS

On May 10, 1979, cracking and otherdistress was observed in the westbridge (the first bridge to be built). Thebridge structure was essentiallycom-plete at that time, except for parapetsand the wearing surface. All the precastsegments and cast-in-situ closures werein place and all post-tensioning hadbeen completed. The launching trusshad been moved off the bridge a fewdays earlier. The bridge superstructurewas supported on its permanent bear-ings at all piers; all tie-downs at piershad been removed. Cantilever erectionof the second bridge had not yet begunbut six segments adjacent to an abut-ment had been erected on falsework.

There was obvious evidence of dis-tress at and near the joint between a

pier segment and the cantilever seg-ment immediately adjacent to it (seeFig. 22). In the east web of the box gir-der, the bottom of the male shear key atthis joint had crushed and the innersurface of the key had spalled overmuch of its area. The outer surface ofthe web of the pier segment had de-laminated and spalled over a large areabetween the key and the bearing.These conditions are shown in Figs. 22,23, and 24.

There appeared to have been a rela-tive vertical movement or slip of about1/2 in. (13 mm) in the east web at thedistressed joint. The magnitude of thisslippage could be inferred from the gapthat appeared at the top of the key (seeFig. 24). There was no evidence of anyrelative vertical displacement at thejoint in the west web.

The upper surface of the top slab did

42

Fig. 22. Damaged girder.

Fig. 23. Damaged web seen from outside.


Fig. 24. Damaged key seen from insidethe box girder.

not show any sharp step at the joint, noteven above the east web. The relativemovement at the web, combined withthe absence of any noticeable move-ment at the top of the slab, indicatedthat there must be a horizontal delami-nation in the slab, probably in the planeof the longitudinal post-tensioning bars.This was confirmed by testing withdelamination-detecting equipment.

Shortly after the distress in the girderweb joint was discovered, cribbing wasplaced between the pier top and thegirder under both webs. The cribbing isvisible in Fig. 22.

Cause of Joint ProblemThe examination of conditions at the

distressed joint revealed immediatelythat the epoxy in the joint had nothardened properly and was soft andplastic. Epoxy that had extruded out ofthe joint could easily be pulled off byhand and molded with the fingers.Subsequently, non-hardened epoxy wasalso found in a large number of otherjoints in the bridge.

As mentioned earlier, in the sectionon design details, the web keys werenot designed to serve as the mainshear-transfer device between seg-ments; the keys were intended only tosupport the weight of a few segmentsduring curing of the epoxy. Undergreater loading, after proper curing ofthe epoxy, the epoxy was required tosustain a combination of shear andnormal stress over the entire height ofthe joint. The soft epoxy that was foundin the distressed joint and in manyother joints was obviously incapable ofsustaining any significant shear loading.The soft and viscous material could, infact, be expected to function as a lubri-cant between adjacent segments, thusforcing the web keys to support the en-tire shear loading on the joint. The re-sult is severe overstressing of the keys.

All observed conditions at the site ofthe distressed joint were completelyconsistent with this failure mechanism.The key and web damage was of thetype that could be expected to resultfrom overloading of the key. It is esti-mated that the shear force on the jointat the time of the failure was about 1500kips (6675 kN) (750 kips at each web),which is about 65 percent of themaximum design shear loading on thejoint.

Repair ProcedureThe repair or correction scheme that

was devised for joints that containednon-hardened epoxy is shown in Fig.25. This technique was developed byAlfred Benesch & Company and ac-cepted by the contractor. (Note that thecontractor had the option of proposingan alternate solution but chose to adoptthe Benesch scheme.)

The correction technique consists es-sentially of the insertion of stainlesssteel pins into the joint through thethickness of the girder web, which al-lows shear transfer to take place bybearing on the sides of the pins. The

44

STAINLESSSTEEL

PIN(TYPICAL)

A1'4-

A PININ -^

2"s HOLE OUTSIDEFACE OF

4.1 WEB

SECTION A-A

Fig. 25. Correction scheme for joints that contained non-hardened epoxy.

1151 PINS SET WITH EPDXYIN 2' HOLES

. k Ig(+) GAP FILLEDWITH CAULK

L eN

Fig. 26. Test specimen.

1.94-in. (49 mm) diameter pins are setin 2-in. (51 mm) diameter cored holeswith epoxy. The number of pins re-quired varies with the shear loading onthe joint. The maximum number wasdetermined to be 18 in each web atjoints near the piers. In low-shear areasnear midspan, it was determined thatpinning of joints that had soft epoxywould not be required since the keysalone could support the load.

The repair design was based on anaverage shear transfer of about 60 kips(267 kN) per pin. To verify the appro-priateness of this design value, a simple

test was conducted in IDOT'slaboratories. The test specimen andmethod of loading are illustrated in Fig.26. The specimen is a full-sized two-pinassembly that represents a part of ajoint in a girder web. Equal' shear andnormal loading is imposed on the jointin the specimen. The design loading of60 kips (267 kN) shear per pin is equiv-alent to a test load of 170 kips (756 kN)on the specimen.

The specimen failed at a load of 337kips (1500 kN). Failure occurred bysplitting of the concrete at a pin. Thismode of failure is not possible in the


Fig. 27. Installation of pin.

actual structure. At initiation of thesplitting failure, there was no indicationof incipient crushing or any other modeof failure. The joint capacity — for fail-ure modes that are possible in thebridge — could, therefore, be expectedto be even higher than indicated by thetest.

All joints, except those in very low-shear areas [located more than 90 ft(27.4 m) from any pier and 30 ft (9.1 m)from any abutment], were investigatedfor defective epoxy. This investigationincluded the removal of four cores fromeach joint for examination and testingin the laboratory. Defective epoxy wasfound in 81 joints. All 81 of these jointswere repaired by insertion of pins. Thetotal number of pins installed was about1500.

One of the steps in the pin-installation procedure (insertion of thepin) is shown in Fig. 27. There werefew problems during the work. At somejoints, interference with reinforcingbars required that one or two pins beeither eliminated or inserted only part-way into the web. All repair work wasdone from inside the box girder. As

shown in Fig. 25, the holes for the pinswere stopped 1 in. (25.4 mm) short ofthe outside of the web. There was someconcern that holes might break through;however, this did not turn out to be aproblem. From outside the box girder,there is no evidence of the existence ofthe pins. The damaged web and key(Figs. 22, 23, and 24) were repaired bypatching with epoxy mortar. The deckdelamination above the damaged webwas repaired by pressure grouting.

Result of RepairFollowing the repair of joints that

were found to have non-hardenedepoxy, there were no further jointproblems in the bridge. The repairedbridge has been subjected to heavyloading for 2 years. Most of the seg-ments for the second bridge weretransported over the repaired structure,as were more than 2 million tons ofearth fill. These loadings includednumerous applications of loads greaterthan the design HS20 truck. There hasbeen no indication of joint distress. Thesecond bridge has now been com-pleted, with no change in joint designor details. There has been no repetitionof the joint problems that occurred inthe first bridge.

SUMMARYThe Kishwaukee River Bridges were

the first precast segmental bridges to bebuilt in Illinois and the first segmentalbridges anywhere in the United Statesto be erected using a launching truss.Another signficant feature of the projectis that contractors were given an un-usual degree of freedom to choose theirown construction methods and details.

Construction had to conform to cer-tain strict requirements for protection ofthe local environment. Aesthetics werean important consideration in the de-sign of the project. Despite these con-straints, the Kishwaukee bridges did

46

not cost the owner any more than con-ventional bridges of similar spanlengths built with less regard foraesthetics and the environment.

In the first of the two bridges, epoxyin many of the joints between segmentsdid not harden properly. A correctiveprocedure that involved installation ofsteel pins was carried out at thesejoints; this procedure was completely

successful in restoring the structure.The Kishwaukee River Bridges have

received awards from the U.S. Depart-ment of Transportation and NationalEndowment for the Arts (jointly), theAmerican Consulting Engineers Coun-cil, the Prestressed Concrete Institute,and the Post-Tensioning Institute.

The bridges were opened to publicuse the summer of 1982.

CREDITS

Owner: Illinois Department ofTransportation.

Engineer: Alfred Benesch & Company,Chicago, Illinois.

Special Consultant: BVN/STS,Indianapolis, Indiana.

General Contractor: Edward Kraemer &Sons, Inc., Plain, Wisconsin.

Contractor's Engineer for DesignModifications: Dywidag Systems

International, USA, Inc., Lemont,Illinois.

Precast Concrete Fabricator: J. W.Peters & Sons, Burlington,Wisconsin.

Post-Tensioning Supplier: DywidagSystems International, USA, Inc.,Lemont, Illinois.

Erector: Heavy Construction Services,Belvidere, Illinois.


design and construction of the kishwaukee river bridges · 2018. 11. 1. · james k. iverson...

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