by S. Mojtahedi and G.
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.