SOFT-STORY EFFECTS ON THE BEHAVIOUR OF SEISMICALLY ISOLATED BUILDINGS UNDER NEAR AND FAR-FAULT EARTHQUAKES

In this study, the effects of soft-story on the seismic behaviour of 3-story and 8-story isolated buildings under near and far-fault earthquakes were investigated. Four different structural models with two different 1st story height were designed: One with 1st story height of 3m and 2nd with 1st story height of 4.5m to capture soft story effect. The prototype fixed base buildings were converted to seismically isolated buildings by introducing rubber isolators at base level. Analyses were conducted by using two different isolation systems (QW7.5Tb3 and QW7.5Tb4). The modelling of conventional fixed base prototype seismically isolated buildings and their modal analyses were conducted on finite-element program SAP2000, whereas, modelling of seismically isolated buildings and nonlinear time-history analyses were conducted using 3D-BASIS program. The four accelerations records has been used for the time-history analysis. Floor accelerations, story shears and inter-story drift ratios were the key structural responses considered. The analysis results showed seismic isolations can be used as a viable mitigation method for the buildings with soft-stories under near and far-fault earthquakes. Based on the results obtained, it is interesting to note that all types of buildings whether with soft story or with typical story height show the same acceleration trend and close values (except top floor) for all types of isolation systems and earthquakes considered. In addition, both 3 and 8-story buildings suffered increase in interstory drifts beyond the limits defined UBC 97 under earthquakes containing long period pulses.