Modern Steel Construction February 2012 : Page 45y.P. Kim THE SEISMIC RETROFIT of the Antioch Toll Bridge in North-ern California consists 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 also was neces-sary to install steel bracing in the tall piers to make the pier portal frames stiffer. Caltrans owns and operates Antioch Toll Bridge, but the fund-ing came from the Bay Area Transit Authority (BATA), which also managed oversight of the retrofit construction. The total steel used for the cross bracing was 1,850 tons, all of which was fabricated and prime painted by Brooklyn Iron Works, Inc., Spokane, Wash. Eighty-two single-surface friction pendulum isolation bearings were supplied by Earthquake Protection Systems, Inc., Vallejo, Calif. 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. Antioch Toll Bridge is one of the last two toll bridges to be ret-rofitted in Northern California. It 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 the latest seismic design criteria and an extensive geotechnical investigation, Caltrans concluded that the bridge needed to be retrofitted. The bridge’s average daily traffic is 15,000, a relatively small number compared to other toll bridges in Northern California. However, because it crosses the San Joaquin River, which is an important navigational channel, its seismic retrofit is based on the Safety Evaluation Earthquake criteria with a 1,000-year return probability. The project-specific SEE design criteria are based on “No Collapse” with permissible damages in parts of the pier pile groups and the deck expansion joints. The analysis of the existing bridge exposed several deficiencies. First, there is a possibility of shear failure in the existing columns and the bent caps. Second, the existing rebar couplers at the base of the columns could fail prematurely. Third, the existing pile foun-dation system could fail undermining the stability of the bridge. Steel braces were added to stiffen the columns of the 20 tallest piers as part of a seismic retrofit on the antioch Toll bridge over the San Joaquin river in northern California. In addition, the existing pin hanger hinges could fail due to possible misalignment of the girders. Although the existing superstructure carries only two traffic lanes and is relatively light , isolating the super-structure proved to be an effective solution. Single-surface friction pendulum isolation bearings were selected for the design due to the restricted vertical clearances. Two sizes were used in order to accommodate different magnitudes in loading conditions. The larger bearings are 7.2 ft in diam-eter, 9.2-in. thick and have 23 in. of maximum displacement capacity. The smaller ones are 5.8 ft in diameter, 7.2-in. thick and have 20 in. of maximum displacement capacity. By isolating the superstructure, the base shear at the piers dropped between 23% and 79%. Similar reduction in shear demand in the bent caps was observed. In addition, it reduced the tensile forces in the column vertical rebar. This will eliminate concerns about premature failure of the existing rebar splices by keeping the forces in the rebar within the yield limits. Although the retrofit reduces forces going into the pile foundation, some pile failures are still expected. Most of the pile failure will be in the exterior battered piles that will form multiple plastic hinges. Some interior piles will fail, in some piers, but based on the project-specific “No Collapse” criteria the performance of the substructure is defined as acceptable. This not only reduces the construction cost but also saves the existing river environment from any disturbance. yong-Pil Kim, P.e., is a senior bridge engineer at Caltrans. he has 23 years of bridge design experience and is the project engineer for the design of seismic retrofit the antioch Toll bridge project. Two sets of steel cross bracing were installed to stiffen each of the taller piers, then painted brown to match the weathering steel superstructure. february 2012 MODERN STEEL CONSTRUCTION ➤ ➤ 45 Publication List Using a screen reader? Click Here |
