ASCE 7-05 Wind Code Method 1

[Heck83]

I am having trouble understanding which way the force is acting. There is a clause that states, "plus and minus signs signify pressures acting toward and way from the projected surfaces, respectively;" however the horizontal and vertical forces on a windward roof stay negative until the slope reaches like 25 degrees (transverse in figure). Also, the arrows in figure 6-2 are shown going in both directions.

This seems completely counterintuitive with a roof nose-down into the wind. Can someone please help me?

Thank you so much in advance!

 

[mike20793]

Wind pressures act normal to the roof. If it's positive, it's acting towards the surface; if it's negative, it's acting away from the surface.

 

[mike20793]

Sorry, I didn't actually look at the figure before answering. For this method the pressures do not act normal to the roof. This method is a simplified procedure and is often very conservative for design.

 

[structSU10]

When the roof pitch is shallow the wind produces a vacuum on the roof surface as it passes over. Its due to the abrupt change from the vertical surface, and the wind doesn't conform to the profile of the roof very quickly. As the roof pitch increases, it allows for a more 'gradual' transition for the wind to react to, and starts to lead to the wind pushing against the roof.

 

[Heck83]

The wind pressures are specifically said to be vertical "projection": see note 8 in figure 6-2.

Also, I stated the pos/neg toward/away clause, I was asking about the arrows pointing to and away in some, but not all, of the zones.