Roof Pressue under a overhang ASCE 7-16

[engreg]

I have a roof with a large overhang, 12'-15'. In the attached diagram from ASCE 7-16 The value Ps,(pressure from the underside of the overhang)is the wall pressure value from the wall below the over hang. I believe this value is from wind hitting the wall and running upward to continue its path. Does anyone have any justifiable thoughts on how far out to allow the distance to go away from the wall onto the overhang? For a 15ft overhang the values seem excessive, and I wouldn't expect the pressure to effect the overhang as much farther from the building. Any thoughts are helpful.

 

[driftLimiter]

Theres no attachment.

Wind pressure can easily act on a horizontal surface regardless of proximity with building or vertical walls.

ASCE 7 provides sufficient guidance to design these things without making a judgement call like you are suggesting.

 

[engreg]

Attached now. Where are the guidelines you are talking about. Is it your opinion that the wall pressure needs to be applied over the entire overhang, regardless of width or height of the wall its connecting to.

 

[driftLimiter]

Well we have roof overhangs (which you have found).

And we have ASCE 7-16 Section 30.11. Which covers canopies) Your judgement to decide which one applies better.

My point is ASCE 7 doesn't say you can just ignore wind pressure on these elements if they are a certain size.

Intuitively, if you continue with your thinking... The wind hits the wall, then it has to flow out and around the overhang before it crests the roof and continues down the stream. The presence of a building surface doesn't absorb the wind flow, it deflects it. A bigger overhang I would think is more susceptible to wind uplift forces not less.

 

[kissymoose]

I'd use 30.11 for "attached canopies"

Ah, you beat me to it DL

 

[engreg]

I am familiar with the canopies section, and although I doubt it would be allowed in my scenario. My interpretation is there must be wall above the "overhang/canopy" to allow for the use of those values. With no wall a typical overhang will have wind moving a lot faster over the top than below it, creating a very high net uplift. This is why your canopy force isn't nearly as high as your overhang force.