Design-Construction Feature

The Twelve Mile CreekPrecast PrestressedSegmental Bridges

Kris G. BassiHead, Design SectionStructural OfficeMinistry of Transportation-And CommunicationsToronto, Ontario

Watone L. LinDesign Engineer (Central)

Structural OfficeMinistry of Transportation

and CommunicationsToronto, Ontario

T he Twelve Mile Creek Bridges, lo-cated on Highway 406 linking the

City of St. Catherines, Ontario, to theQueen Elizabeth Way, were completedin 1982. For the last two years thebridges have been used for access andhauling fill before being opened to thepublic this past October.

The route of the new highway, whichfollows the valley through which thecreek flows (as shown in Fig. 1), wasselected for economy and minimum so-cial and environmental impact. TheTwelve Mile Creek originally formedpart of the Welland Canal, connectingLake Erie to Lake Ontario. Today, it ispart of a fast flowing tailrace for agenerating station.

The twisting alignment and profile ofthe new divided highway at the creekcrossing was dictated by an adjacent in-terchange, site terrain, poor soil condi-tions, and property constraints. The size,number, and location of piers in thewaterway also were restricted and therequirement for a mimimum verticalclearance above the creek had to be met.

In addition, the proximity of the adja-cent interchange required part of theroadway on the bridge to be flared toaccommodate the ramps. Therefore,where the divided highway crosses thecreek, two bridges, one for the north-bound lanes and the other for the south-bound lanes, were necessary to meet thespecified requirements.

30

SynopsisDescribes the overall design of the bridges plus the

fabrication and erection of the segments. A specialfeature of these structures is the use of transverse ribsalong both segment faces to accommodate the deckflare. Problems encountered during construction andtheir method of solution are also discussed.

(Stru

ctural OfficeOffice

AI-Ba=iSenior Structural Engineer

Ministry of Transportationand CommunicationsToronto, Ontario

Osmo E. RamakkoHead, Structural Section

F

Northwestern RegionMinistry of Transpoilation

and CommunicationsThunder Bay, Ontario

Several alternative types of bridgeswere investigated during the prelimi-nary design stage. The precast post-tensioned segmental concrete boxgirder bridges were selected for theirability to economically satisfy the com-plex geometry, restricted constructiondepth, and the variable roadway widthrequirements.

Description of BridgesThe bridges consist of a 594 ft (181 m)

long northbound lanes structure and a1310 ft (399 in) long southbound lanesstructure. The northbound lanes struc-ure is on a horizontal curve with three

continuous spans of 172, 250, 172 ft (52,

76, 52 in) while the southbound lanesstructure is on a horizontal S-curve withsix continuous spans of 155, 250, 250,250, 250, 155 ft (47, 76, 76, 76, 76, 47 m).The southbound lanes structure is con-siderably longer than the northboundIanes structure because neither fill or anembankment retaining wall at the southapproach was permitted to encroach onthe waterway. The layout of the bridgesis given in Fig. 2.

Both bridges were designed for three12 ft (3.66 rn) lanes of traffic; two 3 ft 3in. (0.99 m) shoulders; and 1 ft 8 in. (0.51m) safety barriers giving an overallwidth of45 $10 in. (13.97 m). However,to accommodate the on and off rampsfrom the adjoining interchange, the

PCI JOURNAUNovember-December 1984 31

Fig. 1. Aerial view of the completed bridges.

decks are flared towards the northabutments to a maximum of 48 ft 8% in.(14.85 m) for the northbound lanesstructure and 54 ft 4 in. (16.56 m) for thesouthbound lanes structure. Bothbridges are on a slight grade towards the.north.

Each bridge consists of a single cellbox girder having a constant depth of9 ft3 in, (2.82 m) and a constant out to outweb width of 25 ft (7.62 m). The crosssection of the superstructure is given inFig. 3. The extremely shallow construc-tion depth (span to depth ratio of 27:1)was dictated by the roadway profilewhich was controlled by both the

nearby interchange and the restrictedsoffit elevation required for creek flow.

Two spherical rotational bearingswith TFE sliding surfaces were pro-vided at each abutment and pier withthe exception of the bearings at Pier 1for the northbound lanes structure andPier 4 for the southbound lanes struc-ture. Those bearings were fixed fortranslation.

A special design feature of the bridgesis the transverse ribs located along bothsegment faces as shown in Fig. 4. Thedecision to use transverse ribs wasprimarily due to the need to flare thedecks towards the north abutments

32

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where the normal deck cantilever over-hang of 10 ft 5 in. (3.17 m) increases to amaximum of 14 ft 8 in. (4.47 m). Thetransverse post-tensioned ribs with a 7in. (178 mm) thick top slab were ex-tended to accommodate the deck flareand resulted in a 10 percent reduction inthe segment weight, Without the ribs,the deck flare would have required aconsiderably thicker top slab.

The two bridges were constructed bythe balanced cantilever method. Thenorthbound lanes bridge has 74 precastsegments and three cast-in-place closuresegments. The southbound lanes bridgehas 163 precast segments and six cast-in-place closure segments.

The pier and abutment segments are 6ft 8 in. and 6 ft (2.03 and 1.83 m) long,respectively, The first segments adja-cent to the pier segments are also 6 ft 8in. (2.03 m) long. They are followed byfour segments, each 7 ft (2.13 m) long.The thickness of the bottom slab for thefive segments adjacent to the pier seg-ment varies front 2 ft 4 in. to 9 in. (686 to

229 m) and then remains constant for theremaining segments which are 8 ft5 in.(2.57 m) long.

The abutment and pier segments in-clude transversely post-tensioned soliddiaphragms with access openings. Theconcrete in the diaphragms was placedafter erection of these segments to keepthe segment weights within the capacityof the launching truss.

Multiple shear keys without any re-inforcement and spread over the seg-ment faces were provided to resist thevertical shear forces during construc-tion. They were chosen over a singlelarge shear key due to the lack of spacein the shallow webs where anchoragesfor the draped longitudinal tendons alsowere located.

Longitudinal cast-in-place fasciabeams were provided at the edges of thedeck to cover the transverse post-tensioning anchorages and to give thestructure an aesthetically pleasing ap-pearance. The fascia beams also help todistribute the wheel live loads.

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Design ConsiderationsThe bridges were designed for all

normal AASHTO' loads. The longitudi-nal analysis of the box girders was car-ried out using the BC computer pro-gram. 2 Three groups of tendons (19/0.6strands) were used for longitudinalpost-tensioning (Fig. 5). The function ofthe first group, where the tendons aredraped and anchored in the webs, was toresist the negative moments during con-struction by the balanced cantilevermethod. The tendons for the secondgroup, located in the girder bottom toresist the positive moments for the con-tinuous structure, are draped and an-chored in the deck slab. The third groupof tendons, located in the top slab, wereprovided to resist negative momentsalong the span arising from live load inadjoining spans.

The transverse design was based on afinite element analysis and a modifiedversion of the procedures given in thePrecast Segmental Box Girder BridgeManual. 3 Transverse post-tensioningtendons (110.6 strands) were provided inthe ribs as well as in the deck slab. Thebottom slab of the box girder was rein-forced with mild steel reinforcement.

The design for shear in the webs ofthe box girder was based on the CEB-FtP Model Code for Concrete Struc-tures. 4 A combination of vertical post-tensioning consisting of 1 in. (25 mm)diameter Dywidag bars, stressed fromthe top, and mild steel reinforcementwas used in the first six segments oneach side of the pier to resist shear. Theremaining segments were provided withmild steel stirrups.

All piers consist of 10 ft (3.05 m) di-ameter round columns with post-tensioned pier caps. Circular single col-umn piers were chosen to suit the skewcrossing and to minimize any effect onthe creek flow, a concern of the nearbyupstream generating station. Piers I and2 of the northbound lanes structure werealigned with Piers 4 and 5, respectively,

of the southbound lanes structure asthey are located in the fast flowingwater. The construction sequence ofthese piers was established from a scalemodel study of creek flow to minimizeflow restriction and scour.

The piers are supported on eitherspread footings or footings with pilesdriven to bedrock. The abutment foot-ings are supported on piles driven tobedrock.

The design strength of the concretewas 6000 psi (41.3 MPa) for the precastsegments and 5000 psi (34.4 MPa) forthe pier caps. No tension was allowed inthe design of the prestressed concretebox girder and pier caps during con-struction and under AASHTO 1 loads.

Fabrication of SegmentsThe segments were precast in a tem-

porary all-weather building near thebridge site. Two short-Iine casting bedslocated on the same base line, one toproduce the standard 8 ft 5 in. (2.57 m)long segments and the other to producenonstandard segments, were used. Thesurvey tower to control the horizontaland vertical alignments of the segmentswas located between the two castingbeds.

Each casting form consisted of a fixedvertical bulkhead, soffit, two wingforms, and a retractable mandrel carry-ing the expanding interior forms. Eachsegment was cast between the fixedbulkhead and its adjacent or countercastsegment. Horizontal and vertical align-ment was achieved by adjusting theorientation of the countercast segmentprior to casting the new segment.

A tolerance of ±16 in. (1.59 mm), con-sidered to be too fine by the contractor,was enforced both horizontally and ver-tically for the setting up of the counter-cast segment priorto placing concrete inthe new segment. Due to the move-ments caused by the concreting opera-tion, the resulting as-cast tolerance was-t%a in. (4.76 mm). Although it was time

PCI JOURNAL/November-December 1984 37

consuming to meet the initial toleranceof ±lhs in. (1.59 mm), the resultingbridge alignment was satisfactory.

The reinforcing steel cages, with thepost-tensioning ducts in place, wereprefabricated in a wooden mock-up ofthe segment before moving them intothe casting beds.

The forms were stripped when theconcrete strength reached 1500 psi (10.3MPa). The countercast segment wasmoved into a storage area and the newlycast segment was then moved into thecountercast position.

The segments were vertically andtransversely post-tensioned and groutedin the storage area after the concretereached a strength of 5000 psi (34.4MPa).

An average of 1.5 segments per daywere produced over a period of about 10months. The maximum segment weight,excluding the cast-in-place diaphragms,was 72 tons (65 t).

Erection of Segments

The abutment segments and the adja-cent segments (up to the first cast-in-place closure segment) were erected onfalsework using a crawler crane. Theentire balanced cantilever over Pier I ofthe southbound lanes structure, which issituated over the creek shoreline, waserected by a crawler crane using a mov-able temporary support and stabilizingconcrete blocks.

The remaining segments wereerected by a launching truss. The steellaunching truss which had previouslybeen used on two other straight seg-mental bridges in Ontario had a lengthof 377 ft (114.9 m) and an approximateweight of 463 tons (420 t).

Major modifications were required toenable the truss to travel and erect seg-ments on the horizontally curvedbridges. The launching truss stabilizedthe balanced cantilevers, delivered seg-ments, and supported the work plat-forms and post-tensioning hardware.

Initially, two segments were erectedduring a normal working day. This in-creased to four segments per day by thetime the northbound lanes structure wascompleted.

The temporary stressing system usedto clamp the segments onto the growingcantilevers, until permanent tendonscould be installed, was a combination ofstrand and high strength bar tendons lo-cated in either existing ducts or in ductsespecially provided for this purpose.Due to the presence of the transverseribs, the large contact area at the seg-ment joints was provided with addi-tional short prestressing bars to providethe required 30 to 50 psi (0.21 to 0.34MPa) temporary contact pressure.

Epoxy adhesive was used to bond thesegments during erection. Two formu-lations were used; one for faster settingduring cool weather and the other forsetting during warmer weather. A dili-gent inspection, testing, and curing pro-cedure in accordance with AASHTOspecifications' was followed to ensuresuccess.

After completion of cantilevers ineach span and removal of all loads fromthe spans, stabilizing clamping beamswere used to connect the two canti-lever tips or the cantilever tip in the endspans to segments resting on temporaryfalsework. The concrete in the closuresegments was placed during periods ofstable ambient temperature, which wasusually around midnight. When theconcrete in the closure segment reacheda strength of 1000 psi (6.9 MPa), four ofthe tendons were stressed. The re-maining tendons were stressed whenthe concrete strength reached 4000 psi(27.6 MPa),

Problems and SolutionsDuring precasting of the segments

and erection of the superstructure sev-eral problems were encountered. Dis-cussed below are the difficulties andtheir method of solution.

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Precasting

During the separation of the counter-cast segments from the new segments,some of the shear keys were damaged.This was due to the method of separa-tion employed which caused the seg-ments to slightly rotate relative to oneanother, resulting in unanticipatedforces on the shear keys when the con-crete strength was only 1500 psi (10.3MPa). This problem was resolved (but

not until about 20 percent of the seg-ments had been cast) by adjusting themethod of separation, increasing thedepth of shear keys from 4 to 6 in. (102 to152 mm), and careful application of thebond breaker.

The damage to the shear keys neces-sitated a reassessment of the capacity ofthe remaining undamaged shear keys.Tests were carried out on a number ofconcrete blocks with 4 in. (102 mm)deep shear keys. A minimum shear

PCI JOURNAL/November- December 1984 39

Fig. 7. Completed northbound bridge with southbound bridge under construction. Notemislocated Pier 4 with Pier 5 in the foreground.

capacity of 2.5 ^J{ psi (0.2 ,' MPa)was established from these tests and wasused in assessing the shear capacity ofthe undamaged shear keys.

Most of the damaged shear keys werein the webs. A finite element analysiswas used to check the shear distributionat the remaining keys located in the topand bottom slabs and is shown in Fig.6. The analysis confirmed that onlyabout 20 percent of the shear force wastransferred through these keys. It was,therefore, decided to hold the segmentswith the damaged keys in place until theepoxy cured since the keys could not berelied upon to transfer shear forcesacross the joint. This resulted in a slow-down of segment erection.

The fixed bulkhead had a 5 in. (127mm) protruding ledge which formedpart of the bottom pallet for the segmentforms. During separation, large pieces ofconcrete often were broken off by theledge. These breaks were subsequentlyrepaired but caused fit-up problemsduring erection and led to leakage dur-

ing grouting of the tendons.The longitudinal tendon ducts were

displaced between the segment facesduring concrete placement even thoughthey were well fixed and inflatable rub-ber hoses were inserted into the ducts.This resulted in excessive friction dur-ing post-tensioning.

The transverse tendons in two seg-ments were accidentally grouted beforethey were stressed. In the process ofremoving these segments from the stor-age area, the boom of the lifting cranecollapsed and severely damagedanother two segments. Since these foursegments were to be located near the tipof one of the cantilevers, it was onlynecessary to recast these four segments.

The original design called for thetransverse reinforcing bars crossing theanchorage recesses for longitudinalpost-tensioning in the top slab to extendinto the recesses and he spliced by fieldwelding after longitudinal post-tension-ing. However, for construction it wasdecided to place prebent bars through

40

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42

Fig. 10. Typical pier segment with cast-in-place diaphragm and erection oS adjacentsegment.

the recess formers and bend them downto form a lap splice after longitudinalpost-tensioning. This caused thebent-up bars to interfere with construc-tion and many of them were broken off.In retrospect, the original design detailwould have been better.

ErectionAs noted earlier, the longitudinal ten-

don ducts had displaced between thesegment faces during precasting and thefriction loss was higher than anticipated.since low-relaxation strands were usedon this project, it was decided to in-crease the jacking force from 0.08f f to0.85 f„ for subsequent tendons to com-pensate for the higher friction loss.

Due to sharp kinks at some segment

joints, some strand wires did break.Where it was not possible to increasethe jacking force for subsequent ten-dons, especially near the ends of thecantilevers, the temporary prestressingtendons connecting the last few seg-ments were left in place and grouted.

Provision should be made in all seg-mental bridge designs to provide addi-tional prestressing to compensate forany excessive friction or for futurestrengthening.

Due to the breakage and patching ofthe concrete in the bottom slab at thesegment joint during segment separa-tion from the bulkhead and possiblyother reasons, numerous interconnec-tions and leaks were discovered duringthe process of water pressure testing theducts prior to grouting. The original de-

PCI JOURNAL)November-December 1984 43

Fig. 11. Delivering segment on completed cantilever for erection by launching truss.

sign called for the negative momenttendons in the cantilevers to be groutedafter the completion of each balancedcantilever. However, due to the possi-bility of duct interconnection, thegrouting was delayed and the tendonswere protected against corrosion bycoating them with a water soluble oilprior to installation.

The northbound lanes structure wasgrouted in one stage after completion ofsuperstructure erection. The south-bound lanes structure was grouted intwo stages due to the approachingwinter. The grouting for the first stageextended to the vicinity of Pier 3. Thegrouting for the second stage, which in-cluded the remaining tendons, was car-ried out in December during coldweather. The structure was heated frominside the box to prevent freezing of thegrout. The required heat exceeded thetemperature differential between theinterior and exterior of the box girder as-sumed in design and fine cracks de-

veloped in some segments on each sideof Piers 4 and 5.

Where heating for grouting purposesis likely to be required, the designshould allow for the anticipated thermalg_ rradient.

The vertical alignment of the canti-levers was controlled by insertion ofstainless steel wire mesh between thesegment faces and the resulting canti-lever tip elevation was within 1 in. (25mm) of the theoretical value. Atone clo-sure segment, the clamping beams be-tween the cantilever tips moved duringthe concreting operation, resulting in astep of about 2 in. (51 mm) across theclosure segment.

The alignment control pins in top ofthe deck along the segment centerlinewere set about 2 in. (51 mm) from thesegment faces. Since the horizontalalignment computations were based onthe actual segment lengths, rather thanthe pin to pin lengths, the erectedcurved cantilever tips curled inwards.

44

Fig. 12. Underside view of completed bridges showing the transverse ribs.

The maximum horizontal alignment de-viation at the forward tip (with the reartip in the correct position) was 6 in. (152mm), This error was split between twoclosure segments by rotating the bal-anced cantilever in plan and the devia-tion is unnoticeable.

A serious problem on this project wasthe discovery of a survey error, whichhad located Pier 4 of the southboundlanes structure about 3 ft 6 in. (1.07 m)east of its correct location, as the canti-levers at Pier 3 were nearing completion(Fig. 7). Since the alignment of thebridge was curved, the problem couldbe resolved by altering the horizontalalignment of the bridge between theforward cantilever at Pier 3 and thenorth abutment. By allowing some ten-sion in the top ofthe post-tensioned piercaps and reassessing the pier columnand footing design, it was possible toshift the pier segment at Pier 4 eastwardfrom its theoretically correct location by1 ft 9 in. (0.63 ni) with a further easterly

bearing shift of 9 in. (0.32 m) relative tothe segment and completing the bal-anced cantilevers as usual, as shown inFigs. 8 and 9.

The completed cantilever at Pier 4was then rotated to line up with the pre-viously completed cantilever at Pier 3and the closure segment was concreted.The pier segment on the correctly lo-cated Pier 5 was also shifted eastward by1 ft 9 in. (0.53 m) from its correct locationwith an easterly bearings shift of 1 ft(0.30 m). The completed cantilever atthis pier was then rotated to bring theforward cantilever tip to its correct posi-tion. On inspection, the shift in thecurved alignment is not noticeable.

It was fortunate that the bridge was ona curved alignment and some adjust-ments could be taken up by the piersand the cast-in-place closure segments.With any other type of construction, itprobably would have been necessary todemolish Pier 4 and reconstruct it in itscorrect location, resulting in increased

PCI JOURNAUNovember-December 1984 45

Fig. 13. View of completed bridge.

costs and considerable delay in com-pleting the bridges.

Figs. 10 through 13 show variousphases of the bridges during construc-tion and after completion.

Closing CommentsThe Twelve Mile Creek Bridges were

the first precast segmental concretebridges to be constructed by the OntarioMinistry of Transportation and Com-munications. The restricted construc-tion depth and the curved and twistedalignment combined with flared decksnear the north abutments, provided achallenge that was satisfactorily met byusing precast prestressed concrete seg-mental construction.

The experience gained and the les-

sons learned from the design and con-struction of these bridges have en-hanced the future of this type of con-struction in Ontario.

The total cost of the bridges (in Cana-dian dollars) was $8,970,000 or $100/ft2($10761m 2). The cost of the superstruc-ture was $88/ft 2 ($947/m2).

In 1984 the Twelve Mile CreekBridges received national awards fromboth the Prestressed Concrete Instituteand the Post-Tensioning Institute.

The bridges were opened to traffic inOctober 1984 on completion of Highway406 to the Queen Elizabeth Way.

CreditsOwner: Ministry of Transportation and

Communications, Ontario.

46

Engineer: Structural Office, Ministry ofTransportation and Communications,Ontario.

Special Consultant to Owner: Precon-sult Canada.

General Contracator and Manufacturerof Precast Segments: Kilmer VanNostrand Company Ltd., Downsview,Ontario.

Special Consultant to Contractor: DRCConsultants, New York City, NewYork.

Post-Tensioning Supplier: CanadianBBR (1980) Inc., Toronto, Ontario.

References1. Standard Specifications for Highway

Bridges, American Association of StateHighway and Transportation Officials,Washington, D.C., 1977.

2. Bridge Construction (BC) Computer Pro-gram, Europe Gecti Etudes, France.

3. Precast Segmental Box Girder BridgeManual, Published jointly by the Pre-stressed Concrete Institute, Chicago, Il-linois, and the Post-Tensioning Institute,Phoenix, Arizona, 1978.

4. CEB-FIP Model Code for ConcreteStructures, Comite Euro-International duIB^ton, Paris, 1978.

NOTE: Discussion of this paper is invited. Please submityour comments to PCI Headquarters by July 1, 1985.

PCI JOURNAL.lNovember-December 1984 47