Wind Load Cases 1 thru 4 (Fig. 27.3-8 in ASCE 7).

[Brad Larsen]

Hi,

I'm curious if people that are using the directional procedure for wind loading ever have Cases 2, 3, or 4 control. I'm usually doing residential, and I think these code provisions are there because of the design of the Citibank building in NYC. I solved the equation symbolically for a rectangular 2-story box and get a loading of 0.8625*P for one shear wall, and 0.6375 for the other (where P is the total load in Case 1). So Case 2 will never control, at least not for rectangular boxes with resistance elements on all sides.

Has anyone else come to the same conclusion?

 

[phamENG]

For simply wood structures with flexible diaphragms? Pretty much. But if you get into rigid or semi-rigid diaphragms with odd shear wall layouts that require the resolution of torsion through the diaphragm with just a simple case 1, you can get some interesting results with the others.

 

[Brad Larsen]

Hi phamENG,

Yes, I should've included that. A simple wood structure with a flexible diaphragm. As I mentioned, I believe these load cases came from the design of the CitiTower in NYC, which has rigid diaphragms and no supports on the corners (or maybe no support on just one corner, it's been too long!). The building was found to be structurally deficient by a grad student studying the unique design. It was designed by only looking at Case 1 (before there was Cases 2, 3, or 4). Thanks for your confirmation.

On a similar note, I'm only seeing Case 3 to be relevant to hold down design, as two walls can share a common hold down. Would you agree?

Brad

 

[phamENG]

I know quartering winds were the source of the problem with CitiTower, but I don't know if that caused the change in the building code or if a change in the building code prompted somebody to consider it - I don't know enough about the history of the code development to agree or argue with you.

Generally speaking, yes. Though the increase on the hold down is only about 6% for a square building which would be the worst case. That's usually not enough to make a difference, but might be if you're right on the line between hold downs.