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Steel Shots: California Bridge Gets a (Seismic) Makeover
Posted by Tasha Weiss on February 3, 2012 at 2:10 PM.


Adding steel cross braces to stiffen tall concrete piers made isolation bearings an effective seismic solution for the retrofit of the Antioch Toll Bridge in Northern California. Photo: Y.P. Kim


If you’re a bridge in California, you’re going to need your outfit updated every so often to prevent any damage to your apparel. The Antioch Toll Bridge in Northern California recently received a seismic makeover and is one of the last two toll bridges to be retrofitted in the northern part of the state.


The seismic retrofit of the bridge consisted of replacing the existing bearings at all 39 piers and at the abutments with seismic isolation bearings. In order to make the isolation bearings work effectively, it was also necessary to install steel bracing in the tall piers to make the pier portal frames stiffer. The 1,850 tons of steel for the retrofit was fabricated and prime painted by AISC member Brooklyn Iron Works, Inc., Spokane, Wash.


The main structure is 8,650-ft long with 40 spans arching over San Joaquin River. The midsection of the bridge rises as high as 147 ft to allow for ship passage. The superstructure consists of two weathering steel plate girders that are continuous over the piers. The girders are in excellent condition, having formed the expected uniform protective outer coating with no degradation in structural capacity.


The Antioch Toll Bridge was constructed in 1978, so the lessons learned from the San Fernando Earthquake of 1971 were implemented in the original design. For this reason, the bridge was long considered to have sufficient earthquake resistant features and deemed safe. However, reevaluating the bridge based on on the latest seismic design criteria and an extensive geotechnical investigation, Caltrans concluded that the bridge needed to be retrofitted.


The seismic retrofit based on isolating the superstructure was a simple but effective solution. Implementing this scheme by adding steel cross braces to the concrete pier frames was an ideal match. Shop fabricated segments of the steel braces were field assembled with bolted connections and the bracing was easily integrated to the existing concrete frame by connecting the two different elements through a cast-in-place concrete pedestal.


Due to steel’s light weight, the additional weight of the bracing could be accommodated within the capacity of the existing foundation. Not requiring a foundation retrofit meant big savings in the construction cost and also minimized the disturbance to the sensitive environment.


You can read more about the seismic retrofit of the Antioch Toll Bridge in the February issue of MSC, available now.

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