The coupled bending and torsional vibrations of a relatively symmetric 22-story reinforced concrete building in Reno, Nevada are studied. Analytical results are compared with observations obtained during the nuclear explosion FAULTLESS and to ambient vibration data. The fundamental periods of vibration observed during FAULTLESS were (TNS = 1.42, TEW = 1.81, TTORSION = 1.12 sec), and the calculated periods were (TNS = 2.14, TEW = 2.07, TTORSION = 1.90 sec). It was estimated that between 25 and 45 per cent of the total available nonstructural stiffness was required to explain the differences in the observed and calculated fundamental periods.

Each floor diaphragm in the system was allowed three degrees of freedom-two translations and a rotation. It was found that coupled torsional motions can influence the response of structural elements near the periphery of the structure. Strong-motion structural response calculations comparing the simultaneous use of both components of horizontal ground motion to a single component analysis showed that the simultaneous application of both components of ground motion can significantly alter the response of lateral load-carrying elements. Differences of the order of 45 per cent were observed in the frames near the ends of the structure. Also, it was shown that the overall response of tall buildings is sensitive not only to the choice of input ground motion but also to the orientation of the structure with respect to the seismic waves.

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