SPECIFICATION FOR THE DESIGN OF - Transcon Steel

Commentary on the Prescriptive Method for One and Two Family Dwellings - 2004 89
slope and 8’ wall studs, subjected to 110 mph wind speed, Exposure Category C.
From Table C2.1 of Section 1, the following wind pressures are obtained for the given
wind speed, exposure and roof slope:
Roof pressure
Roof corner pressure
Main building pressure
Main building corner pressure
= 11.7 psf
= 14.6 psf
= 24.1 psf
= 30.3 psf
Calculate corner area width (2a), where “a” equals 10% of least width or 0.4h (whichever
is smaller) but not less than either 4% of least width or 3 feet.
a = 10%(40) = 4’
a = 0.4(30) = 12’
a = 3’
Use a = 4’
2a = 8’
Shear = (60’–8’)(8’/2)(24.1 psf)+8’(8’/2)(30.3 psf)+(60’–8’)(20’)(11.7 psf)+20’(8’)(14.6 psf)
= 20,486 lbs.
Roof diaphragm load =
(20,486 / 2)
40
= 256 plf
IBC Table 2306.3.1 (ICC, 2000a) provides recommended shear values for wood structural
shear panel diaphragms. For 7/16” OSB with 8d-nail spacing, the unblocked shear value
is 230 plf (multiplied by wood species reduction factor). The Light Gauge Steel
Engineering Association Technical Note No. 558b-1 (LGSEA, 1998) provides allowable
shear values for unblocked diaphragms. For 7/16” OSB with No. 8 screws spaced at 6
inches the unblocked shear value is 272 plf.
Using the LGSEA diaphragm value, the diaphragm in this example would be adequate.
F6 Roof Diaphragm Design (Second Example)
Check the adequacy of the roof diaphragm in Section F5 for a 30x60 ft two-story building.
Calculate corner area width (2a), where “a” equals 10% of least width or 0.4h (whichever
is smaller) but not less than either 4% of least width or 3 feet.
a = 10%(30) = 3’
a = 0.4(30) = 12’
a = 3’
Use a = 3’
2a = 6’
Shear = (60’– 6’)(8’/2)(24.1psf) + 6’(8’/2)(30.3psf) + (60’– 6’)(20’)(11.7psf) +
20’(6’)(14.6psf) = 20,321 lb
Roof diaphragm load =
(20,321/ 2)
= 339 plf > 272 plf (not acceptable)
30
The roof diaphragm must be blocked.