MWFRS Wind Load Dispute

[EngSD]

I am designing a simple gable roofed building using ASCE7-10 envelope procedure (Figure 28.4-1 low-rise building). In the design of the MWFRS the negative pressure on the windward side of the roof was greater than the suction on the leeward. Since the net pressure on the roof was in the opposite direction of the walls I neglected it in my design following Footnote #6 in Figure 28.4-1.

The way that I approached this has come into question by another engineer. Their interpretation of the footnote is that only the wind pressure on the surface of the roof that is in the opposite direction of the wall can be neglected. Essentially saying that you should neglect the wind pressure on the windward side of the roof that is in the opposite direction of the wall pressure but you have to take into account the suction on the leeward side of the roof. Their justification for this is that if you analyze the same building using the Directional Procedure (all heights method Chapter 27) the net pressure on the roof is in the same direction of the wall pressure.

Can someone tell me if I am in the wrong on this and am being unconservative in my design. Thanks.

 

[JAE]

I think your interpretation is correct.

The footnote simply, and clearly, says that you either use ALL of the forces given (even if the roof pressure is opposite) or you neglect the "roof forces" totally.

It does NOT say "neglect the roof forces that are opposite the walls only".

Not sure what your engineer compadre is thinking here but I think they are too conservative.

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[EngSD]

Thanks for the reply. Do you know of any text examples that might help with my argument? I looked at the ASCE7 Wind Load Design Guide but all that showed was how to determine the wind pressures and didn't go into summing the pressures to design your wind resisting system.

 

[DaveAtkins]

I agree with JAE.

This is why the 10 PSF minimum on the entire projected area usually controls the design of a one story building with a gable roof.

DaveAtkins