Honors Geometry

Nov. 1, 2005

Almost 16 years after the Loma Prieta earthquake, commuters in the city of San Francisco are finally driving over the new Central Viaduct. For many years, the fate of the Central Viaduct was at an impasse because of equally vociferous and opposing groups of interests. Finally, with the passage of Proposition I in 1999, the remaining single level of the original viaduct was set to be demolished and replaced by a single-level viaduct, continuing from Van Ness Avenue and touching down at Market Street.

Almost 16 years after the Loma Prieta earthquake, commuters in the city of San Francisco are finally driving over the new Central Viaduct. For many years, the fate of the Central Viaduct was at an impasse because of equally vociferous and opposing groups of interests. Finally, with the passage of Proposition I in 1999, the remaining single level of the original viaduct was set to be demolished and replaced by a single-level viaduct, continuing from Van Ness Avenue and touching down at Market Street.

Due to a highly political profile, this project had to meet a very tight schedule and many requests from local interest groups. They were pressuring to have a special structure that is visually appealing and less intrusive to its surroundings.

Type casting

There were some major puzzles to solve during the type selection process. The main challenge was how to provide the vertical clearance for the traffic below the viaduct during and after the construction. Since this viaduct is an extension of the existing Highway 101 North, the vertical profile is tied to the existing highway.

Another difficulty was how to span the superstructure over a very busy intersection without falseworks. The viaduct had to be built over an intersection with a span length of 190 ft without supports in between. Besides the clearance problem, we also had to make sure that the new structure’s foundation would be at a clear distance from the existing Bay Area Rapid Transit (BART) tunnel under Mission Street. An analysis needed to be submitted to the BART authority proving that the increased earth pressure from the adjacent pile shafts under seismic forces wouldn’t cause any damage to the BART tunnel.

It was obvious that a conventional cast-in-place concrete box girder couldn’t be built because of the massive falseworks required. One span in particular, over the Mission-Duboce intersection, had to be built without falseworks. Even incorporating precast girders to span the intersection was not feasible, because cantilevering sections had to be built from the adjacent spans. The only way to build the cantilevering sections was by supporting the formwork from above, and its technical details, cost and safety risks made its feasibility doubtful.

The use of precast concrete girders with integral bent caps had to be discarded as well. Integral bent caps are required to maximize the vertical clearance for the street traffic below that runs parallel to the bridge spans. This system requires post-tensioning continuity tendons after all the girders are in place. However, because of the horizontal curvature of the alignment, lateral thrust from the prestressing tendons was a problem that was very difficult to resolve.

An ideal solution turned out to be using continuous steel girders with post-tensioned concrete integral bent caps. This system was first used in the state of Tennessee in the late 1970s. However, this is a relatively new system in California. This system made possible the construction of a complicated structure with minimum intrusion to the traffic and also provided the required vertical clearances through the bents.

The design of the steel girder superstructure was only feasible with the recent acquisition of a powerful computer program that helped analyze a very long bridge with complicated geometry. Attention to precise detailing was important, because holes had to be predrilled and cut in the girder webs to accommodate the reinforcements and the prestressing ducts at the bent caps. Assembly of the steel girders and the construction of bent caps required a high degree of precision, and the quality of workmanship of the subcontractors was one of the main factors in successful completion of the project.

Coincidentally, steel girders are much lighter than concrete girders, and the inertial force transmitted to the substructure is significantly less than concrete during a seismic event. This was particularly helpful when getting approval from the BART authority for the construction of the pile-shaft foundation adjacent to the existing BART tunnel under Mission Street.

The 10-span viaduct was designed without any intermediate hinges. Unlike concrete, steel only experiences temperature shortening. To accommodate extreme temperature shortening, PTFE bearings were installed on top of the columns at the two adjacent bents from both ends of the bridge. Internal steel pipe shear keys are embedded next to the bearings for transverse restraint of the superstructure under seismic load.

Although the bent cap’s prestressing was designed for final service loads, the contractor opted to take advantage of the partially built bent caps to carry the construction load and the wet concrete weight during the deck pour, without additional falseworks.

Falseworks were used only to set the steel girders in their final configuration and also to support the weight of the bent-cap concrete during its construction. After the bent caps were poured just below the deck slabs and attained the designed concrete strength, they were partially post-tensioned to support the construction live load and the weight of the deck slab. Thus, no additional falseworks were necessary during the deck pour. After the deck concrete slab was poured and attained the designed concrete strength, all the bent caps were post-tensioned an additional amount of force to support final design service loads.

Feasting on feedback

Unlike most other projects, this project required extensive technical support from the design group during the construction. Because of an unusual structural system and complicated details, Caltrans’ construction inspection team had to rely frequently on the feedback of the design group for technical complexities.

One of the major design changes during the construction was the contractor’s request to redesign the bent-cap post-tensioning. They wanted to utilize the partially built bent caps to carry the deck slab weight and the construction live load during the deck pour. It was in the interest of all parties involved to make this scheme work. The design group was responsible for checking the contractor’s redesign of the bent-cap post-tensioning. It not only saved the contractor from additional falseworks, but more importantly, it improved safety with less intrusion to the traffic.

Another important collaboration was during the deck pour. Because the steel girders deflected differently from each other during the deck pour, it required an extensive analysis on deflection to figure out how to best set up the Bid-Well machine. This was critical in order to have the final deck grade match that of the contract plans. Although it was the contractor’s responsibility to build it correctly, Caltrans’ team had to investigate the factors involved to assure a correct deck pour.

Bent on aesthetics

When steel girders follow the changing curvature of the bridge alignment, it enhances its visual appeal. Besides reducing the girder depth by using continuous girders, it also allowed the girders to conform to the changing bridge alignment naturally. Unlike most of the conventional steel structures, where the girders are usually placed over the dropped bent cap, the use of integral bent caps helps even more the visual impression of a continuous flow of the girders through the spans. Although it is a long bridge with 10 spans, there are no intermediate hinges. All the temperature shortenings of the superstructure is accommodated by the bearings on top of the columns at the bents adjacent to the ends of the bridge. The girders are continuous without any intermediate joints and the visual continuity of the girders adds to its appeal.

Span four has a deeper girder depth and its adjacent spans have girders tapering linearly to match the smaller girder depth of the rest of the spans. This gives the illusion of constant girder depth throughout the bridge from most perspectives and avoids any abruptness in its visual fluidity.

The integral concrete bent caps also contribute to the visual impression of naturally connected bridge elements. Unlike conventional dropped cap bents, the integral bent cap shows a solid interconnection between different bridge elements. The vertical space underneath the bridge is relatively constant, and that helps with the sense of fluidity along the spans. Circular columns were chosen as requested by a citizen’s advocacy group. They were concerned with possible criminal activities in the area. Circular columns could reduce the probability of someone hiding behind them. Furthermore, the initial bridge layout was carried out with the objective of minimizing the location of columns on the sidewalks for the same reason. Additional aesthetic treatments were incorporated as requested by the city of San Francisco. Yellow color was added to the concrete mixture in the columns, abutment, bent caps and concrete barriers. In addition, alternating bands of rough finish texture and smooth finish pattern were applied on the columns and on the abutment walls.

Seismic solid

Steel girders are much lighter than concrete girders. Therefore, the inertial forces exerted on the substructure are much smaller under seismic actions.

Conventionally constructed steel-girder bridges don’t provide a proper load path for the seismic forces to get to the substructure. This is especially true at the bents, where traditionally studs were omitted on top flanges. With the integral concrete bent caps, the inertial forces of the deck slabs and the girders are directly transmitted to the columns and subsequently to the foundation.

Lack of intermediate hinges and any other joints, except at the beginning and end of the bridge, makes the structure solid and redundant under seismic forces. The columns at the bents adjacent to the bridge ends have bearings on top, and the columns are restrained transversely with internal steel pipe shear keys for seismic forces.

Building a viaduct over congested city streets calls for a structural system that is least intrusive to the traffic lanes during its construction. In the replacement of Central Viaduct, the use of continuous steel girders in combination with post-tensioned integral concrete bent caps solved a variety of construction constraints. Temporary supports were required only for the girder set up and the construction of the bent caps. This is an ideal structural type for a site with heavy traffic and tight vertical clearances, especially through the bents.

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