Rocking Response of Unanchored Building Contents Considering Horizontal and Vertical Excitation

Abstract

The seismic performance of unanchored slender contents at various floor levels within a building has received little attention in the literature, despite earthquake reconnaissance reports routinely emphasizing the overturning of contents as a pervasive problem. This paper investigates the rocking response of building contents in a four-story concentrically braced frame building, focusing on how the variation in horizontal and vertical floor accelerations in the building affects the planar pure rocking demands (sliding and bouncing are not considered). A cascading dynamic analysis approach is taken whereby nonlinear response history analyses of a three-dimensional (3D) building model to a suite of ground motions provides the floor accelerations, which are in turn used to develop rocking spectra for assessing demands on unanchored building contents of various sizes and aspect ratios. The effect of the vertical floor excitation on the peak rocking response is found to be negligible for most practical purposes, especially for slender components. Consequently, rocking demands on an object are not affected by where it is placed on a specific floor. Rocking demands on an object increase at higher floors because horizontal floor accelerations are amplified with elevation, but the difference in rocking demands from one floor to the next are significantly more pronounced for stocky than for slender objects. Finally, the paper develops analytical fragility curves for rocking and overturning limit states using the normalized peak rotation angle as the demand parameter and a physically motivated floor intensity measure based on the peak floor velocity, block size, and slenderness.