SEISMIC BEARINGS FOR LARGE SPAN BRIDGES

One of the most effective strategies in enhancing (or turning) the seismic response of a bridge to seismic events is the effective use of isolation bearings. Implementing this strategy for long span bridges requires the use of large bearings (up to 3 m [10 ft] in diameter), designed to have properties specific to each application. Several types of bearings are currently available for this purpose which include (1) friction pendulum isolation bearing, (2) slider with elasto-plastic springs, (3) elastomeric isolation bearings (e.g., lead core - rubber bearing), (4) elastomeric with metallic dissipaters, etc. Use of these types of bearings on large bridges requires that they are designed to carry as much as 10,000 kips of vertical load and allow for displacements up to 1.2 m [4 ft], i.e., 2.4 m (8ft) total. Imbsen & Associates, Inc. has incorporated isolation bearings in the seismic retrofit design of several major bridges, which include for example: The I-40 Mississippi River Crossing near Memphis, Tennessee, the Golden Gate Bridge in San Francisco, and the Benicia-Martinez Bridge in California. The elastomeric isolation bearing was used in the Golden Gate Bridge and the approaches on the Benicia-martinez Bridge, and the friction pendulum bearing has been proposed for the I-40 and main span on the Benicia-martinez Bridge. Analysis methods for incorporating isolation in retrofit design is presented below, followed by a discussion of their specific applications to the three bridged mentioned above.