This paper describes the design process by which the major aseismic building codes account for torsional effects in the dynamic response of asymmetric buildings to strong earthquake ground motion. Earthquake-resistant design of conventional buildings requires that the forces affecting the lateral resisting elements of the structure be estimated for specified loading conditions. In cases where the building design induces irregularities in the horizontal stiffness or mass distribution, the earthquake response consists of coupled sway and rotational effects which combine in the worst cases to increase significantly the design loadings in peripheral walls or columns. Building codes employ empirical techniques to est imate the design shear and torque affecting each building storey, taking into account the effect of modal coupling. The paper outlines these provisions as given in the USA, Canadian, Mexican, New Zealand, Eurocode 8 and CEB (Euro-International Committee for Concrete) design codes and makes a review of current practice in the light of recent research results based upon simplified dynamic analysis of torsional coupling effects.

Recommendations for improving some of the shortcomings of the seismic torsional provisions are described, in which use is made of the concept of effective eccentricity to match the dynamic results with an equivalent static design approach. The influence of a building's natural frequencies on torsional coupling effects is examined with regard to the difficulties encountered in determining accurate values when using simplified design models. It is suggested therefore that improved building design should endeavour in the first instance to avoid the worst-case situations in which coupling of lateral and torsional response induces forces in excess of design values. For this purpose the results of simplified parametric studies as presented in this paper are a valuable aid to conceptual building design.

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