Approach Spans to theWest Seattle Bridge

144.

4Thomas A. KanePrincipal-in-Charge

and Project ManagerAndersen Bjornstad

Kane Jacobs, Inc.Seattle, Washington

James E. CarpenterProject EngineerAndersen Bjornstad

Kane Jacobs, Inc.Seattle, Washington

John H. ClarkBridge EngineerAndersen Bjornstad

Kane Jacobs, Inc.Seattle, Washington

Synopsis

Some of the largest precastprestressed concrete girdersused to date in the UnitedStates are currently being in-stalled on the West SeattleFreeway Bridge in Washing-ton. This report describes thedesign/construction systemadopted for the project's Har-bor Island, West Interchange,and East Interchange units.

C onstruction efficiencies inherent insome of the largest precast pre-

stressed concrete girders used to datein the United States are speeding theWest Seattle Freeway Bridge towardsits scheduled 1985 completion. Thiscomplex $87 million urban freewayproject consists of four major units asillustrated in Fig. 1.

The West Interchange connects theexisting West Seattle Viaduct and sur-face streets to the main span crossing theWest Waterway (main navigation chan-nel). The Harbor Island unit carries theroadway across the industrial area andEast Waterway. The East Interchangeunit connects the project to the existingSpokane Street Viaduct, Alaskan WayViaduct, and surface streets.

The segmentally constructed, post-

C

OG4 HARBOR ISLAND F

4 ILl

WEST SEATTLE y,Q

WEST MAIN ARB R I EAST

INTERCHANGE SPAN APP OAG INT'CHGE

5600'

Fig. 1. Map of project area showing location of West Seattle Bridge approach.

Table 1. Summary of approach contracts.

ContractNumber Description

Deck Area(sq ft)

Girders

Number Lin. ft

5 Harbor Island Approach 168,125 161 24,183

6 West Interchange 245,719 242 30,216

7 East Interchange 222,730 294 27,291

Note: 1 sq ft = 0.09 Ins : 1 It = 0.305 in.

tensioned concrete box girders com-prising the three-span West WaterwayCrossing have been described else-where. 1g This paper focuses on the pre-cast prestressed concrete girder systemused for the Harbor Island, West Inter-change, and East Interchange units,The magnitude of the project is indi-cated in Table 1, which lists the num-ber and lengths of girders and roadwayareas for the main units.

Girder DesignSeveral site constraints combined to

result in a clear need to maximize thespan length while still preserving theeconomy inherent in precast pre-stressed girder construction. For exam-ple:

I. Soil conditions required expensiveand heavy pile foundations.

2. Surface traffic conditions and pub-lic desires favored a layout which min-imized the number of columns and pro-vided an "open" appearance.

3. Construction phasing to accommo-date traffic routing through the siteduring construction severely restrictedthe possible pier locations.

The principal design issues faced inthe development of the approach unitswere aesthetics, economy, constructionstaging, and design for good seismic re-sponse. The solution, which evolvedfrom these considerations, is a series ofrigid frames made up of precast pre-stressed concrete girders and cast-in-place reinforced concrete columns, capbeams, and roadway slab.

PCI JOURNAL/November-December 1983 59

0

^123P Axp itLEp. AExp. Jt

102 102 116 120 121 151 154 164 164 158 I 189 189

(31) (31) (35)! 137) 1 (37) (37) (46) (47) (50) öT (48)1(58) ! (58)

Fig. 2. West Interchange plan (top) and elevation (bottom). Span lengths given in feet (m).

31011 -

2 1 - 4

M 120 ?S 120 _

Draped Post-tension

Stroight Pretension's i

2'- r^

Fig. 3. Girder Section showingmodifications (M120) to standard section(S120),

The construction sequence wasplanned so as to maximize the devel-opment of dead load continuity mo-ments at the piers. The rigid frameswere an essential feature of the designfor proper seismic response. Fig. 2shows the plan of the West Inter-change, which consists of four separateframes and two ramp structures whichframe into the main roadway.

Variations of the basic design themefor the units were (1) different girdersections for shorter spans in areas ofsharp curvature; (2) shorter framelengths; and (3) the insertion of simplespans where required to decoupleseismic response motions betweenframes.

The largest standard girder sectionavailable was the Washington StateDepartment of Transportation 120Series girders which had previouslybeen utilized for spans up to about 145ft (44 m). To allow the longer spansneeded for this project, two modifica-

tions to this section were made whichallowed easy adaptation of the existingstandard forms (Fig. 3).

1. The web width was increased from5 to 6 in. (127 to 152 mm), thus in-creasing the shear capacity of the sec-tion and allowing more room for con-crete placement around the post-ten-sioning tendons.

2. The flange width and thicknesswere increased to provide better stabil-ity during handling and erection.

Using the modified section, the max-imum girder length of 154 ft (47 m)was established after consultation withlocal precasters. Longer and heaviergirders would have been beyond thecapabilities of existing transportationequipment and would have made erec-tion more difficult. These limitationsestablished an upper limit on spanlength for this structure type. Horizon-tal clearance requirements over surfacestreets and railroad required a 185-ft (56m) span adjacent to the main spanunit which was achieved by wideningthe typical 10-ft (3 m) cast-in-place capbeam into a 70-ft (21 m) long cast-in-place reinforced concrete box girderover the pier.

Prestressing of I-girders is providedby a combination of straight preten-sioned strands in the bottom flange anddraped post-tensioned tendons. Thestraight pretensioned strands furnishadequate prestress for removal from thepretensioning bed and handling in theyard. Some of the straight pretensionedstrands were debonded by plasticsleeves near the end for stress control.Release strengths required were lessthan would have been required for afully pretensioned design. Cambergrowth is better controlled since post-tensioning can be delayed until higherstrength and modulus of elasticity areachieved. The turnover time for thebeds was thus held to 24 hours. Post-tensioning operations were off the criti-cal path and were performed at the yardstorage locations.

PCI JOURNAUNovember-December 1983 61

An essential element in achieving theutmost structural efficiency and in sat-isfying the seismic design requirementswas the construction sequence. Thissequence was designed to obtain signif-icant dead load negative moments atthe piers through the sequencing of theslab pours. Staging the slab casting se-quence so that the sections over thepiers were cast before the sections overthe central portions of the slab resultedin a larger dead load negative momentthan would result if the slab were cast

in one pour. This additional negativemoment reduced the required positivemoment at the midspan of the girders,but more importantly it prevented mo-ment reversal (i.e., positive moments atthe piers) under seismic response con-ditions. It is difficult to provide con-structahle, efficient details to resist pos-itive moments at the piers in precastgirder construction.

The construction sequence used isillustrated in Figs. 4 and 5. Poor foun-dation conditions made it necessary to

Step 1. Construct pier up to construction joint at cap beam soffit.

Step 2. Erect girder and cap beam falsework.

Step 3. Erect girders.

Fig. 4. Construction sequence.

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Step 4. Cast cap beams.

Step 5. Cast slab over piers.

Step 6. Cast slab over midspan portion.

Step 7. Cast barrier curbs and complete.

Fig. 4 (continued). Construction sequence.

PCI JOURNAUNovember-December 1983 63

Fig. 5. Girders erected on falsework priorto casting cap beams and deck slab.

i1'Or

Fig. 6. Shear-friction reinforcement ingirder ends.

support the erected girders on steelfalsework founded on the permanentpile caps. This specification require-ment prevented premature overloadingof the shear-friction connections be-tween the girders and the cast-in-placecap beam due to falsework settlement.

Close attention was given to the de-tailing of reinforcing steel projectingfrom the ends of the girders. This rein-forcement had to be closely coordinatedwith cap beam reinforcing to avoid in-terference. The resulting detail is theoverlapping 180-degree hooks shown inFig. 6. The reinforcement required forpositive moment (due to seismic loads)where the girders framed into a pierwith an expansion joint at the far sideusually required additional mild steelat the bottom flange. The bottomstraight pretensioned strands were ex-tended in the cap beam in all girders.Fig. 6 also shows the shear keys whichwere developed to meet design shearrequirements. Dapped-end girders atexpansion joints had additional shearkeys on the sides where the end dia-phragm framed in.

Plans of detailing for the cap beamand girder interface included a scalemodel of the cap beam and all requiredreinforcement (Fig. 7). This modelproved to be a helpful aid in preventingreinforcing steel conflicts. As a result ofthis prior planning, the project has ex-perienced a minimum of field problemsof reinforcing steel conflicts.

ConstructionConcrete strength for the girders was

specified at 7000 psi (49 MPa). Thisstrength requirement is met routinelyby precasting plants in the area. Trans-fer strength for pretensioning was 4000to 5500 psi (28 to 38 MPa) dependingupon the girder details. Strength at ap-plication of the post-tensioning was re-quired to he 6000 psi (42 MPa).

The mix designs used for girdermanufacture were predicated upon the

64

Fig. 7. Cap beam scale model.

Table 2. Typical concrete mix designs.

Amount per Cubic Yard

Contract 5, 7 Contract 6Material

Cement, Type III 605 lbs 605 lbs 752 lbsWater, total 249 lbs 212 lbs 300 lbsDry, fine aggregate (F.M. 3.1) 1190 lbs 1435 lbs 1176 lbsDry, coarse aggregate (7/a in.) — — 1743 lbsDry, coarse aggregate ('/z in.) 2105 lbs 1985 lbs -Water-reducing admixture 42 oz 30 oz 40 ozSuperplasticizer — 60 oz -

Slump 3-5 in. 3-5 in. 3-5 in

Note: 1 1b = 0.45 kg; 1 ox - 28.35 g; l iu. - 25.4 mm.

pretensioning transfer strength, sincethis strength was the critical require-ment for girder removal from the cast-ing bed. Some of the typical mix de-signs used are shown in Table 2.Weather conditions prevalent at thetime of casting were the final determi-nant in mix design selection.

Erection of the 75-ton (68 t) girdersrequired two 200-ton (181 t) cranes, asshown in Fig. 8. Girders were trans-ported by tractors with steerable reartrailers, as shown in Fig. 9.

SummaryThe precast girder approach struc-

tures on this project were successful inovercoming the traditional aestheticobjections to precast girder structures.This success was due to the attentiongiven to (1) achieving a harmoniouspattern and arrangement of girders inthe structure as viewed from below; (2)maximum span lengths; and (3) the de-tails providing cap beam soffits flushwith the girders (Fig. 10).

PCI JOURNAUNovember-December 1983 65

Fig. B. Erection of girders.

Fig. 9. Girder transportation using steerable rear trailers.

66

Fig. 10. View from beneath structure.

Preliminary design studies clearlyindicated the economy of precast girderconstruction over competitive structuretypes and these studies were reflectedin the bids received. Precast girderconstruction also facilitated the mainte-nance of traffic through the area duringconstruction by minimizing the false-work required.

REFERENCES

1. Kane, T. A., Carpenter, J. E., and Clark,J. H., "Concrete Answer to Urban Trans-portation Problems," Concrete Interna-tional, August 1981, pp. 85-92.

2. "Girder Duo Cantilevers in Union," En-gineering News Record, November 25,1982, pp. 26-27.

3. "Record Girders Trucked to Site," En-gineering News Record, March 24, 198.3,p. 69.

Credits

West Seattle Design Team: AndersenBjornstad Kane Jacobs, Inc., Seattle,Washington; Parsons Brinckerhoff,Seattle, Washington; Tudor Engi-neers, Seattle, Washington; Kramer,Chin and Mayo, Seattle, Washington;and Contech Consultants, Seattle,Washington.

Special Consultant: Dr. A. H. Mattock,University of Washington, Seattle,Washington,

Girder Manufacturers: ConcreteTechnology Corporation, Tacoma,Washington, and Prestressed Con-crete Products, Woodinville, Wash-ington.

General Contractors: Kiewit-Grice: (ajoint venture), Seattle, Washington,and Moseman Construction, Seattle,Washington.

Owner: City of Seattle.

PCI JOURNALINovember-December 1983 67