Experimental Investigation of the Effect of Multiple Earthquakes on Woodframe Structural Integrity

Woodframe (light-frame wood) structures make up the vast majority of the United States building stock including those in areas of high seismicity. Their life cycle is unique in that when they are damaged, repair is often nonstructural in nature consisting of drywall repair, and possibly wall replacement. The main reason for this is that most woodframe structures are residential and life safety is typically implied through prescriptive means. Thus, one important question that has yet to be answered is: What is the effect of previous earthquakes on the performance or integrity of a woodframe structure during the next earthquake? Because the primary lateral force resisting system in western style woodframe construction is the wood shearwall, an attempt is made herein to answer this question, albeit in part, based on a series of assembly-level tests conducted at Colorado State University on (1) isolated wood shearwall assemblies; and (2) corner assemblies consisting of a shearwall and short transverse wall. Both types of assemblies were tested with and without drywall and subjected to a series of six earthquakes of increasing intensity. As expected, the effect of the drywall was quite significant, reducing the peak drift of the isolated shearwalls by 30% and the corner assemblies by over 50%. Interestingly, the corner assembly without drywall was most severely damaged by the repetitive simulated earthquakes. In contrast, the corner assembly with drywall suffered very little damage simply because of the added stiffness and strength, so peak displacements were below 1% wall height. Woodframe assemblies appear to perform quite well, only suffering moderate degradation, when subjected to subsequent earthquakes provided peak drifts are kept below 1.5%.