Downward roof wind pressures
Downward roof wind pressures
(OP)
Per strength level combos, there is a load combination (1.2D+1.6Lr+0.5W) where the wind load is always additive to your worst case gravity loading on your roof. This wind load per components and cladding (i.e. for a joist or purlin) is always going to have a value that is around 0.2*wind load and end up with a higher vertical loading that cannot be ignored.
Has anyone thought about this? How are you guys handling this load case in your gravity design?
Has anyone thought about this? How are you guys handling this load case in your gravity design?






RE: Downward roof wind pressures
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RE: Downward roof wind pressures
Codes just give a guideline .... thats all
If you need more combinations or other combinations...do it ...
best regards
Klaus
RE: Downward roof wind pressures
DaveAtkins
RE: Downward roof wind pressures
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RE: Downward roof wind pressures
JAE is right, that is the 0.2 factor I was talking about. For typical wind speeds, that 0.2 factor can become to be around 7-10 psf of additional load on your purlins. But then, you don't need to apply those loads on your girders as their tributary areas will most likely exceed the 700 sq ft components and cladding limit and can be designed using MWFRS, for which there is no downward pressures for flat roofs.
I just wanted to get a feeling of how you guys are handling this problem when designing in your software of preference.
Thanks
RE: Downward roof wind pressures
One issue I have often run into is ASCE 7 prescribes minimum wind pressures to be used for positive C&C pressures. These are typically higher than what you would calculate using the pressure coefficients. For example, using ASCE 7-10 for a 30' tall flat roof building in a 115 MPH wind zone you may calculate a positive wind pressure of around 11 psf. However per ASCE 7-10 30.2.2 the minimum pressure would be 16 psf. I don't have the strongest background on the origin of this minimum pressure but you seem to be unnecessarily penalized using these minimums.
RE: Downward roof wind pressures
DaveAtkins
RE: Downward roof wind pressures