DEVELOPMENT OF NEW LOS-ANGELES SEISMIC ANALYSIS CRITERIA FOR TALL BUILDINGS - SITE-SPECIFIC CONSIDERATIONS

Many tall buildings are planned and constructed in the city of Los Ang eles. Significant building code changes as a result of recent earthqua kes will complicate the design and plan checking for new buildings, no t only in Los Angeles, but throughout all seismic regions in the Unite d States and around the world. This paper reports on new efforts to de velop seismic analysis criteria for Los Angeles. The initial efforts h ave been in the areas of the design basis ground motions, near-source effects on ground motions and building design, and the first yield lim it state. A study of the design basis ground motions in the Los Angele s area indicates that the present code-defined level of ground motion may underestimate the seismic forces in design. Near-source effects wo uld affect practically all of the heavily populated and built-up porti ons of Los Angeles due to the many known and suspected active seismic sources in the region. Studies of the ground motions suggest that a re turn period of about 40 years may be reasonable to estimate the earthq uake demand for the first yield limit state.