Commercial

Fixing One of America’s Busiest Bridges

How San Francisco’s new Bay Bridge was repaired, with traffic flowing, after 32 anchor rods snapped.

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When one of the country’s busiest bridges needs repairs, you don’t stop traffic. Not even when highly publicized reports regarding the failure of 32 anchor rods called into question the new San Francisco-Oakland Bay Bridge’s ability to withstand an earthquake.

Even with the broken rods (or bolts) on the bridge’s eastern span, the California Department of Transportation’s seismic safety advisers did not want to delay the new East Span’s Labor Day opening. After all, there were more serious safety concerns about the old Bay Bridge, which was still in use even though it sustained damage by an earthquake in 1989. The East Span replaces the old bridge span connecting San Francisco to Oakland.

The Federal Highway Administration concluded that temporary seismic elements would make the East Span safe for traffic. Therefore, inserting large steel plates, or shims, into each of the four bearings allowed the bridge to open while the retrofit was in progress. Repair work began in late July 2013. The East Span opened to traffic Sept. 2.

The permanent fix

The defective anchor rods were supposed to secure the bridge deck to the concrete crossbeam support. After extensive collaboration and analysis, engineers designed a fix. It involved a complex series of steel saddles and post-tensioned jacket walls that would provide the required clamping force to secure the bridge deck framing to the crossbeam, or cap beam.

General contractor American Bridge/Fluor Enterprises Joint Venture, Pittsburgh, retained concrete contractor Conco, Concord, Calif., to assist with the repair work. Conco was the logical choice based on the company’s earlier performance with other critical and technically challenging components of the new bridge structure, says Brian Petersen, the GC’s project director.

The saddles wrap over the top of the shear keys and concrete cap beam, with steel tendons inside the saddles spreading down either side of the beam. Before installation, workers spent several weeks prepping the cap beam. They chipped concrete on either side of the shear keys to allow room for the saddles, drilled into the cap beam to allow steel tendons to pass through, and cut into the face of the cap beam to allow a better connection with the new concrete jackets.

Working around obstacles

The retrofit involved a large amount of challenging concrete work associated with temporary support structures such as the steel shims, says Jim Klinger, Conco’s project manager. “Almost all our concrete work items were custom, one-of-a-kind scopes that we will probably never see again.”

The first task was to install a steel-framed trestle platform to give work crews access to the site and eliminate the potential for materials to fall into the San Francisco Bay. Because the bridge was open to traffic, materials had to be delivered by boat. Once the platform was complete, crews roughened the crossbeam surface and epoxied more than 3,000 drilled dowels into place. The dowels were used to secure the new concrete and post-tensioning cables to the face of the crossbeam.

Reinforcing steel and post-tensioning cable ducts were then installed by Harris Salinas Rebar, Livermore, Calif. Conco crews followed, placing custom-made post-tensioning trumpet blockouts. The tapered steel wedges were attached to the post-tensioning cables to secure them after tensioning. The trumpet-shaped wedges prevent the cables from retracting after tensioning. Crews then set forms for the concrete jacket and placed concrete so that all of the dowels and post-tensioning work is embedded inside the new concrete.

The concrete jacket wall structures were placed in three pours totaling 380 cubic yards. Due to restricted access and rebar congestion, concrete pump trucks had to park on the bridge deck above the worksite. The concrete was pumped down 100 feet and out 100 feet, and then injected into the forms through ports at strategic locations. Conco worked closely with concrete supplier Central Concrete Supply Co. Inc., San Jose, Calif., to develop an injectable self-consolidating concrete (SCC) mix with silica fume that would meet the project’s specifications. They constructed and tested a full-scale mockup to ensure long-distance pumping and injection of the highly flowable SCC mix would work. Once this gained enough strength, the post-tensioning cables were pulled.

In late November 2013, crews finished the third and final injection and successfully completed repairs. Demolition of the old bridge span is now in progress.

About the Author

Victoria Sicaras

Victoria K. Sicaras is a freelance writer and former managing editor of Public Works. She is also an AEC editorial specialist with Constructive Communication Inc. (www.constructivecommunication.com).

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