Inter-story Shear Transfer in Woodframe Buildings

One area of light-frame wood building construction that has seen little engineering research consideration is the mechanism that transfers shear forces between stories. The connection between the bottom of the rim joist and the top plate of the lower wall in conventional construction is of particular interest. This is the connection of concern due to a number of
failures that have been observed in recent earthquake events such as occurred at Northridge, California. The research reported herein analyzed experimentally seven different configurations of inter-story connections commonly found in wood-frame buildings. Each configuration (or scenario) consisted of four tests: one monotonic, or pushover (subject to ramp load), and three cyclic. Other researchers involved in the CUREE-Caltech Woodframe Project (Consortium of Universities for Research in Earthquake Engineering) determined the cyclic testing protocol. The data from the tests done here was used to determine the maximum load that the connection experienced and the associated displacement. Other qualities of the connection that were analyzed determined the energy dissipated in the inter-story connection and any degradation of the connection in cyclic loading versus monotonic. It has been found that any connection exhibiting multiple “planes” of failure performed better (attained higher loads without significant deflection increase) in cyclic loading. This is due to the fact that the forces were resisted by and distributed better throughout the connection rather than having a focus on one line of nails or connectors. Also, connections that were “engineered” performed better by virtue of the fact that any connection designed with a sound knowledge of the force transferring mechanism will have a better chance of survival in extreme loading events. This research provides engineers a better understanding of the force distribution within this system and, in turn, allowing improved life safety of the building. The improved construction of such connections will also enhance the immediate occupancy, that is, the ability of a building to be immediately occupied following a major seismic event. All of this will minimize the possibility of loss of life and limit the damage to such buildings.