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Hi guys, There is a question that I think lost of seismic designer often wonder about that is if there is torsion in first two modes, is that model always considered bad model? I found this in IS 1893:2016 that strictly states this (please see the picture below). “torsional mode of oscillation shall be smaller than those of the first two translational modes along each of the first two translational modes along each of the principal plan directions.” And I wonder, is there any item in IS 1893:2016 tells us about what should we do if we are in case that torsion is inevitable ( like when the owner and architecture don't let us revise a complex structural plan that leads to torsion in first two modes). Please help, thank in advance.

It is general practice to avoid torsional modes. The primary reason is that Engineers want to make their life easy; they do not want to get into the complications offered by structural system with torsion as the main response. Having said that, it is possible to design the structural system with torsion as first mode. The codes around the world penalize the engineer by forcing them to design that structure for amplified forces if torsion is under a certain limit (ratio of drifts at ends< 1.4). If the torsion is more than the limits set by code, the best way to verify such structural system is to put it through a NLTH analysis and use the Performance-Based Seismic design.