Effective live load mass for storage buildings on friction-pendulum isolators

Abstract

This paper presents the results from finite element models of based-isolated storage buildings subjected to ground motion excitation and supporting rigid blocks with the possibility to slide. Main components of the models were first compared and calibrated with the results from numerical solutions, finite element software, and shake table tests. The successfully verified finite element models were then used to assess ASCE 7–16 design provisions for the treatment of storage loads as seismic weight in base-isolated structures. The analyses included multi-story shear buildings of different heights and three-dimensional buildings of three-stories with both regular and irregular plans under combined horizontal and vertical ground excitation. Alternative low and high friction between the rigid blocks (representing the storage live load) and the floor deck were evaluated in the analyses as well as the effect of light versus heavy storage live loads. Ground excitation consisted of 100 records, covering scenarios that range from low to high seismic risk. The combination of cases included in the parametric study led to thousands of nonlinear time history analyses. Selected engineering design parameters (EDP) to conduct the evaluation consisted of isolator deformation and maximum force demands in the lateral load resting system. Calculated EDPs from the detailed models (accounting for inelastic response of the lateral load resisting system coupled with potential sliding of rigid blocks) were compared with calculated EDPs from simplified models with no blocks but having additional floor mass equal to 25% of the design live load. The latter models represented analysis conditions that ASCE 7–16 minimum provisions would allow in consulting practice. This study demonstrates that using the Standard’s minimum provision can lead to: i) significant underestimations of the deformation demand and thus unconservative designs of base isolators; ii) significant underestimations of design forces, and consequently, improper design of the lateral load resisting elements. In order to address these issues, a simple expression recently developed by the authors to estimate the portion of the design live load as seismic weight was also evaluated. It is shown that using the portion of the live load given by this equation in the simplified models with no blocks but having additional floor mass produce very similar EDPs as those obtained from the detailed models with sliding blocks and thus representing a significant improvement over the existing ASCE 7–16 minimum provisions when applied to base-isolated storage structures.