Wind Loads on Overhang

[sturr]

I am designing the attachment of cladding on the underside of a roof overhang using ASCE 7-05 for the wind load determination. The wind load determined by using the GCp factor from figure 6-11B, page 56 (overhang), is the total load on the overhang, including pressure from the upper and lower surfaces (ref: footnote 6).

Does anyone out there know how to divide the GCp factor into separate values for the upper and lower surfaces? I assume it is not 50/50.

In this case the cladding is only subject to wind loads on the bottom of the overhang. I'm always conservative, but I believe using the total load would be excessive.

Thanks!

 

[southard2]

Yep this has been a problem with ASCE wind loads for a long time. I can't speak for the international building code but down here in Florida our Florida Building Code added a provision that stated that the soffits could be designed for the same wind pressure as the walls. It makes sense. I usually use an effective area of less then 10 squared feet. See 2007 FBC 1609.1.



John Southard, M.S., P.E.

http://www.pdhlibrary.com

 

[sturr]

Thank you southard2. I didn't know that about Florida but that is exactly what I have been doing for years. I'm glad someone has come to the same conclusion. My project is in Washington and my use of wall pressures on the soffit is being questioned.

 

[WillisV]

See the attached journal article for a technical resource / backup of your method - the conclusion is to use the wall pressures.

 

[WillisV]

Try Again

 

[sturr]

Thanks WillsV! That is an excellent reference. Maybe ASCE will add the footnotes suggested by the author.

 

[rscassar]

I use the greater of the wall pressure or the roof pressure adjacent to the edge.

I have come accross a similar problem with free roofs which have both ceiling and roof joists and how to distribute roof pressure to upper and under surface.

 

[a2mfk]

Thanks southard, hadn't seen that provision but I just found it. Good to know that what several fellow SEs and myself finally concluded is now official. Used to come up all the time on porte cocheres and large canopies in front of retail buildings...

SECTION 1609 WIND LOADS

1609.1 Applications. Buildings, structures and parts thereof shall be designed to withstand the minimum wind loads prescribed herein. Decreases in wind loads shall not be made for the effect of shielding by other structures.

All exterior wall coverings and soffits shall be capable of resisting the design pressures specified for walls for components and cladding loads in accordance with Section 1609.1.1.