|CUREE-Caltech Woodframe Project
Element 1 - Testing & Analysis
Task 1.4.3. Seismic Behavior of Cripple Walls
PI: Y. H. Rob Chai (University of California, Davis)
Old residential buildings, such as those constructed prior to 1960, were commonly built on cripple walls of 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 were 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 led to irreparable loss of the building.
The objective of this project is to characterize the in-plane seismic behavior of cripple walls, and is intended to contribute towards the performance-based design of woodframe structures. Design values for the lateral strength and ductility capacity of cripple walls will be characterized through full-scale experimental testing. Evolution and distribution of damage within the wall, which are important for performance assessment, will be identified at various stages of testing.
Test specimens with details representative of braced cripple walls constructed on level and sloped ground, and on walls finished with and without stucco were investigated. Full-scale specimens of 12 ft length were subjected to a combination of gravity compression and in-plane reversed cyclic lateral displacement. Figure 1a shows the test setup for a stepped cripple wall finished with stucco. Parameters in the test matrix include the height of the wall, slope of the footing, gravity load, stucco finish, and loading history. Level walls of 2 and 4-feet height, and stepped walls of 1:3 and 1:2 slope were investigated. Gravity loads representative of single and two-story homes were considered.
The total number of cripple wall tests in the project was 28, and as of December 8th 2000, testing has been completed. Figure 1b shows a comparison of the lateral force-displacement response of two similar specimens finished with and without stucco. These results show significant stiffness and strength increase due to the addition of stucco. The increase in lateral strength, for this case, was about 38% in the downhill direction and 29% in the uphill direction. Damage in the stucco was characterized by diagonal cracking near the steps of the footing and was developed primarily during lateral loading in the uphill direction.
CUREE Report : W:17: Seismic Behavior of Level and Stepped Cripple Walls
Photos from the cripple wall project including a one-page description of the project.
Animation: Comparison of two identical specimens being loaded by ordinary and near-fault ground motions. Animated Simulation: Cripple Wall Test Specimens #20 & 21 - 1:2 Stepped Wall subjected to varying motions. 450 plf gravity, with stucco.