Forced vibration tests and associated analysis of two multistorybuildings are described. In one case, the dynamic properties of thebuilding measured during the tests are compared to those predicted bysimple analytical models. A three-dimensional finite element modelof the second building was constructed for the purpose of evaluatingthe accuracy of this type of analysis for predicting the observeddynamic properties of the structure.
Forced vibration tests were performed on Millikan Library, anine-story reinforced concrete shear wall building. Measurements ofthree-dimensional motions of approximately 50 points on each of sixfloors (including the basement) were taken for excitation in the N-S andE-W directions. The results revealed a complex interaction betweenlateral and vertical load carrying systems in both directions. Theresults also suggest that a significant change in the foundation responseof the structure occurred in the stiffer N-S direction during the SanFernando earthquake. This phenomenon was investigated through theuse of two analytical models of the building which included the effectsof soil-structure interaction.
The Ralph M. Parsons world Headquarters building, a twelve-storysteel frame structure, was also tested. The natural frequencies,three-dimensional mode shapes, and damping coefficients of nine modesof vibration were determined. Other features of this investigationincluded the study of nonlinearities associated with increasing levelsof response and the measurement of strain in one of the columns of thestructure during forced excitation. The dynamic characteristics ofthe building determined by these tests are compared to those predictedby a finite element model of the structure. The properties of primarilytranslational modes are predicted reasonably well; but adequate predictionsof torsional motions were not obtained. The comparisonbetween measured and predicted strains suggests that estimates ofstress obtained from finite element analyses of buildings should bewithin 25 percent of those experienced by the structure for a knownexcitation.
