Evaluation of base-isolated statically-designed steel structures performance under near-field earthquakes

Inspite of the fact that isolation systems have enhanced the structures level of performance in achieving operational to immediate occupancy ones after strong ground motions, the dependency of their behaivior on the input earthquake and high amount of energy contributed to the high amplitude velocity pulses of the near-field ground motions, has limited the application of these elements at sites near enough to seismic sources and with specific design procedures and requirements. ASCE 7-05 prohibits the application of equivalent static procedure in such cases. In this research, two base-isolated steel structure with 5 and 8 stories and special moment frame superstructures have been designed using the equivalent static procedure with LRFD approach determind in AISC 360-05 and their performance is evaluated under three near-field earthquakes Northridge1994, Kobe 1995 and Tabas 1978. The superstructures performance is generally satisfactory under Kobe 1995 and Tabas1978, however the structure needs a brief change in the 7th story sections to satisfy operational level of performance under Northridge 1994 earthquakes. On the other hand the dynamic responses of the superstructures are discussed and although the inter story drifts are within the code limitations, the superstructure accelerations are not reduced enough.

Keywords:

base isolation , superstructure performance , nonlinear dynamic analysis , near-field ground motion