A consideration of the torsional response of building frames
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History of elastic static procedures for the seismic analysis of torsionally unbalanced building structures is briefly reviewed. It is suggested that the provisions of NZS 4203:1976, accounting for modal coupling, are based on inconsistent interpretation of results from well known two-degrees-of-freedom models. An alternative dynamic procedure is described which, while retaining the basic two-dimensional features of NZS 4203:1976 torsional provisions, is equivalent to three-dimensional modal spectral analysis. The procedure also results in a substantial simplification of the analysis compared with standard dynamic computer techniques now available to the structural engineer.