Executive Summary : | The translational components of the strong motion are always accompanied by the rotational components during a seismic event, due to travelling wave effect. But these rotational components are not commonly recorded. Past earthquakes additional stresses due to torsional response led to structural damage during earthquakes The torsional ground motion amplifies the detrimental effects in both asymmetric/irregular and symmetric buildings. Most building seismic design codes do not explicitly consider the seismic loads due to the torsional component of rotational ground motion. This is possibly because of lack of direct recordings of rotational motion and the historically popular belief that this motion is ‘too' small in magnitude. Similarly, IS 1893 (2016) do not consider the effect of torsional ground motion. Eventhough, construction of ‘torsionally flexible' buildings is not permitted as per IS 1893 (2016), it does allow ‘torsionally irregular' buildings. Additionally, IS 1893 (2016) defines accidental eccentricity, which has to be accounted for in symmetric as well as torsionally irregular buildings. It is implied that the accidental torsion takes into consideration of the torsional component of earthquakes, uncertainty in mass and stiffness distribution, ignoring the stiffness of non-structural elements, and the non-uniform yielding of the lateral load resisting elements. However, the provision of accidental eccentricity in IS 1893 (2016) is examined from the reference to the performance of the building under the action of torsional ground motion. The objective of the present study is to evaluate the adequacy of accidental torsion design requirements in IS 1893 (2016) and to study the seismic performance of code-compliant torsionally irregular buildings under the combined action of translational and torsional components of ground motions. The performance of the building is evaluated in the seismic risk framework. The study is limited to mid-rise special RC frame buildings with fundamental periods in the range of 1 sec to 2 sec. The proposed research will be useful to understand the seismic risk of buildings designed as per current Indian Standards. The principal outcome of the research will be a robust methodology to determine the low-frequency torsional component of ground motion that is consistent with the recorded horizonal translational components. This will enable the seismic assessment of the Irregular Structures using three components of ground motion via non-linear time-history analyses. |