Experimental damage observations and component fragilities for wood-framed houses with cripple walls

Abstract

Assessment of damage to homes following a significant earthquake has found that the primary reason for major structural damage and failure is due to inadequate lateral bracing of cripple walls and inadequate sill bolting of the cripple wall to the foundation. Since the 1970s, methods to retrofit weak cripple walls and improve sill anchorage have been developed and implemented on many dwellings to improve their seismic performance, most notably available are the Federal Emergency Management Agency (FEMA) P-1100 guidelines. To assess the seismic vulnerability of existing and retrofitted cripple walls, experimental programs were performed and documentation of the evolution of damage during these experiments were produced and correlated to the structural capacity and residual drift of the specimens. Using the documented damage evolution, existing damage fragility functions were revised for wood light-frame sub-assemblies. In this article, an overview of the cripple wall experimental program is provided, including the variables investigated. Documentation of the damage evolution during these experiments is presented. Damage states based on repair triggers associated with imposed displacements of the experiments are defined. Finally, a discussion of how the damage observations were used to develop probabilistic component-level damage fragility functions for cripple-wall dwellings is provided.