Wind Pressure on Roof

[Xaviera]

Hi everyone,

I am an architectural student that has just started learning about wind engineering. I have a bit of a problem trying to understand how the wind pressure works in a building. In what situation will the roof pressure act alone on a building?

 

[JedClampett]

I'm not clear on your question. Upward wind pressure is always going to be resisted by the roof's dead weight. The codes (IBC; ASCE 7, etc.) prescribe combinations and load factors. For example, one downward wind combination is DL + .75 LL + .75 WL. So the a portion of the downward wind pressure is added to the dead weight and a portion of the live load. There are many combinations and they change based on the type of stresses you're comparing them to.
So this is a complicated question and you need to have the proper references to answer it. I don't think this is what you wanted to hear, but that's why we make the big money.

 

[Xaviera]

Dear JedClampett,

Thank you for your reply.I was actually wondering about a situation like the image below.

I'm not too sure in what condition will cause the pressures to act in such a way.

 

[JedClampett]

Outward pressures are caused by two effects:
[ol 1]
[li]The aerodynamic effect of the wind traveling over the building, like a wing,[/li]
[li]The inherent leakage of doors, windows and other openings (intentional and not) into the building. Wind gets in, but it can't get out. Depending on the size and position of the intentionally open ones, the pressures can be relatively small (less than 20%) or be over one half of the wind pressures.[/li]
[/ol]If you're asking why some upward wind acts at the bottom chord of the truss, it might have a ceiling covering it.

 

[hokie66]

Those two diagrams, if the pressures are to scale, represent wind loading when the wind is blowing parallel to the ridge. The first one with no openings on the windward (gable end) wall, and the second one with an open end or substantial openings.

 

[Xaviera]

Dear JedClampett,

Thank you for your explanation. I could kind of understand it now.

 

[Xaviera]

Dear hokie66,

Thank you for your reply and explanation. I could understand it better now.

If that's so, does that mean that if the pressure acts like the image below:

There is an opening in both the leeward and windward wall only? And for the image below, there are openings on the windward and side walls only?