asce 7-93 wind

[haynewp]

What MWFRS roof coefficients should be used with an "open" gable type building according to asce 7-93? Code seems vague in this respect. There is a clear difference with SBC open condition roof coefficients as compared with enclosed or partially enclosed. I wouldn't think ASCE table 11 "other strucures" would apply either, since I have a leeward roof.

thanks

Will

 

[JAE]

Will:

I guess I would read Table 11 as applying to your condition. The term "monoslope" to me implies a constant slope, verses an arched condition. The coefficients from Table 11 would apply normal to each surface. The footnote 2 indicates a requirement to assume a variation in wind by varying your true roof slope by 10 degrees in either direction (up or down)and reading the resulting pressure coefficients out of the table.

I agree, its kind of vague, but this is how I would approach it. Anyone else have an opinion?

 

[haynewp]

I initially started using Table 11 but I noticed that the windward roof coefficient in my case (9.5 degree slope) would be about +0.9, and leeward -0.9. In comparison, SBC- 97 uses -0.7, -0.7 or -0.3, -0.8 for windward and leeward! That's when I thought that Table 11 must have been generated without a leeward face as to generate such a large positive windward coeff (ie. wind blows directly into and not over roof surface). Any other thoughts?
Thanks for responding JAE.

 

[JAE]

Will:
How about this -
With a 9.5 deg slope, you would use coefficients from the table for 0 degrees and for 19.5 degrees (based on the +/- 10 degree variance in wind horizontality) also, footnote 1 indicates that the forces are to be directed "inward or outward". To me, this would mean your coefficients would be either:
a) + (outward) for BOTH windward and leeward at the same time, or
b) - (inward) for BOTH windward and leeward.

This would then create two forces (see Table 4: F = qz x Gh x Cf x Af.) Af is the area of one monoslope roof (either windward or leeward). Assuming the wind perpendicular to the ridge:

One force would be a windward force applied to the windward slope a distance of X from the windward edge. It would be normal to the windward roof (actually directed upward and somewhat against the wind direction).

The second force would be a leeward force applied to the leeward slope a distance of X from the ridge line (which is the windward edge of the leeward monoslope roof). It also is normal to the leeward roof and is directed upward and somewhat with the wind direction).

The above is how I would apply ASCE 7 to an open structure with a dual pitch roof. I noted that ASCE 7-98 (the latest) still uses the same table (Now called Table 6-9).

Does this make sense?

 

[haynewp]

Yes, thanks again JAE.

 

[Ron]

JAE/Haynewp...
I have used similar approach to JAE's last description for applications of open canopies (walkway covers)with gable roofs when using 7-88 or 7-93. In 7-95, you can use Fig. 6.5b as a variant to this (factors will be a bit higher) since for small roof area, Zone 2 will likely prevail. Fig. 6.5b is referenced from the notes of other factor tables when they don't know what else to do with it!

Ron