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Selection of linear seismic analysis method

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I was working on seismic design of steel structure.

There are two methods for linear seismic analysis 

1. Lateral force method and

2. Modal response spectrum analysis 

Now my structure is not satisfying all criteria for both methods i.e.,

First mode of vibration criteria is satisfied for 1 st  method while regularity criteria is satisfied for 2 nd  method.

So I am confused which method Shall be used.

Please guide me.

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You should do modal response spectrum analysis if your structure is irregular.

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Thank you very much for your reply.

My structure has no setbacks, but irregular in the sense of its lateral stiffness and storey weight.

Its lateral stiffness and storey weight is not decreasing from bottom to top of structure.

So still I have to go for modal response spectrum?



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