P Delta Effects Etabs 1

[Ganesh Persaud]

Hello everyone, my question is, how do you know which values has to be used from Etabs for the story forces, displacements etc?
I exported the Etabs results into excel, and sorted it with respect to load combination and stories, but I’m not sure of which values I should use?

Any advice on this?

Thank you

 

[milkshakelake]

I'm not really sure what you mean. ETABS can export different tables for joint drifts, story drifts, and displacements. Are you asking how to use these values per code?

There are different codes for seismic and wind. Your local building code can tell you which thing to look at. ETABS is just a (very approximate) tool to get those numbers.

Are you looking for seismic or wind drift per your code?

 

[Ganesh Persaud]

Milkshakelake, yea I’m asking how to use these values as per code. I’m calculating the p delta effect as per asce 7-05 12.8.7, and I’m not sure of which values to use in my excel calculation.

 

[milkshakelake]

There's an option to add P-delta effects into ETABS directly. I don't remember exactly; sorry for the quick response, but it's in the menus. Look around. And make sure you use the right load combination in ETABS for the case that you want.

 

[milkshakelake]

Go to define -> P-delta Options. If you're using the iterative solution, you might have to run the model twice: one with strength level combinations and one with service level combinations. It's good to compare the results anyway.

https://wiki.csiamerica.com/display/etabs/P-Delta+analysis+parameters

 

[Ganesh Persaud]

Hey, Question, how do I know which is the correct load combination?

 

[milkshakelake]

I use strength-level (factored) load combinations for strength analysis and seismic drift. I use service level (unfactored) load combinations for wind drift. I take full dead load and 50% of the reduced live load. I don't have a reference for that, it's just what I do and the reasoning is based off of how to calculate long-term deflection for concrete, though I also apply it to steel buildings.

So let's say the strength level load combination is 1.2D+1.6L. I'd use 1.2D+0.8L to be conservative. Where D includes dead load, superimposed dead load, and facade load, and L is the reduced live load. But live load reduction in ETABS doesn't really work, so I just use 1.2D+0.5L.

 

[Ganesh Persaud]

Thank you very much