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ETABs Drift Calculation

ETABs Drift Calculation

ETABs Drift Calculation

(OP)
I am performing some drift reviews on a building I am working on and I am able to reproduce a lot of the drift values that ETABs seems to autocalc.
One point in particular is not matching my hand calcs and I was wondering what is going wrong with either my hand calc or ETABs. Of course this is the one that will push me into extreme torsional irregularity if it is truly per my hand calc.


Image 1, Deflection in X direction, 1.288202”, drift recorded = 0.002043


Image 2, Deflection in X direction, 1.053346”



Story height = 16'

Hand calc drift value = (1.288202”- 1.053346”)/(16’*12) = 0.001223, again ETABs drift = 0.002043

I am off by a factor of 1.67, what's the difference?



S&T

RE: ETABs Drift Calculation

(OP)
After talking with tech support, the issue is that I am reviewing response spectrum analysis results and that I cannot merely look at the combined response, but have to review individual mode responses and combine logically.

The person I talked with stated that the value ETABs has returned does look at each mode response and calculates the correct drift parameter. They also attached the following article:

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

After reading the article though, it does not sound as if ETABs does this automatically. I will follow the article and see if it matches the ETABs value of 0.002043.

S&T

RE: ETABs Drift Calculation

ETABS internally multiplies displacements/drifts by q factor, as defined by EC8 (EN1998).

RE: ETABs Drift Calculation

Quote (sticksandtriangles)

After talking with tech support, the issue is that I am reviewing response spectrum analysis results and that I cannot merely look at the combined response, but have to review individual mode responses and combine logically.

There's a chapter in Chopra's second edition of Earthquake Engineering that covers why you cannot consider the deformed shape of a response spectrum to be accurate. I believe it's called 'avoiding a pitfall' or something similar.

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