Calculation of Roof Joist C&C Effective Wind Area per ASCE 7-10

[brockjd]

For a mono-slope, 2-span, continuous roof joist with an eave overhang at one end (7' span/15' span/3.5' overhang), what is the ASCE 7-10 Component & Cladding Effective area for the joist and the overhang?

With regard to the overhang, the closest examples I've found are contradictory. The example in Guide to the Wind Load Provisions of ASCE 7-10, with a 41.2' span between walls and 7.2' overhang, calculates the effective area for for based upon the 41.2' span. The example from a Wood Works seminar, with 20' span between walls and a 2' overhang, calculates the effective area for each based upon their respective individual lengths.

I've emailed ASCE for clarification, and at their usual glacial pace I may receive an answer in a year or so! I also contacted two wood truss manufacturer's to find out how they calculate the C&C loads for the continuous top chord of a truss with an eave overhang - their reply - "we use whatever the design program tells us to use.

Does anyone on this forum know the answer or where I can find it?

Thanks

 

[JAE]

brockjd,
One way to look at this is to keep in mind the concept of C&C wind loads. These are based on the concept that structural "things" that receive wind from smaller areas will tend to get higher maximum wind pressures due to the extreme variability of true pressures across a surface. So a single screw fastener of a wall panel may see very high pressures at random times due to that variability.
Conversely a 25 ft. horizontal wind girt will see the variations - some high peaks and some low - across a large area where the peaks and valleys of wind pressure will average out and result in smaller bending moments, shears, etc. on that girt.

For your case, the overhang bending moment results from the wind pressure only on the overhang. Thus, I would tend to use the 3.5 ft x joist spacing as your area for checking the bending of the cantilever.

For the remainder of the joists, I think you could look at the full length x spacing. Not sure I would go to the trouble of breaking a continuous joist into parts.

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[Mjkkb2]

What he said above