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Hi Asif, Why don't you do a FEA analysis in any standard software and compare the peak stress with the max allowed by the code? Thanks.

I was designing high rise building and designed the piles and pile cap on dynamic load cases (RSx, RSy). However the vetting engineer told me to check pile reactions in static cases as well (EQx, EQy). Now I have confusion as to why i should check pile reactions on static case when code tells me to do dynamic analysis on structure? The vetting engineer couldnt give me satisfactory explanation, that is why i turn here. Thank you.

Box girders would span between columns. What do you have in mind for the region of middle strips? I think you might end up spending more on man hours, shuttering and probably reinforcement. If you do want to make foundation light, go for post-tensioned floor systems. Structural engineers, in Pakistan, are very reluctant to adopt this widely accepted system, around the world, in buildings. If they can afford to spend more on labor, engage it on this. It will help developer, as you will be able to add additional floors for the same total height of building.

Waslaam, Your first two modes are not torsion. Torsional mass participation (Rz) is negligible in first two modes. Your first two modes are predominately translational in nature. See the attached table for reference.

There is no satisfactory answer. There is no logical answer as well. For high rise buildings, the equivalent static seismic force distribution along the height of building is incorrect. Vetting engineer wants to be on the safe side; he wants the foundation to be safe against both set of forces.

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.

The pop-up window, displaying the table after right-click, shows the DC ratios against all the load combinations. The DC ratio visible on ETABS screen is the worst among all the load combinations. You should be able to find this in the pop-up window.