Making earthquakes tremble

News April 24, 2002
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Long-span suspension bridges with spans over 2,300 ft have not been built in the U

Long-span suspension bridges with spans over 2,300 ft have not been built in the U.S. for the past 36 years when the 4,260-ft main span Verrazano Narrows bridge opened to traffic in 1964. In 2004, the Carquinez Bridge, with a main span of 2,388 ft, will be ready for action.

The bridge, which spans the Carquinez strait about 20 miles northeast of San Francisco, is located within a few miles of several active faults. While other famous suspension bridges like the Golden Gate Bride and the San Francisco-Oakland Bay Bridge are undergoing major seismic retrofit, Carquinez Bridge is the first bridge in the U.S., located in a potentially high seismic risk area, to be designed to present-day stringent seismic design standards.

The first Carquinez Bridge, designed by David Steinman and Charles Derleth, was built in 1927 by the American Toll Bridge Co. It replaced a heavily used ferry service and cleared a bottleneck on the highway between San Francisco and Sacramento. The state of California acquired the bridge in 1941 and in the face of growing traffic needs designed and constructed a second Carquinez Bridge in 1958. Completion of the second bridge was part of an ambitious project that transformed a two-lane road into the modern eight-lane interstate highway of today. Both bridges are multi-span cantilever truss bridges with maximum spans of about 1,180 ft.

Considering the potential for seismic activity in the Carquinez strait, safety initiatives prompted the California Department of Transportation (Caltrans) to commission retrofit studies of the existing structures. These studies were carried out by a joint venture team of HNTB and CH2M Hill. As a result of these studies, replacement over retrofit was considered the best "overall value" for the 1927 bridge due to several reasons: the bridge had severe seismic deficiencies; its main members required rehabilitation due to fatigue and corrosion; the concrete deck required replacement; and the lane width were substandard with no shoulders. The state chose to replace the 1927 bridge and the 1958 bridge was considered for seismic retrofit for continued service.

Caltrans chose the joint venture team of De Leuw, Cather & Co., Steinman Boynton Gronquist & Birdsall (both firms are now part of Parsons Transportation Group) and OPAC Consulting Engineers, San Francisco, to design the replacement bridge. A type selection study was undertaken by the consultants to choose between cable stayed and suspension bridge alternatives. The study concluded that a two-tower suspension bridge would cost about the same as a three-tower cable-stayed bridge. However, for reasons of better seismic performance, shorter construction schedule, better aesthetics and less construction risk, the suspension bridge alternative was chosen for final design.

For more on the story, read the May issue of ROADS&BRIDGES.

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