Segmental Box Girders for the High Level West Seattle Bridge, C. K. Yu

Home , West Seattle

Segmental Box Girders Rising 150 ft (46, State of Washingi span of the West: for the High Level example of efficiei long-span structur West Seattle Bridge the structure’s deE

girders. Only the superstructm of the two concrete altemate covered in this article. Fig. 2 shows the general plan vation of the concrete bridge, w a span arrangement of 375-5 (114-180-114 m). The superstn rigidly connected to the tw( piers. The bearings at the end i free to slide in a longitudinal c but restrained laterally. The ceni are designed to withhold the t L ture, creep, and shrinkage defor of the center span. Ching K. Yu, P.E. The Manager overall outlines of the si are Contech Consultants, Inc. identical for both the cast-i Seattle, Washington and precast schemes. Howev

or many years two bascule bridges The initial planning for the ensuing Fover the west waterway of the replacement project has been described Duwamish River were the only direct previously.1 link between West Seattle, Washington, As shown in the vicinity map (Fig. 1), and the downtown area. Busy maritime the entire project was divided into four traffic and an ever-increasing vehicular units: West Interchange, Main Span, volume on the bridges caused serious Harbor Island Approach and East Inter traffic congestion, making the need for a change. A paper published in the high level structure apparent. November-December 1983 PCI JOUR On June 11, 1978, a fully-loaded NAL describes the design and con 12,000 ton (10,900 t) freighter slammed struction of the three approach structure into the north bascule bridge, severely 2units This article presents a detailed damaging the superstructure and pow discussion of the main span. erhouse wall and rendering the bridge Structures selected for final design in inoperable. This accident left only the cluded three types: (1) steel box girders south bascule bridge, with its four-lane with an orthotropic steel plate deck, (2) capacity, to handle all traffic, creating an cast-in-place segmental prestressed even more acute need for a high level concrete box girders, and (3) precast replacement bridge. segmental pre stressed concrete box

52 PCI JOURNALJJUIy-August1984 Rising 150 ft (46 m) over the Duwamish River in the State of Washington, the recently completed main span of the West Seattle Bridge is an excellent example of efficient use of prestressed concrete for a long-span structure. This article presents highlights of the structures design and construction.

girders. Only the superstructure design minimum concrete strengths at 28 days of the two concrete alternates will be are 5000 and 6000 psi (35 and 41 MPa) covered in this article. for the cast-in-place and precast alter- Fig. 2 shows the general plan and ele- nates, respectively. This allows a thin- vation of the concrete bridge, which has ner bottom slab for precast segments at a span arrangement of 375-590-375 ft the center piers. Otherwise, all the con- (114-180-114 m). The superstructure is crete dimensions were kept uniform for rigidly connected to the two center both construction methods. Unless piers. The bearings at the end piers are otherwise stated, the description free to slide in a longitudinal direction hereinafter applies to both superstruc- but restrained laterally. The center piers ture alternates. are designed to withhold the tempera- The bridge section is composed of twin ture, creep, and shrinkage deformations single cell boxes of trapezoidal shape of the center span. (Fig. 3). The total deck width of 104 ft 5 The overall outlines of the structure in. (32 m) provides a roadway of six traf- are identical for both the cast-in-place fic lanes and shoulders. Following a and precast schemes. However, the parabolic curve at its soffit line, the

GG^ HARBOR ISLAND SEATTLE sy WEST SEATTLE

WEST INTERCHANGE MAIN SPAN 4 HARBOR ISLAND EAST 1340 \ I INTERCHANGE 5600 Fig. 1. Vicinity map of West Seattle Bridge.

PCI JOURNAL/July-August 1984 53 AU1 f Pier 15 Pier 16 Pier 17 C Pier 18

Duwamish River West Waterway o

West Seattl [QBarrier

Seattle Expansion \ o Expansion Joint \\ Joint

375-0 59 -0" 375-0" Span

S 00

125 -0" 125-0" o Clearance Envelope

.I. tai /pp rox. Existing I Channel Ground Future Elev.0.00 Future City Datum Waterway

Fig. 2. General plan and elevation of main spans. C- 0 WSB Line C

52-21/2 52- 21/2^ z D Box Girder Box Girder C

D 2" Wearing Surface 2.0/. Cc 2.0

OD co A O rn o _ HL.N r N — m U H I 0I I

I ^TYp. 0 U 24-II1/4" 1 N m ^ v a

0 0 co n

17-83/4"

Section Section At Piers 16 and I7 At Mid-Span

Fig. 3. Typical sections of main spans.

C 55-0"

151-0 5-0° 15-0 5-0 5-0°

12151<121 5K 121.5 K12,51<2151< 121.5K 121.5K

8 Axles at 21.5 K each; 172 K Total Transver wheel spacing 0-8 O.C. Tire pressure 80 psi, width 12"

Vehicle Load

57-6" .HHHH.3.0 1

Eauivalent Uniform Load

Fig. 4. Assumed overload due to vehicle load and equivalent uniform load.

depth of the girder varies from 30 ft (9.1 cluded a temperature differential of 10 m) at center pier to 12 ft (3.7 m) at deg F(6 degC) warmer or5 deg F(3 deg midspan. The slightly arched form of the C) cooler at the outside than the inside superstructure provides not only effi- of the box. A temperature for the top slab cient use of concrete materials and pre- 18 deg F (10 deg C) warmer or 9 deg F (5 stressing, but is also aesthetically ap- deg C) cooler than the rest of the section pealing. was incorporated into the longitudinal design. In addition to the interstate highway Design Criteria bridge loadings specified in AASHTO, In general, the structural design fol- Article 1.2.5(G), the bridge was also de- lows AASHTO Standard Specifications.3 signed for an overload vehicle or its For subjects not covered by AASHTO, equivalent (Fig. 4). other commonly used codes for seg- Allowable concrete stresses in tension mental construction were adopted.4•5,6 specified in AASHTO, Article 1.6.6, In application of AASHTO loading were modified to account for frequent combinations, the transverse design in- segmental joints in the superstructure

56 Table 1. Allowable tensile stresses in psi (fe)for cast-in-place construction.

All tension force No additional resisted by additional Loadingcombinations reinforcement reinforcement Dead+ Live or 0 O

Table 2. Minimumcompressive stresses in psi for precast construction (longitudinal design only). Deck slab Deck slab local stresses local stresses Other Loading combinations not included included area Dead ÷ Live or 200 100 30 Dead + Construction All other combinations 30 30 30

Note: 1 psi = 0.006895 MPa. )overn

ad and to provide better control over deck deck was specified. An additional slab concrete cracking. Table 1 shows superimposed dead load of 25 lb per sq 9quivalent uniform load. the allowable prestressed concrete ten ft (1.2 kN/m2) of deck surface was de sile stresses for cast-in-place construc signed for as a future surfacing allow tion, applicable to the design of both ance. transverse deck and longitudinal cross emperature differential of 10 sections. :gC) warmer or5 deg F (3 deg The precast design’s allowable stress Construction Sequence it the outside than the inside for the transverse deck section remains As shown in Fig. 5, the suggested con Atemperature for the top slab the same as in Table 1. However, in rec struction sequence may be divided into .OdegC)warmeror degF(5 ognition of reinforcement discontinuity four stages: ler than the rest of the section at segmental joints, allowable lon Stage 1: Casting the pier table over orated into the longitudinal gitudinal tensile stress requirements are both the north and south columns at the 9 more stringent. In fact, minimum com east side the river. Post-tensioning all of ion to the interstate highway pressive stresses are required for this di the tendons in the pier table and placing dings specified in AASHTO, rection (Table 2). Allowable compres two sets of form travellers in position, .5(G), the bridge was also de sive stresses for each construction are in one pair for each box girder. For the r an overload vehicle or its conformance with AASHTO require purpose of reducing the unbalanced (Fig. 4). ments. pier moment, the pier table is offset a le concrete stresses in tension A dense concrete or latex-modified half segment length from the centerline in AASHTO, Article 1.6.6, concrete wearing surface of 2 in. (51 of the pier. iified to account for frequent mm) nominal and 1Y2 in. (38 mm) Stage 2: Performing the balanced free joints in the superstructure minimum thickness over the bridge cantilever construction by casting 16 ft 6 ‘I PCIJOURNAL/July-August1984 57 in. (5 m) segments on alternate sides of Progress of the two parallel girders the pier table. Stressing half of the should be approximately the same, with transverse tendons in each girder from a difference of no more than two seg- the outboard edge of the deck slab, as ments. This is to minimize possible mis- shown in Fig. 6. The other half will be alignment of the continuous transverse stressed only after the longitudinal clo- tendons due to concrete creep effect. sure between the two girders is cast. Stage 3: After the balanced cantilevers

375-0" , 590,-0° 375-0

t North Columns

r- j L c5outh Colurns Stage I - Build East PierTable Forroveller Span Th Pierl6 CE Pier 17 (West (East)

Stage 2-Free Cantilever Const ruct East Side

^i Temporary 1 Supports Stage 3- Complete East half of Bri dge

.J ® 4a ^ r Longitudinal Closure Pour Closure Pour Diaphragm 0

Ex ponsion Expansion Joint Joint

Stage 4-Complete the whole Bridge

Fig. 5. Suggested construction sequence of main spans.

reach their limits, moving the center the precast units, the segment length is span form travellers to the west pier reduced to between 6 ft (2 m) at pier and table, installing temporary supports near 12 ft (4 m) at midspan. the edges of the side span cantilevers and continuing segmental construction Prestressing System until the girders land on the end pier. Stage 4: Repeating the construction as Longitudinal tendons consisting of 12 described above for the west half of the x 0.6 in. (15 mm) diameter strands were bridge. Pouring the center and lon- specified in the design. As shown in Fig. gitudinal closures, stressing all the con- 7, the cantilever tendons are anchored, tinuous transverse tendons and com- three at each web, on the bulkhead face pleting other work. of each segment, the continuity tendons The construction sequence for the at buttresses built up from the slab. The precast scheme is basically the same as cantilever tendons follow the profile described above. However, to facilitate grade and the continuity tendons follow transportation, handling and erection of the soffit line.

Stress after completion Longitudinal Closure Pour of Segment

II 4-1 A

Stress after Longitudinal Closure Pour Plan

WSB Line Box Girder Box Girder 3-6°(Closure)

Discontinuous ons Ti

Section A-A

II Stressing End I Dead End

Fig. 6. Plan and section of transverse tendon details.

PCI JOURNAL/July-August 1984 59 rn 0

26-1/4 Box Girder 1 1/4" Dia.Deformed Bar Longitudinal Tendons Symm. abt. (Transverse Tendon) Anchor Plate

!." °O oo 0 00000 o 00 0 001 000000° 0 12x0.6°Dia. Strands ° 1 (Cantilever Tendon) °

11 1 1/a° Dia. Bar (Vertica 11 Web Tendon )

11 Typ. Stressing Buttress 11 Anchor Plate

1 ^ o ;.. 0 0 0 0

12x 0.6" Dia. Strands (Continuity Tendons)

Fig. 7. Tendon arrangement inside box segment., Straight tendons are preferred for strand and wire systems providing their ease of installation and ability to equivalent effective forces are also per- minimize friction losses. They also pre- mitted. vent the necessity of having a wide web to accommodate the anchor plates, as Corrosion Protection they are all anchored away from the webs. For corrosion protection of reinforcing Additional longitudinal bar tendons bars and prestressing tendons, a are installed across the deck slab of the minimum concrete cover of 1Y2 in. (38 closure segments. They are required to mm) was specified for the deck slab in resist the negative moments induced by addition to the wearing surface. Epoxy live loads before creep-induced mo- coating was required on all top mat deck ments settle in. bars as well as web bars having less than For transverse and vertical tendons, 3½ in. (89 mm) concrete cover, mea- 1 1/4 in. (32 mm) diameter deformed high sured from the top of the monolithical- strength bars are preferred. The screw ly-cast deck slab. To eliminate the need type bar tendon anchorages reduce the for epoxy coating long web bars, a spe- seating loss to about lho in. (2.5 mm), cial detail was developed (Fig. 8). rendering them most effective for One of the following measures must short-length prestressing. However, be taken to protect the transverse ten-

PCI JOURNAL/July-August 1984 61 dons, depending on the type of tendons Design Considerations used: (1) epoxy coating the deformed Although sections subjected to pre- high strength bar tendons, (2) using stressing forces were designed accord- plastic ducts with spiral corrugations, ing to allowable stress requirements, either polyethylene or polyvinyl- their ultimate strengths must be chloride, (3) using epoxy coated metallic checked by load factor design, as ducts. specified by AASHTO. For the lon- Because the longitudinal tendons are gitudinal direction, allowable stress located well below the riding surface, checks are performed at all construction galvanization of the metallic ducts is stages, i.e., when a new segment is considered adequate for their corrosion added, a form traveller advanced or re- protection. moved, additional tendons stressed and Special Requirements a closure poured. In addition, stress conditions at completion of the bridge, The following prestressing-system 18 months after completion, and at in- related requirements were specified: finity must be evaluated to insure the 1. Pull-out tests: If epoxy-coated flat soundness of the structure with all pre- or plastic ducts are used for transverse stress losses and creep effects consid- tendon sheathing, pull-out tests must be ered. performed to prove the bonding- In the transverse direction, the cross strength adequacy of the ducts. The section was analyzed as a frame struc- tests should show that a force equal to 40 ture. In order to take longitudinal dis- percent of the ultimate tensile strength tribution of wheel loads into considera- of the tendon, 0.4 fpu A,,, can be trans- tion, influence surfaces prepared by ferred from the tendon through the duct Homberg7 were used to determine the to the surrounding concrete at a length equivalent beam fixed end moments of 2 ft 6 in. (0.76 m). A total of 12 pull-out prior to the frame analysis. The method tests should be conducted of which 10 is described in detail in the PTI Box must pass the bonding requirement. Girder Bridge Manual." 2. Lift-off tests: As short tendons are The load factor method was used for apt to lose too much prestressing force designing reinforcement in the webs due to slip at anchorages, tendons short- and bottom slabs, whereas allowable er than 20 ft (6 m) must be equipped stress design was used to determine the with an anchorage device which allows transverse tendon spacing and profiles. lift-off checks and restressing. This re- Since only half of the transverse tendons quirement applies to most vertical ten- run continuously across the longitudinal dons. closure, dead load stresses and second- 3. Water freezing prevention: Covers ary moments introduced by the discon- and plugs should be provided to prevent tinuous tendons need to be estimated. rain water from entering vertical ducts. This is done by using a separate struc- As a complete seal of ducts at deck level tural model consisting of only one box is always difficult, some water will in- girder section. evitably be collected, and its sub- Vertical bar tendons were selected as sequent freezing could cause cracks in the main reinforcement for shear force. the webs or delamination of the bottom However, in applying AASHTO design slab. Unless a complete seal can be equations specified in Article 1.6.13, achieved, the ducts should be filled half of the nonprestressed reinforce- with a mixture of glycol antifreeze and ment required for transverse flexure de- water when the temperature falls below sign is considered effective in resisting 45 F(7C). shear forces. This is justified, since po-

62 Approach Structure Main Span I ft 1rD

End Pier

D=8+0.02L+0.08H D1 = D/4 where D and D i are in inches. L Length of bridge deck between expansion joints, in feet. H= Pier height in feet

Fig. 9. Minimum gap and seating length requirement.

tential shear failure normally originates minimum seating length, D, was estab- at the half-depth location of the web lished in accordance with Article 4.9.1 where transverse bending stresses are of ATC-6, and the gap distance, Dl , was not critical.9 predicted by response spectrum For seismic design, recommendations analysis." This provision stipulates that developed by the Applied Technology temperature induced movements must Council (ATC-6)10 were followed. The be added to D, in determining the de- design objective is to ensure that in an sign expansion joint gaps. earthquake, with a 50-year probability of To provide a positive horizontal link- occurrence, the bridge will remain usa- age between the main span structure ble for emergency passage, although it and the approaches, earthquake re- may suffer repairable damage. This cor- strainers were installed at each expan- responds approximately to designing sion joint. A sufficient number of earth- the ultimate strength of the bridge for a quake restrainers were selected at load equivalent to that induced by an each expansion joint to provide a total earthquake of a 500-year return period. ultimate strength equal to 15 percent of The structure is designed to allow plas- the weight of the main span structure. tic hinge formation at the pier top and The length of the restraining cables is bottom only when its failure mechanism such that they will accommodate with- is reached. out rupture to the total joint movement Special consideration was given to the during an earthquake. In order to trans- earthquake requirements for expansion mit this restraining force to the remain- joints and restrainers. Fig. 9 schemat- der of the superstructure, additional ically shows the gap and seating length tendons were provided at the weak parts as specified in the design. The of the side spans.

PCI JOURNALJJuly-August 1984 63 Contract Awarding with an orthotropic steel plate deck. No bids were received for the precast con- A total of six bids was received on crete superstructure alternate. The bids October 1, 1980: five for cast-in-place for the concrete structure varied from box girders and one for steel box girders approximately $24 to $28 million, and

375-0" 590- 0 375-0" CE North Columns

^S olumns O Formtroveller

Stage I - Build East Pier Table

(jSpon Pier 16 Q Pier 17 (West) I (East)

104 -S

Stage 2 - Free Cantilever Construction, East Side

L .l Closure Pour O

East Half of Bridge Stage 3 - Complete Falsework

Expansion Joint O Closure Pour Expansion Joint

Stage 4 - Complete the whole Bridge

Fig. 10. Actual construction sequence.

64 Table 3. Comparison of tendon system from as designed and as built condition.

Tendon location As designed As built Longitudinal 12 x 0.6 in. (15 mm) 0 strands 19 x 0.5 in. (13 mm) (A strands Transverse Epoxy coated 1/a in. (32 mm) 4) 4 x 0.6 in. (15 mm) 4) strands deformed bar with epoxy coated flat duct Vertical 1 in. (32 mm) 4) deformed bar 6 x 0.5 in. (13 mm) 0 strand

the sole steel bid was approximately $40 designer was used in construction. million. The winning bid put the con- 2. As permitted in the specifications, struction cost of the structure at about the contractor had the option of select- $170 per sq ft ($1935 per m 2). A break- ing any post-tensioning systems pro- down of the bid is as follows: viding they met the requirements Common Items (mobilization, specified in the special provisions. utilities, etc.) ...... $3.3 million Table 3 compares the designed systems Substructure (steel pipe piles, with the contractors choices. foundation and reinforced concrete Although the contractor replaced the pier columns) ...... $9.1 million 12 x 0.6 in. (15 mm) diameter strands Superstructure (post-tensioned with 19 x 0.5 in. (13 mm) diameter structure) ...... $11.4 million strands for the longitudinal tendons, the pattern, distribution, and anchorage location of the tendons remain the same as I Construction Modifications shown in the design drawings. The Basically, the bridge was built in ac- spacings of vertical and transverse ten- cordance with the conforming cast-in- dons were adjusted to provide the place design. However, the following minimum effective forces required by modifications were furnished by the the designer. contractor: 3. As described previously in the con- 1. Instead of casting the two box gird- struction sequence (Fig. 5), the designer ers separately, the contractor elected to recommended that the unbalanced parts pour the complete bridge section all at of the side spans be built segmentally by once. This was accomplished by tying the form travellers, with the help of two parallel form travellers together temporary supports. However, the con- during concrete casting. This modifica- tractor elected to construct that portion tion eliminated the need for leaving a of the side span totally on falsework longitudinal closure between the two (Fig. 10). To allow room for adjusting box girders. Since all transverse tendons misalignment, a 3-ft (0.91 m) wide clo- were made continuous, any possible sure was provided between the end of misalignment of the transverse tendon the side span cantilever and the ducts was avoided. This simplified the falsework-supported portion. Since the construction but required a more inten- contractor dismantled the form travel- sive labor force, reducing flexibility of lers in the proximity of the east end clo- manpower allocation. sure prior to making the closure pour, Fig. 10 indicates the construction se- clamping the ends together proved to be quence selected by the contractor. The difficult, and concrete block counter- girder cross section was kept the same as weights were placed on the higher side that shown in Fig. 3. The segmental of the two free cantilevers to achieve length of 16 ft 6 in. (5 m) specified by the vertical alignment.

PCI JOURNALIJuly-August 1984 65 Concluding Remarks placing reinforcing bars and tendon ducts, concreting, stressing the tendons The construction went smoothly with and grouting the ducts, their pro- only a few delays at the beginning of the ductivity dramatically improved. Near job. The first two segments took about 2 the end of construction of each balanced months to complete. Once the crews be- cantilever, it was not uncommon for two came familiar with the repetitive opera- segments to be completed within a tions of advancing the form travellers, week. A special feature of this project was the close involvement of the consultants in nearly all aspects of the construction. Continuous review of construction progress and daily field inspection were provided by the design engineers. This approach proved to be effective in ensur- ing that the construction met the intent of the contract documents. There was one incident which exemplifies the importance of careful field inspection. Inspectors noticed that the brownish color of a freshly cast segment appeared to be abnormal. It was later discovered that an undue amount of fly ash had been accidentally mixed into the concrete by the supplier through a mechanical failure in the mixing process, reducing the concrete strength considerably. Fortunately, the mistake was caught Fig. 11. West Seattle Bridge, halfway before any forward segments were cast through construction. and corrected by jackhammering off the

Fig. 12. West Seattle Bridge, nearing completion.

66 Fig. 13. West Seattle Bridge, completed and in-service.

defective segment. Had additional seg- 2. Kane, T. A., Carpenter, J. E., and Clark, ments been poured without the correc- J. H., "Approach Spans to the West Seat- tion, devastating consequences could tle Bridge," PCI JOURNAL, V. 28, No. 6, November-December 1983, pp. 58-67. have resulted. 3. Standard Specifications for Highway Figs. 11 through 13 show various Bridges, 12th Edition, American Associ- views of the bridge during construction ation of State Highway and Transporta- and after completion. The bridge was tion Officials (AASHTO), Washington, opened for traffic in November, 1983. D.C., 1977 (with 1978 and 1979 Interim During the last 8 months the structure Supplements). has performed with total satisfaction. 4. ACI Committee 318, "Building Code Requirements for Reinforced Concrete (ACI 318-77)," American Concrete In- I Acknowledgments stitute, Detroit, Michigan, 1977. 5. CEB/FIP Joint Committee, Interna- The following are those chiefly re- tional Recommendations for the Design sponsible for the realization of the West and Construction of Concrete Struc- Seattle Bridge, and their areas of re- tures, Comit6 Europeen du BE ton/ sponsibility: Federation Internationale de la Pr6con- Owner: The City of Seattle, trainte, Cement and Concrete Associa- Washington; Bruce Wasler, Project tion, London, 1970. Manager. 6. Post-Tensioning Manual, Third Edition, Post-Tensioning Institute, Phoenix, Prime Consultant: West Seattle Arizona, 1981. Bridge Team, composed of: Anderson- 7. Homberg, Hellmut, Fahrbahnplatten Bjornstad-Kane-Jacobs, Inc., Kramer, mit Veraenderlicher Dicke, Springer- Chin Mayo, Inc., Parson Brinckerhoff, Verlag, New York, N.Y., 1968. and Tudor Engineering Co.; Thomas A. 8. Post-Tensioned Box Girder Bridge Man- Kane, Project Manager. ual, Post-Tensioning Institute, Phoenix, Contractor: Kiewit-Grice. Arizona, 1978. Superstructure Subconsultant: Con- 9. Jungwirth and Baumann, "Shear Design tech Consultants, Inc.; M. C. Tang and J. with Dywidag Prestressing System," In: B. Louie, Principal Designers; Paul Finsterwalder Festschrift, Verlag G. Braun, Karlsruhe, 1973. Brallier, Construction Inspector. 10. ATC-6 Seismic Design Guidelines for Highway Bridges, Applied Technology REFERENCES Council, Berkeley, California, 1981. 11. Mahoney, T. F., and Clark, J. H., "Seis- 1. Kane, T. A., Carpenter, J. E., and Clark, mic Design — West Seattle Bridge," J. H., "Concrete Answer to Urban Trans- Paper Presented at the Northwest Bridge portation Problems," Concrete Interna- Engineers Seminar, Olympia, Wash- tional, August, 1981, pp. 85-92. ington, October 4, 1983.