ASCE-7 Wind Load Cases

[vincentpa]

For determining roof pressures for MWFRS in Design Load Cases 3 & 4 (Fig. 6-9 p. 54), are the roof loads additive for the different wind directions i.e. 0.75Px + 0.75Py for the roof as well as the walls?

 

[jike]

I believe so. Aren't these the loadings that were added to the code because of the discovery of the weakness in the Citicorp building in New York?

 

[chichuck]

I disagree with jike. I think that these two load cases are intended to be a simple approximation of wind on an oblique axis. The two 0.75 factors look like they are a (conservatively high) approximation of 0.7071, which are the components in the two orthoganal directions of wind at a 45 degree angle at the corner of a square. I further think that these should be used only for analyzing lateral force resisting systems, like braces or shear walls. Diaphragms in floors or roofs might also be included as well. But, the uplift on a roof, for a wind blowing at the corner of a square building would not be as high as you are suggesting. The code requirements direct you how to find roof uplift, this is dependant on the length and width & height of building; for diagonal, I'm afraid I cannot offer any suggestions on "proper" or "correct" parameters for wind on a diagonal. But I'm convinced that it doesn't get to be 0.75*uplift for long + 0.75*uplift for short directions.

Regards,

chichuck

 

[jike]

Chichuck:

I think that we are saying the same thing. The diagonal wind was the problem with the Citicorp building for the bracing and shear walls. It is my understanding that these are for wall loads, not roof uplift

 

[vincentpa]

My questions stems from the the design of pre-engineered buildings where the wind loads control the design of the foundations for uplift. Combining the roof loads for the two directions would have a significant impact on the uplift forces on the column and hence the foundation/anchor bolt design.

 

[cap4000]

vincentpa

You have a very valid concern especially for pre-engineered light buildings.I believe based on the wind testing studies I have seen regarding "Quartering Winds" at say 30 to 60 degrees in the corner of the building, it is definitely occuring at the immediate areas of the roof in both directions. To quantify it, the suction/uplift forces on the roof perimeter strip edges in both directions can easily exceed a Cp coefficient of negative 4 and get quick spikes as high as negative 10 or 20. Its referred to as "Conical Vortices" on the roof. A great book for you to get is "Wind Loading on Structures" by Holmes. Its on page 172 and 173. As I see it, C&C loading issue for sure, MWFRS probably so in your case. Good Luck.