External Pressure Coefficient (GCp) & Internal Pressure Coefficient (GCpi) interaction for C&amp

[KelleyA]

Hello all,

I'm looking at designing some components and cladding (Metal studs) for a project right now. The building is enclosed, class B, flat roof and with a Velocity pressure of 31.3 psf according to equation 30.3-1. I'm using ASCE 7-10 section 30.4 to solve for my values

I'm currently trying to figure out how the two pressure coefficients in the title interact with each other. The code seems to read as if you take the two positives and put them together for your windward, and your two negatives and put them together for your leeward. So in this case you would get the following values:

Windward: p = 31.3 (.9 - .18) = 22.5 psf
Leeward: p = 31.3 (-1.26 - (-.18)) = 45.1 psf

However, one of my coworkers says you simply make the worst case positive and negative forces, like so:

Windward: p = 31.3 (.9 + .18) = 33.8 psf
Leeward: p = 31.3 (-1.26 - (.18)) = 45.1 psf

Does anyone have some insight as to which way is the proper way to solve for these values? Thank you.

 

[MrHershey]

Technically you're both half right (though your arithmetic doesn't match your final answer on the leeward side; solving your equation gives -33.8 psf, not -45.1 psf).

In theory you should be checking both faces with both positive internal pressure and negative internal pressure. So the windward face would be 31.3 x (0.9 +/- 0.18) and the leeward face would be 31.3 x (-1.26 +/- 0.18), assuming your qz and GCp values are correct.

Your coworker is just taking a shortcut to get the worst-case values for both faces. Your windward value still technically needs to be considered, as does 31.3 x (-1.26 + 0.18) = 33.8 psf for leeward. But both are less than the values your coworker's getting with his/her shortcut, so they won't ever govern.