The authors have presented an interesting and thought-provoking study on the e!ect of orthogonal or x-direction elements on the torsional response of yielding asymmetric one-storey structures under bidirectional excitation. The authors conclude that the orthogonal elements do not materially a!ect the post-yield response when the torsional rigidity of the system is unchanged, i.e. equal to that of a similar system but without these elements. About 10 years ago the writer and his co-workers conducted a similar study on the response of code-designed asymmetric structures under unidirectional (y-direction) excitation [2,3]. They also concluded that the torsional-to-lateral frequency ratio was a very important parameter and that the distribution of the torsional rigidity among the y-and x-direction elements had only a minor e!ect on the response, provided the total torsional rigidity of the system was kept intact. That study also showed that for a given value of (the ratio of torsional rigidity of elements in the y-direction to the overall torsional rigidity) the peak ductility demand of the y-direction elements falls somewhat with increasing strength of the x-direction elements. In other words, as observed by the authors, rotational strength, and not only rotational sti!ness, lowers the torsional response. However, this bene"cial e!ect is bounded by the minimum strength level of the x-direction elements that is su$cient to keep them in the elastic range.
The need for bidirectional studies arises from the fact that the x-direction excitation reduces the strength available to the elements in the same direction to resist the torque due to eccentricity. The resulting lower torsional strength can lead to larger ductility demand. The results of the authors' study suggest, however, that this e!ect is minor. The reason for this behaviour may be found in the fact that, because of low correlation, the response peaks in the two orthogonal directions are not simultaneous. If this conclusion can be generalized, it supports the view that the orthogonal elements should be considered in the unidirectional models routinely used in practice to analyse asymmetrical structures, thereby settling a long-standing controversy [4].