Seismic Behavior of Level and Stepped Cripple Walls

Old residential buildings, such as those built before 1960, were commonly supported on short wooden studs between the foundation and first floor framing. For many of these buildings, the inertial force associated with the horizontal earthquake ground motion must be resisted entirely by the cripple wall and wall-to-foundation connections. Past earthquakes in California have shown that unbraced cripple walls are particularly vulnerable due to insufficient lateral resistance, even for moderate levels of ground shaking. A typical mode of failure involved the building shifting laterally over the ’soft’ cripple wall causing the building to drop vertically, resulting in fractured sewer, water and gas lines, and often leading to irreparable loss of the building.

The objective of this project is to characterize the in-plane seismic behavior of cripple walls, and to contribute towards the performance-based design of woodframe buildings. Twenty eight cripple wall specimens with details representative of current construction and retrofit were tested using displacement histories recommended for the CUREE-Caltech Woodframe Project. Level cripple walls of 2 and 4 feet height, simulating the retrofit condition, and stepped walls of 1-in-3 and 1-in-2 slope, simulating the current construction, were investigated. Other parameters in the test program include gravity load, stucco finish, and loading history. Damage evolution as well as its distribution within the wall is identified at various stages of loading. Test results are compared with design values currently prescribed in the design code.