C&C Wind Load Effective Area

[medeek]

As I'm going through the "Guide to the Wind Load Provisions of ASCE 7-10" by Mehta and Coulborne I noticed an inconsistency with the regard to the way the effective area for a roof panel is determined. In the one example the roof panel is a 4x8 plywood panel on 24" o/c spaced trusses with the eff. area given as 32 ft². On the other example the roof panel is 2x8 spanning across trusses that are 48" o/c with the eff. area given as the larger of 4 x 2 = 8ft² or (4*4)/3 = 5.3 ft².

If the 4x8 sheet of plywood is spanning the trusses with the long axis perpendicular to the trusses (case 1) shouldn't the span then be 2 ft. The effective area would be the larger of either 2x4 = 8 ft² or (2*2)/3 = 1.33 ft².

When dealing with windows, doors and roof panels do you consider the actual area of the component or consider some spanning distance and then compare this distance squared divided by 3 to the trib area? I guess what to use as the effective area for some components had got me a little confused.

 

[JAE]

Are the two examples designing the same "thing"? If the design is for the plywood itself, then perhaps the whole 32 in^2 is appropriate.

If you are checking the nailing, a smaller trib area.
If you are checking the rafter then another one as well.



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[Ron]

Agree with JAE...it depends on which part of the load application you are considering.

 

[medeek]

I'm often dealing with high wind locations and I like to check that the roof sheathing will be sufficient for out of plane loads (SDPWS 2008 Table 3.2.2). My concern is that I am calculating the correct wind force to compare against the allowables of this table. 90% of time the roof sheathing is either 7/16 OSB or 15/32 CDX APA rated plywood. The sheathing is typically installed with the strength axis perpendicular to the trusses or rafters. If I consider the span to be the 2 ft. between trusses then the effective area is only 8 ft^2 whereas if I consider the entire area of the panel then the effective area is 32 ft^2. The C&C wind loads are dependent on the eff. area and the difference between 10 ft^2 and 32 ft^2 is actually quite significant.

 

[DaveAtkins]

I think 32 SF is OK for checking the plywood and fasteners. A strict interpretation of the Code would result in designing just about any fastener on a typical project for the 10 SF C & C wind load--I just think this is too conservative.

For example, I do a lot of design of cold formed steel wall stud framing. I use the tributary area for an individual stud (usually H^2/3) to determine the C & C wind load for checking the stud AND connections. A typical slide clip connection which connects a stud to a steel superstructure will be fastened with 2, 3 or 4 fasteners. Should I check the slide clip for the 10 SF C & C load, since an individual fastener will have a tributary area less than 10 SF? Strictly speaking, yes, but I don't--too conservative.

DaveAtkins

 

[dcarr82775]

I don't think of the span length necessarily, but the member. I am not going to fail a piece of OSB on the 2ft span, an entire sheet will have to come into play, so I use the full 32 ft^2. If I have a 30ft long roof truss I wouldn't expect the truss designer to check C&C pressures on a piece of chord between panel points (I sure wouldn't do it either).

 

[medeek]

The consensus here seems to be that interpreting the code to rigidly is over conservative and hence some engineering judgement comes into play.

 

[medeek]

Here is what I get for the C&C wind loads based on the preceeding conversation:

C&C Wind Loads - Job 2014-017

Note that the wind forces on the roof panel in zone 3 (overhang) is way up there 88 psf (ASD), but I suppose this is realistic.

Based on this and the APA publication Q225G the allowable for L/240 for 7/16" OSB is only 51 psf. This would then require a thicker sheathing (ie. 15/32 Plywood) which gives a 81 ps allowable for the same spacing and deflection requirement. Note, that this allowable is still below the 88 psf however I think some judgement needs to come into play here otherwise this roof would cost a fortune to sheath.