Use of Simplified Nonlinear Response History Analysis to Inform Equivalent Lateral Force Procedure: A Hybrid Approach to Base-Isolated Building Design

摘要

Design of base-isolated structures using the Equivalent Lateral Force procedure of ASCE 7 is limited to regular buildings with moderate ground motions. When the limits of the procedure are exceeded, nonlinear response history analysis is usually implemented to capture the dynamic interaction of the superstructure and the isolation system using site-specific ground motions. This paper provides an example of a hybrid approach for designing base-isolated structures by conducting a detailed nonlinear response history analysis of a simplified isolator-building system to generate an envelope of story forces to be used in the design of the building using a fixed-base, static analysis approach. This allows the engineer to take advantage of the potential reductions in base shear and maximum displacement permitted by a dynamic analysis while preserving the simplicity of a static analysis for design. The resulting vertical force distribution from the simplified model is also compared to the new vertical force distribution for isolated structures included in ASCE 7-16. Results of nonlinear response history analyses using the simplified isolator-building model provides the maximum isolator design displacement, forces for the design of the superstructure lateral system, and anchorage of nonstructural components. The analysis also applies tri-directional acceleration input (as a peer review requirement) in order to capture the vertical demands on the isolation system and building components, and the potential increase in lateral forces and accelerations due to coupled vertical-horizontal response of the selected isolator type. The system behavior using vertical excitation is compared to that of horizontal excitation only and both are compared to the proposed code vertical force distribution.