Story drifts from STAAD with dynamic analysis

[DAVE9999]

How do you obtain story drifts from STAAD with a dynamic analysis? Should you use the node displacements? Or the PRINT STORY DRIFT command? And do you need to factor up displacements Cd/I (see ASCE 7-05 12.9.2)

Thanks

 

[mike20793]

I'm not totally familiar with the STAAD output, but I usually find it's easier to look at the individual story drift rather than look at global drifts. In fact, I often use a drift ratio based on the allowable drift. If you are comparing your story drift to the allowable from Chapter 12, then yes you need to increase the elastic story drifts by the factor you mention. I find it much easier to just look at a drift ratio and not worry about increasing the elastic drifts.

 

[DAVE9999]

Thanks, you mention elastic drifts but this is for a dynamic (or modal) analysis

 

[mike20793]

The section you reference directly states, "The values for displacement and drift quantities shall be multiplied by the quantity Cd/I." Unless you are using a nonlinear analysis, the drifts you're getting are elastic, so you increase them based on the plastic response of the SFR system.

 

[SethGuthrie]

Story drift should still be OK for dynamic response spectrum analysis. Here's a related post you might be interested in about getting uncombined modal results:

https://communities.bentley.com/products/structural/structural_analysis___design/f/5932/t/87016