Overturning Fragility of Base‐Isolated and Conventional Freestanding Building Contents in Variously Designed Moment‐Resisting Frames

ABSTRACT Freestanding building contents represent a large portion of non‐structural components in the built environment and therefore receive significant attention. However, research using realistic friction coefficient to investigate their seismic fragility is still scarce. In addition, limited efforts have been made to understand whether a well‐established technique (e.g., seismic isolation) is beneficial towards protecting unanchored building objects under floor motions. To address this knowledge gap, this study first presents an analytical formulation for a base‐isolated freestanding rigid block system, including: the exact nonlinear equations‐of‐motion, transitional criteria between different motion modes, as well as the approach to handle impact. The developed analytical model is then validated by comparing responses predicted from a different numerical model in which the penalty stiffness method is used to handle contact and impact. Subsequently, fragility curves of freestanding building contents located at different floors of four code‐compliant steel moment‐resisting frames are developed under a suite of fourteen pairs of ground motions. These fragility curves consider different slenderness angles of blocks, friction coefficients, floor locations, strength reduction factors, as well as whether or not using base‐isolation. The influences of these parameters on the overturning probabilities of freestanding building contents are compared and analyzed.