Auto Wind Load Calculation - ETABS ASCE 7-05

[EngineerRam]

Hi,

From my understanding, ETABS auto-calculates wind lateral loads based on the projected area of the diaphragm on the main wind force resisting system. ETABS never asks to specify the GCPi factor. Does ETABS always assume a fully enclosed structure? If not, how would I specify partially enclosed? I also ran a trial run, where I set a story grid 50' higher than the top of my building (no walls were extended up to this story). I included this story as my top story in my Wind Load Pattern Exposure height. My lateral wind loads were much higher than before adding this grid story. If this is how ETABS works, doesn't that mean that wall openings are not considered?

Thanks.

 

[Trenno]

For global lateral analysis, which ETABS is predominately used for, the internal pressures cancel out and do not affect story shears.

 

[MrHershey]

Two questions, two answers:
1. As Trenno noted, your internal pressures cancel out. If you have negative interior pressure you'll be pulling inwards on both windward and leeward surfaces an equal amount (and pushing outwards for positive interior pressure). Add them together and you get zero net force on the lateral system.
2. Would have to play around, but I would guess that the reason your lateral wind loads go up is because the leeward component of your wind load is constant over the building height and is based on the roof height. By artificially increasing your top story, you're telling ETABS that your roof is much higher than it really is and thus get higher wind loads on your leeward surfaces. You should notice that if you have no diaphragm at the top story, you'll get no wind load there, but your other wind loads will still increase because you've told ETABS that your roof height is higher.