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Fundamental Dynamics and Performance Assessment of Three-Dimensional Seismic Isolation
Civil and Environmental Engineering
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Seismic isolation is an effective technique used to mitigate effects of ground shaking and achieve higher seismic performance in building structures. Recent research has suggested that vertical excitation has significant effects on the structural response during an earthquake. In this dissertation, the dynamic response and control of buildings with 3-dimensional (3D) isolation systems is explored. First, the fundamental dynamic response of a simplified model consisting of a rigid block resting on isolation bearings is studied. The overall structural response of the 3D isolated block is evaluated through drift ratio, vertical and horizontal displacement and acceleration amplification factors. A parametric study is carried out to evaluate the effect of different site conditions, building aspect ratio and 3D isolation parameters on the structure and bearing response. The results show that effective 3D isolation is achieved by designing the system with vertical isolation period in the range of 0.5 – 1.0 sec. Also, the horizontal (TH) and vertical (TV) isolation periods should not be closely coupled, and TH should be selected to be much longer than TV. Structural responses are seen to increase for increasing site class (increasingly softer soil) over the entire parameter range. Increasing horizontal ζH and vertical damping ratios ζV, leads to decreasing drift ratios and vertical bearing displacement. However, increasing h/b leads to an increase in rocking, which consequently impacts the relative drift ratio between the top and the base and the vertical bearing displacement.Next, a concept is proposed for 3D isolation that combines elastomeric bearings to resist horizontal ground shaking in series with bilinear liquid spring (BLS) - controllable magnetorheological fluid dampers (CMRD) to resist vertical shaking. The simplified rigid block model is extended to predict the response of the building isolated with BLS-CMRD devices under earthquake loading. BLS-CMRDs are simulated through a combination of nonlinear springs, viscous and hysteretic (semi-active) damping. A new Disp/Vel-based Control strategy is proposed that adjusts the input current according to the instantaneous vector combination of feedback displacement and velocity of the damper. Two variations of the control strategy are explored. First, with Linear Current Variation, the current is activated when a threshold lower bound vector magnitude is reached, and maximum current is applied when a threshold upper bound magnitude is exceeded. Second, the simplified ON-OFF strategy uses a single threshold vector magnitude that triggers the maximum current to turn on when the instantaneous vector magnitude exceeds the threshold, and turn off otherwise. Results of this analysis show that for ground motions that exceed the design level, Disp/Vel-based Control is effective to moderate the level of energy dissipation, keep device vertical displacement within the design stroke limit, and attenuate vertical acceleration below PGA. In addition, Disp/Vel-based control reduces all responses relative to the well-known clipped optimal strategy used for structural control.Finally, the promising findings regarding 3D isolation are validated through a detailed study of the structural response of a real multi-story frame building with 3D isolation. Three hypothetical steel buildings of different aspect ratio with special concentric braced-frame lateral systems have been designed for this purpose. The seismic response of the buildings with fixed-base, horizontal (Hz) isolation and 3D isolation are compared to evaluate the effectiveness of 3D isolation to mitigate the vertical ground shaking. Also, responses predicted by a 3D rigid block model are compared to the flexible building response to identify the limitations of rigid block in predicting both global and local structural responses. The simplified rigid block model estimates reasonably well the global structural responses, but does not account for slab vibrations. Overall, 3D isolation with a relatively short isolation period (0.5 sec) is found to be adequate to significantly mitigate the vertical acceleration in a flexible frame building, including the amplification at mid-slab relative to adjacent columns, without compromising the usual reductions in horizontal story drifts and floor accelerations. Aspect ratio was shown to insignificantly influence the structural response of 3D isolated buildings for the relatively short, stiff buildings considered in the study.