drift results using response spectrum case not logic

[ELSARWAN]

Dear experts,
i want to know how to check drift results using dynamic analysis ( response spectrum analysis ) , the traditional method that going to show tables in etabs and select show drift then see the drift in RSA in x or y direction will not work because (as i think ) etabs calculates story drift from maxi displacement for each mode shape which is not correct
, i.e if i have two story building etabs calculates the max. displacement for the second story from mode shape and then calculates the max. displacement for the first story from another mode shape and then get the drift and this is shouldn't happen.
- the right way is to calculating the drift from each mode shape and get the maxi,
so i want to ask the experts here how they dealing with this issue.
thanks in advance

 

[MrHershey]

ETABS does not compute drift as max displacement from any one mode. The drift is based on a combination of modes, as the actual response will be in a combination of modes rather than any single mode.

That said, you can break things up into modes if you wish and recombine yourself. CSI provides documentation for doing this on their wiki, though it's geared towards checking for torsional irregularities:

https://wiki.csiamerica.com/pages/viewpage.action?pageId=4554971.

But the same process could be followed for drifts if desired.

I typically don't bother as it's relatively uncommon for adjacent floors to be moving in different directions with enough magnitude to constitute a drift issue.

 

[ELSARWAN]

thanks a lot for your kind response,
i asked my question because some times when i check drift from show tables>>>show drift i got non-logical answers for drift from etabs specially when i have extension of some columns in roof floor.
i will check this method that you mention in your answer .
thanks