COMPARATIVE RESPONSE OF MID-RISE (9-STORY) BASE-ISOLATED ANDCONVENTIONAL STEEL MOMENT RESISTING FRAME BUILDINGS
AdvisorRyan, Keri L
Civil and Environmental Engineering
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Base isolation systems are preferably applied in low-rise buildings since the vibration period of the structure can be shorter, so that it performs more rigidly to maximize the benefit from the isolation system. Mid-rise and high-rise base-isolated buildings are expected to have different and variable response characteristics compared to low-rise base-isolated buildings, and may not perform as well as low-rise or relatively stiffer base-isolated buildings. However, in this study, the benefit of seismic isolation in a 9-story is comparable or even better than a low-rise (3-story) isolated building that was investigated thoroughly in a previous study (Sayani et al. 2011) with respect to peak floor acceleration, peak story drifts, and peak plastic rotation. A pair of 9-story isolated and conventional buildings were designed by Forell/Elsesser, using the same design philosophy as 3-story buildings that were investigated earlier. Both 9-story and 3-story buildings were designed to satisfactorily meet the current building code standards (ASCE 2005). OpenSees and SAP 2000 were used to develop analytical models of the 9-story buildings for independent purposes. The SAP model was used primarily for model analysis and to perform a design check with standard response spectrum analysis procedures. The OpenSees model was used for nonlinear pushover analysis and nonlinear response history analysis to suites of ground motions representing various probability of occurrence events. Eigenvalue analysis was carried out on the various building models to evaluate their elastic dynamic properties in ii both the OpenSees and SAP models. Pushover analysis of the OpenSees model was carried out under an inverted triangle load pattern to determine the base shear capacity and post-yield behavior. Prior to performing model or dynamic time history analysis, response spectrum analysis (RSA) and equivalent lateral force (ELF) procedure were performed to verify the design of the 9-story buildings. Finally, nonlinear dynamic analyses were carried out to investigate the responses for 9-story isolated and conventional buildings under 50/50, 10/50 and 2/50 year earthquakes. The major observations and conclusions from this analysis are as follows. The design objectives for the 9-story isolated and conventional buildings have been met. The 9-story isolated building performs much better than 9-story conventional building with respect to peak floor acceleration, peak story drifts, and peak plastic rotation. Demands in the mid-rise isolated building can be predicted with high confidence relative to conventional building even for ground motion intensities at MCE level. The yielding of superstructure of isolated building may lead to slightly larger but still acceptable dispersion of drifts, while the dispersion of acceleration remains very low. The responses of the 9-story and 3-story isolated buildings were also compared to investigate the influence of height as the isolation system designs are comparable in the two buildings. The total displacement, which is a function of the fundamental period of the system, was observed to be about the same in the 9-story and 3-story isolated buildings. Thus, the drift demands were much lower in the 9-story building as they were distributed over the height more effectively. In the MCE event, 3-story isolated building was sensitive to outliers in extreme motions that produce very large drift demands, while this behavior was basically eliminated in 9-story buildings.Due to slight differences in the isolation system design parameters for the 3-story and 9-story building, the influence of this parameter was examined. The major response trends were found to be essentially unaffected by the isolation system parameters. Themodified isolation system based on the design for the 3-story building (effective isolation period and damping ratio) was applied to the 9-story isolated superstructure. The modified system, which had slightly longer isolation period and increased damping, hadlittle influence on isolator displacements but slightly increased drifts and accelerations relative to the original isolation system.