All major building codes employ empirical procedures to account for modal coupling between the lateral and torsional responses of a structure. These procedures are implemented using expressions
defining an equivalent static design torque. The provisions are based largely on the results of parametric investigations of the earthquake response of simple single-storey building models, which are found to
be representative of regular multi-storey structures. This paper presents results obtained by the analysis of the time-history earth- quake response of a single storey mono-symmetric building model, leading to the development of an alternative approach for defining the design torque for torsionally coupled buildings. The procedure is based on the concept of effective eccentricity, in which the design lateral displacements of key structural members on the edge of the building are matched to the results of a dynamic analysis. A close approximation to the dynamic responses is derived over the relevant ranges of the important parameters. These parameters include the ratio of torsional to translational natural frequencies, which strongly influences the magnitude of torsional coupling effects in asymmetric buildings.