CSMIP 00-05

CSMIP 00-05

"Effect of Contraction Joint Opening on Pacoima Dam in the 1994 Northridge Earthquake"

by S. Mojtahedi and G. Fenves

March 2000, 87 pp.

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The response of Pacoima dam in the Northridge earthquake of 1994 is examined using the strong motion records obtained by CDMG accelerographs located in the canyon and on the dam body. A post-earthquake inspection of the dam showed the occurrence of minor cracking and block offset in parts of the dam body. The contraction joints between the cantilever monoliths appeared to have opened during the earthquake because of their clean appearance after the earthquake. The joints closed under static forces after the earthquake, except for the left-most joint which had a permanent opening of two inches because the abutment thrust block slid downstream.

By examining mathematical models of the dam, the seismic response of the dam was found to be influenced by opening-closing of contraction joints and horizontal joints, spatial variation of the seismic input, and amplification of seismic waves due topographical effects. Using a simple assumption for the distribution of free-field motion along the dam-foundation interface and considering the opening-closing of the joints, the response of the dam was computed analytically. The computed response from the model is larger in amplitude than the recorded response, but many of the overall characteristics between the two are similar. The differences between the model and the recorded responses illustrate the uncertainty in many factors affecting the earthquake response of concrete dams, such as input motion to the dam, dam-rock interaction, and energy dissipation.

Study of the CDMG processed records of Pacoima Dam in the 1994 Northridge earthquake and comparison with the analytical results from the models indicated the following conclusions: (i) The contraction joints opened during the earthquake, and the effect of the joint opening is an important factor in the response; (ii) the non-uniform free-field ground motion caused by topographic amplification has a significant effect on the dam response; (iii) the computed response using uniform free-field ground motion does not provide an adequate representation of dam performance; (iv) damping due to foundation rock radiation appears to be important; and (v) the pseudo-static effects of the non-uniform ground motion cause high stresses.