A study of the apparent change in the foundation response of a nine-story reinforced concrete building

Forced vibration tests of the Millikan Library building, a nine-story reinforced concrete shear-wall structure, were conducted in 1966 and 1967, before the San Fernando earthquake, and again in 1974. The measured foundation response of the structure reported for the two tests was significantly different: in the earlier tests, motion of the foundation in the N-S direction contributed only about 3 per cent to the total roof motion, whereas in the more recent tests almost 30 per cent of the roof motion was contributed by foundation compliance. A lengthening of the fundamental period of vibration of 11 per cent was also noted.

The purpose of this study is to examine the indication that the foundation response of the structure may have changed because of the earthquake. To determine whether the observed changes in foundation response are consistent with the change in natural period, two analytical models of the Millikan Library building were developed. Both of these models include the effects of foundation compliance and one includes the effects of shear deformations in the walls of the structure. The results of these simple analyses show the changes of mode shape and period observed between the two tests to be consistent.

From the analysis, and from an examination of what is thought to be minor earthquake damage at the ground floor level of the structure, the authors conclude that the most probable cause of the differences observed in the two tests is the loss of rotational and translational stiffness provided by retaining walls, concrete slabs and other stiff, but brittle elements.