SEISMIC RESISTANT BRIDGE DESIGN IN CALIFORNIA

The design of seismic resistant bridges in California made a dramatic turn in February, 1971. The heavy damage in San Fernando was unprecedented in the history of bridge design in California. In fact California had seen less than $100,000 in earthquake damage to bridges in the 40 year history preceeding 1971. This earthquake created an increased effort to improve the evaluation and design of seismic resistant bridges. A comprehensive program was started immediately to evaluate and improve both the design specifications and details. In 1973 a new criteria was implemented which considered the fault activity in Califonria and the soils at the bridge site as well as the vibrational properties of the bridge itself (5, 6). The California Department of Transportation criteria departed from the traditional seismic design criteria and presented for the first time: site specific response spectra based on active faults in the region; specified reductions for ductility and risk; and modular arrangement of variables for future adjustment. In 1975, The California Department of Transportation criteria was modified and adopted by American Association of State Highway and Transportation Officials for national use in the interim specifications. This paper will describe some of the developments in the design of seismic resistant bridges in the period since 1975. In addition to criteria developments, improved column and foundation design procedures, improved retrofit analysis procedures and the instrumentation of bridges for strong-motions will be discussed. (Author)