Soffit Suction Pressure 2

[mike0123m]

How do I go about calculating the suction pressure on the eaves of a roof to determine the attachment of the soffit material? I know there are tables that spell out the attachment of the soffit based on the wind speed but I would rather calculate the pressure myself and determine the attachment.

 

[hokie66]

Suction on the soffit should be the same as the suction on the wall adjacent to the eave.

 

[Ron]

Suction on the soffit should be the same as the overhang pressure.

 

[DaveAtkins]

I don't think soffits see suction--they just see uplift.

DaveAtkins

 

[hokie66]

Dave,

If there is suction at the top of a wall, there has to be suction on the adjacent soffit. Similarly with positive pressure on wall, then uplift on soffit. You can't go from positive to negative without transitioning through zero.

 

[DaveAtkins]

That seems logical, but I don't think the Code reads that way.

DaveAtkins

 

[hokie66]

Depends on which Code you are talking about. I suppose you mean the US ASCE standard. I am in Australia, and our standard is a bit more specific, but this is really not about a code, it is just common sense.

 

[Ron]

Typically, codes require checking both directions.

 

[kslee1000]

Don't mean any disrespect, I would agree more with Dave on this. Suction is a result of pressure differential between the interior space and the outdoor. The suffit is likely to experience lifting (wind hitting the wall and changes direction upward) than pressure differential.

 

[WillisV]

Agree with hokie66 - suction on the soffit should be the same as that on the wall adjacent to the soffit. Experimental evidence bears this out.

ASCE 7-05 is not clear on the subject; however, for some hard data and conclusions see the paper by Vickery entitled "Component and Cladding Wind Loads for Soffits", ASCE Journal of Structural Engineering, May 2008.


From conclusions section of the aforementioned paper:
"The results of the experimental program clearly indicated that wall and soffit pressures are highly correlated....The results indicate a simple and accurate solution to the soffit loading deficiency in ASCE 7 is to prescribe that the component in cladding pressures (both positive and negative) for use in the design of soffits be identical to the component and cladding loads used for the design of the wall components."

 

[hokie66]

Thanks, Willis. Looks like experimentation bears out common sense in this case.

kslee, suction is just the pressure on one side of a surface. You do sum the pressures on both sides of a wall for design, but a soffit will usually only see the external pressure, as the top of the soffit is in the enclosed roof cavity. You do get upward pressure on soffits on the windward side, but on the leeward and side wall, the soffit pressure is usually suction.

 

[kslee1000]

hokie:

I understand and appreciate your explanation (side walls making sense). Thanks.