Assessment of coupled and uncoupled method on the seismic response evaluation of freestanding building contents

Evaluation of seismic response and performance of non-structural components is of critical importance in performance-based earthquake engineering. Freestanding or unanchored building content represents a large portion of non-structural components and therefore they receive significant attention. This paper critically examines effect of different analysis methods on the seismic responses of freestanding building contents. These analysis methods include simplified uncoupled method (i.e. cascading approach) and fully coupled method. Freestanding building contents are idealized as pure sliding and rocking blocks, respectively. After deriving the equations-of-motion in fully coupled analysis method, their seismic responses are compared using these two methods under analytical pulse excitations. It is shown that even when the mass ratio of block to structure is as low as 0.1%, under certain scenarios, these two methods can give different results. It is speculated these differences may be attributed to the free vibration phase of structure. As such, sliding responses are significantly affected by the structural stiffness while there is no clear trend regarding the rocking response. Similar observations can also be made for real earthquake excitations. Therefore, care should be given when using simplified method to evaluate seismic responses of freestanding building contents.