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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.

Please find the pages from "Design application of Raft foundation by J.A. Hemsley". He is recommending to increase the stiffness of outermost springs to be increased. By how much increase don't mentioned. If anyone has this complete book may please share. Thanks Pages from jahemsley design applications of raft foundations.pdf

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.

Whilst waiting for Waqar's response, here are my thoughts: It is common for geotechnical engineers to allow 20% overstress on bearing pressure due to transient (non-permanent) loading, i.e wind. For allowing 20% overstress of bearing pressures due to dead and superimposed live loading, I would expect the concrete raft to be stiffened along the edges (edge thickening) to assist with the spread over a wider area, and thus averaged out in terms of allowable soil bearing pressure and settlement? With 20% overstress there will be rotation along the raft edges too - that needs to be a design consideration. Also edges of rafts may be subject to erosion or softening of soil from say someone laying a draining pipe along the edge of the raft, or digging an excavation adjacent to the edge of the raft. Think in terms of building foundation performance - over its 50+ years life! The figure below is quiet interesting on simple pad footing case, and how pressure distributes under s a pad (sort of a raft), depending on soil type. Always good to get a geotechnical engineer's opinion on soil bearing pressure matters.

Don't know the finer details of the model - have you tried thickening the edges of the Raft? or local thickening?

it means for noraml building where we are not considering seismic loads there is no need of defining "mass sorce"

Like Rana said, I will just add one more thing. I remember that Live Load only needs to be considered in Mass Source if its a warehouse, or LL=100psf or more. See UBC, for clarification, there is something about the warehouse or 100 psf requirement.