Launch Slideshow

Fixing the West Coast’s Busiest Bridge

Fixing the West Coast’s Busiest Bridge

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    Approximate design of the steel saddle fix for broken bolts. Saddles secure the shear key to the cap beam.

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    Broken shear key bolts.

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    Chiseled concrete awaiting steel saddle.

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    Crews on the pier work platform drill holes for dowels to bond the new concrete jacket to the cap beam.

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    Drilling into the concrete cap beam to prepare for steel saddle installation.

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    Drilling into the concrete cap beam to prepare for steel saddle installation.

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    Dowels and formwork installation at the underside of Pier E2 cap beam. The 1-in.-diamater T-headed dowels were used to transfer saddle anchorage forces to the crossbeam.

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    Epoxy and dowels installation.

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    Crews on the pier work platform drill the concrete cap beam in preparation for the bolt fix.

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    Formwork installation at the bottom of the concrete cap beam.

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    Shear key (middle) awaits broken bolt fix.

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    Shear key with broken bolts sits between two bearings with good bolts.

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    Applying liquid epoxy to seal the existing bolt locations at the shear key.

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    Pre-stressing trumpets. Conco crews installed PT anchorages supplied by Schwager-Davis Inc., San Jose, Calif. They mounted 260 PT bearing plates and trumpets onto custom blockouts. Each of the anchorages accommodate an alignment specific to each tendon, per design specs. The blockouts had to be easily removed after concrete placement to allow timely installation of the PT strand.

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    An upper saddle being delivered.

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    Saddles replace the broken bolts and are attached to the beam with PT cables.

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    Two upper saddles and one lower saddle installed.

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    Trucks on the road deck pump concrete to the bottom of the cap beam. The injectable SCC concrete mix was designed to reach 55 MPa in approximately 10 days. The high strength was specified in the design, and the short duration was requested by the construction team to allow PT to take place asap.

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    Pouring concrete base for the steel saddle retrofit. The geometry of the soffit concrete qualified it as mass concrete, and thermal cracking was a concern given the heat of hydration with the concrete mix. Conco installed internal cooling pipes in the formwork and bay water was circulated through the pipe network during the placing and curing process. This eliminated any potential for internal/external thermal differentials that could cause cracking.

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    Concrete in place at bottom of cap beam, encasing the web of PT cable ducts and steel reinforcing. After all concrete was placed, approximately 2,500 0.6-in.-diameter strands were installed in the ducts and tensioned.

 

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.