PCI DESIGN AWARD WINNER

Design-Construction of Gannon/Goldsworthy Pedestrian Bridge

Richard L. Ames, S.E. Structural Engineer PPL Coffman Engineers Spokane, Washington

Chuck Prussack, P.E. Engineering Manager Central Pre-Mix Prestress Company Spokane, Washington

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Precast/prestressed concrete became the so lution of choice in

replacing an o ld non-functional footbridge at Washington State

University in Pullman, Washington. The new $280,000 pedestrian

bridge has spans of 24, 64, 24 ft (7.32, 19.52, 7.32 m), in which the

two end spans are straight and the middle span is parabolic. Only

seven precast components - three U-shaped girders and four piers

-were needed to build the structure. The end result is a magnificent

bridge that is very functiona l, was erected on schedule with minimal

disruption of traffic and was within budget. This article presents the

owner's requirements and design cons iderations, as well as the

production, hauling, and erection highlights of the project.

The new $280 ,000 Gannon / Goldsworthy Pedestrian Bridge (see Fig. 1) in Pullman, Wash­

ington , is a three-span precast, pre­stressed concrete structure with a total length of 118 ft (36 m). Crossing Stadium Way, the main campus arterial leading to Washington State University , the bridge provides a very functional , safe and aesthetically pleasing elevated pathway for univer­sity faculty and personnel , students , and the public.

The new bridge replaces an o ld

cast-in-place, post-tensioned concrete footbridge that had several deficien­cies. Chief among these deficiencies was that the stem of the tee girder hung below the walkway and was susceptible to damage by trucks and other large vehicles. Also , the non­functioning snow melt system was embedded within the slab, making it difficult to replace. Adding a topping slab to the existing bridge for a new snow melt system was not possible due to the lack of structural capacity in the original structure.

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Fig. 1. The new Gannon/Goldsworthy Pedestrian Bridge, Washington State University, Pullman, Washington .

This article presents the design chal­lenge and solution, and then gives the design considerations, production, hauling and erection highlights of the project.

DESIGN CHALLENGE Due to the deficiencies of the exist­

ing bridge, it became apparent to the Department of Facilities Development of Washington State University that a new pedestrian bridge was urgently needed. PPL Coffman Engineers was approached by university officials to design a replacement pedestrian bridge with the following challenges: • A functional, safe, and aesthetically

pleasing structure was required sub­ject to a quick construction time and tight budget.

• Because the project site was re­stricted, only a minimal staging area was allowed.

• Stadium Way, the main arterial, could not be shut down except at night.

• A sufficient clearance was required in order to minimize the projection of the superstructure below the walking surface to avoid trucks from hitting the bridge.

• The new bridge needed to match the walkways and floor elevations at each end of the bridge for ADA access.

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PRECAST SOLUTION Design of the new bridge began in

August 1994. Although other struc­tural systems were considered, it soon became apparent to the design engi­neers that a precast, prestressed con­crete bridge was the most efficient and economical solution.

To solve the problem, initially a three-span precast, prestressed con­crete structure was designed with span lengths of 24, 64, 24 ft (7.32, 19.52, 7.32 m). The two end spans are straight while the middle span is parabolic (see Fig. 2). Together, the three spans plus approximately a 3 ft (0.91 m) cantilever at each end pro­vide an overall bridge length of 118 ft (36m).

The superstructure for each of the three spans consists of a single pre­cast, prestressed U-girder with bottom and side rails. The walkway width is 7 ft (2 .13 m) and the rail width on each side is 1 ft (0.305 m) for an over­all width of 9 ft (2.74 m). These sec­tions have a 6 in. (152 mm) thick bot­tom and an overall height of 3 ft 1112 in . (0.95 m). The bridge floor plan, el­evation and cross section are shown in Fig. 2.

With this solution, only seven pre­cast components were needed to build the structure - three U-shaped girders

and four piers. Fig. 3 shows plans, ele­vations, and sections of the piers.

This approach ensured quick erec­tion and little disruption to traffic. By using plant cast components, the beam sides and piers could be formed on the ground with simple forming and no form ties. This produced not only eco­nomical forming , but also high quality finishes.

The new bridge section also helped solve the two key dilemmas with the existing bridge, namely, the protrud­ing underside of the superstructure and the faulty snow melt system.

The lines of the precast piers were designed and detailed to flow smoothly into integrally cast handrail and walkway precast, prestressed U­girders. This was done by hiding the ends of the center piers inside the girder section. The result is that the well proportioned piers transition smoothly into the parabolic center girder, creating an aesthetically attrac­tive bridge structure. Most impor­tantly, this design minimized the amount of structure below the walking surface.

In addition, the precast girders pro­vided a platform for the new snow melt system, which consists of insula­tion, wiring, waterproofing and a top­ping slab. If the snow melt system were to fail in the future, all or part of

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Fig. 2. Plan, elevation and cross section of pedestrian bridge.

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Fig. 4. Completed bridge showing its functionality and how well it fits into the campus environment.

the system could be replaced without replacing the entire bridge.

The precast/prestressed concrete su­perstructure was constructed with up­turned side rails on each side and a bottom slab. The bottom slab provided a platform for the snow melt system with the following components: • To this bottom slab was attached a

layer of waterproofmg. • On top of the waterproofing is a

drainage blanket for any water that may penetrate through the upper layers.

• Insulation is placed on the drainage blanket.

• The wiring for the snow melt system is placed on this insulation.

• On top of the waterproofing, drainage blanket, in sulation and wiring, a cast-in-place topping slab is placed. The topping slab joints are all sealed

with urethane sealants to prevent any leakage . Any water that penetrates through the slab will drain down the drainage blanket and into the stainless steel drains at the end.

The main wiring and junction boxes for the snow melt system were hidden in the side rails. If the snow melt sys­tem were to fail in the future, all or part of the system could be replaced without replacing the entire bridge.

The design of the bridge was based on the 1991 Uniform Building Code. The project is located in Seismic Zone 2B. In the design, longitudinal seismic

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forces were taken out at the two inte­rior piers. Lateral seismic forces were taken out at all four piers.

Bid opening was on March 14 , 1995. The notice to proceed with con­struction was given on Aprill3, 1995.

Because of the heavy traffic on the arterial below the footbridge and the busy pedestrian traffic on the bridge itself, demolition and construction of the new structure were only allowed at night. Demolition of the existing bridge was completed in one evening during the third week of May 1995. The piers and superstructure were erected during a day at the end of summer 1995, thus minimizing the shut-down of Stadium Way . The bridge facility was in service by Octo­ber 1995 (see Fig. 4, 5 and 6).

The finished pedestrian bridge has a strong yet beautiful countenance. The sleek and graceful lines of the bridge blend perfectly with the buildings on each side of the street.

PRECASTER'S ROLE

Central Pre-Mix Prestress Company (CPPC) entered the project time line in August 1994 during the preliminary design phase of the bridge. By this time, it became apparent to both PPL Coffman Engineers and Washington State University that a precast con­crete superstructure was the most fea­sible solution.

CPPC was asked to explore the pos-

sibility of providing a parabolic U­shaped superstructure. Within a short period of time, CPPC was able to show that the proposed section was not only feasible but would work very well with a combination of mild rein­forcing steel and prestressing strands.

The new bridge was to have a snow melt system on its deck as well as have sufficient clearance so that trucks would not hit the superstructure. How­ever, the most severe requirement was that the new bridge needed to be erected quickly with minimal traffic disruption. All items considered, a precast concrete system offered the best and fastest solution.

The walkway portion of the bridge is composed of three U-shaped sec­tions, two on straight tangents, and the center on a parabola. Four precast piers were also constructed. CPPC chose to cast the center piers and caps as one monolithic piece.

Because the existing footings were not exactly as shown on the original plans, CPPC had to adjust the spans and a few other dimensions. Therefore, the following dimensions are a little different than those mentioned earlier in the text and shown in Figs. 2 and 3.

U-shaped sections were 25.2, 64, and 28.1 ft long (7.68, 19.51, and 8.56 m) by 9 ft wide x 3.13 ft tall (2.74 x 0.95 m). With the 1 ft (0.305 m) wide sidewalls, a 7 ft (2.13 m) wide walking surface resulted. The weight of a fin­ished U-girder was approximately

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Fig. 5. The graceful and sleek lines of this pedestrian bridge are evident.

1500 lbs per ft (21.9 kN/m). Piers were 9.5, 12.17, 14.4, and 15 ft long (2.90, 3.71 , 4.39, and 4.57 m). Pier weights were 7750, 9210, 17,135, and 17,810 lbs (34.5, 41.0, 76.2, and 79.2 kN).

The walkway components were pretensioned whereas the .piers were conventionally reinforced with mild steel. The shorter walkway segments used eight straight 0.5 in. (12.7 mm) diameter strands and the long walk­way segment used sixteen straight 0.6 in. (15.2 mm) diameter strands, supplemented with #II reinforcing bars.

Using straight strands with a parabolic section left some intermedi­ate areas of the components under­stressed. Those areas were reinforced with #ll reinforcing bars. A plot of moment vs . capacity showed where the mild reinforcing steel was needed. Post-tensioning was initially consid­ered following a parabolic trajectory, but ultimately was pretensioned using a straight trajectory for easier setup and economy.

All precast components required custom forms. Regular flat panel cast-

November-December 1997

ing beds were used with wood side forms as needed. The U-shaped com­ponents were cast by pouring the sides flat, rolling up and bracing, then cast­ing the slab between the sides to form the bottom of the "U." This procedure resulted in the least amount of form­ing, and is a technique that CPPC often uses . It also facilitated a high quality finish on the exterior sides by casting them "down in form. " The parabolic sides were used to support the parabolic formwork for the main span.

Concrete strengths were 4000 psi (27 .6 MPa) at release and 6000 psi (41.4 MPa) at 28 days. Air entrain­ment was used in all the concrete components.

In addition to production, CPPC performed the component design. The manufacture of the precast compo­nents took place during a 3-week pe­riod in the summer of 1995.

CPPC employed a subcontractor to haul the components. The distance to the project site was about 82 miles (132 km). All components were deliv­ered by truck. Standard trailers were

used for all components except for the main span, which was delivered with the same type of setup as a bridge girder, i.e. , the fifth wheel bunk on the tractor supported on one end, with the far end held by a steering trailer.

CPPC did the erection in a day at the end of summer 1995. The entire project had to be completed after school was out for the summer and be­fore classes resumed at the end of September.

Closure pours were made at the girder ends and piers tying the two together. By precasting the piers flat , a high quality finish was achieved.

The precast contract amounted to $70,540, which included production , hauling, and erection.

The overall effect of the end spans transitioning into the parabola and the crisp formwork lines is very attrac­tive. There are several other bridges of varying types cross ing Stadium Way into the campus. However, the Gannon/Gold swo rthy Pedestrian Bridge is by far the most beautiful structure on thi s arterial and has the most attractive lines.

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Fig. 6. The Gannon/Goldsworthy Pedestrian Bridge won an award in the 1997 PCI Design Awards Program.

CONCLUDING REMARKS

The successfu l completion of the Gannon/Goldsworthy Pedestrian Bridge fulfilled the major require­ments of the owner, namely, a highly functional, high quality, aesthetic structure that was constructed on schedule, within budget, and with min­imal disruption to traffic. This could only have been accomplished with the close cooperation of the Department of Facilities Development, Washington State University, PPL Coffman Engi­neers , Central Pre-Mix Prestress Co. , and Hamre Construction .

In June of this year, the Gannon/ Goldsworthy Pedestrian Bridge won a design award in the 1997 PCI Design Awards Program. The jury citation was as follows:

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"What is most impressive about this structure is its very clean lines. It took some effort to form this and erect it because the ceo-

tral span is parabolic, but by doing so they achieved a very nice, clean structure that serves its function very well and blends nicely into the environment. Two key advantages of precast con­crete were the quick erection over the arterial and the ability to re­place the snow melt system in the future."

The bridge has now been in service for about 2 years. The university and the public are pleased with both the functionality and attractiveness of this facility. In a short period of time, this bridge has become an important addi­tion to the university.

ACKNOWLEDGMENT The authors wish to express their

appreciation to Patricia Bergstedt, Se­nior Architect, Department of Facili­ties Development, Washington State University, for her tireless efforts and

gracious demeanor during the con­tract negotiations and in coordinating the work between all parties during the course of this project. Ms. Bergst­edt provided color selection for the bridge, landscaping, contract docu­ments, and other important items.

The photographs used in this article are published through the courtesy of The Commercial Photographers, Inc., Spokane, Washington.

CREDITS

Owner: Washington State University, Pullman, Washington

Structural Engineer: PPL Coffman En­gineers, Spokane, Washington

General Contractor: Hamre Construc­tion, Spokane, Washington

Precast Prestressed Concrete Manu­facturer : Central Pre-Mix Prestress Company, Spokane, Washington

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