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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.

Salam, in buildings we check first three modes normally as far as i know, and we prefer first mode in x or y direction and third mode in torsion, if a building is symmetric with center core shear walls, what could be the reasons that it has first mode in torsion, even center walls has been removed and just checked with columns still first mode is in torsion. Building is -3+G+16, model in etabs 17.1.0 is attached if somebody could have a look at it. what i understand is that symmetric structure should not be going in torsion. if it still goes like this we can design it for torsion or i must add or subtract something to behave with first model in x or y and third mode in torsion, advice is required. Thank you Maskan with loads 03 21_WAQAR_01.$et Maskan with loads 03 21_WAQAR_01.EDB

Dear friends, I have building G+2P+7 floors, it's almost symmetric but the second mode is always torsion, I tried many sh. walls configurations but have the same result any tips Thanks I attached the model e2k, G+2p+7.e2k

Hi there, I am currently working on a 20+ story building (For educational purpose only) having dual resisting system (Moment resisting frames and concrete shear walls located in center). I intend to perform Non-Linear Static Pushover Analysis to capture non-linear response of the structure and to study Demand/Capacity ratios for in-elastic response of structure. However, when I performed Response Spectrum analysis, I got 1st mode of vibration as "Torsional mode", which doesn't make sense to me. Story-drifts are restricted as per code's limitations. Also Torsional irregularity checks are also okay (Considering eccentricity between center of mass and center of rigidity). Base shear for both, Equivalent Lateral Force Procedure and Response spectrum analysis also matches (approximately equal). Since, Pushover analysis is applicable to structures whose response is dominated by its first mode of vibration (Which isn't the case here), although my first mode appears to be in rotation, my gut feeling says its translational. I am attaching Modal participating ratios, any sort of help is appreciated.

Dear All, I received a peer review about the modal analysis I conducted in a high rise building. It states the following : "The preliminary mass participation factors indicate that mode 1 is predominantly translational in the transversal Y direction, mode 2 torsional and mode 3 translational in the X direction. It is not ideal to have torsional as mode 2". 1. What is the effect of Mode-2 on design as we usually concerned with fundamental frequency.? 2. How torsional modifier in ETABS is going to affect in this case? Any addition would be highly appreciated. Thanks in Advance.