Experimental Testing and Seismic Performance Assessment of Wood-Frame Shear Walls with Common Construction Defects

Research Team: Scott Hamel, PE, PhD, and Polly Murray, PhD

Summary:

Building code compliance varies widely within the Municipality of Anchorage, Alaska. There is a subset geographic area in which building codes have been actively enforced since the early 1990s, but many existing structures were built prior to building code enforcement, and a large portion of new construction occurs outside the bounds of the Building Safety Service Area (BSSA). In 2018, a Mw 7.1 earthquake occurred near Anchorage, and caused widespread damage to homes and businesses, particularly in areas where residential building codes are not enforced. This disproportionate distribution of damage motivates this study. Outside the BSSA, and prior to the onset of residential building code enforcement, evidence suggests that houses were and are sometimes built with framing and walls systems that lack all the necessary components or detailing to adequately resist the seismic forces imparted by design-level or larger earthquakes. These deficient systems may be due to either design or construction errors or omissions, or both. The extent to which these structural deficiencies affect the building’s seismic resistance is unknown and is the focus of this work.

This study includes both laboratory testing of code-compliant and deficient components and computational modeling for seismic risk analysis. Testing focused on the cyclic loading of walls to determine the nonlinear hysteretic responses of components for each chosen deficiency. Fourteen configurations of wood frame shear walls typical of construction in Alaska were tested for a total of 37 wall tests, with variation in fastener type and spacing and panel type and configuration. The seismic risk assessment employs incremental dynamic analysis to determine collapse fragilities of houses that feature code-compliant and deficient components.

Testing and modeling illustrated the reduction of strength and stiffness associated with non-code-compliant shear walls, when compared to those built to code standards. Collapse risk was most strongly correlated with the strength of wall panels, and fastener spacing has the largest impact on strength. Houses with half the edge nails (spaced at double design spacing) are 3 to 5 times more likely to collapse during an MCE_R level event.  The results indicate a significant potential increase in seismic risk associated with a lack of design-code enforcement and construction inspections.