SPECIFICATION FOR THE DESIGN OF - Transcon Steel

58 Commentary on the Prescriptive Method for One and Two Family Dwellings - 2004
tabulated pressures (Table C2.1) are perpendicular to the vertical projection of the roof.
The uplift component of the pressure can be calculated as follows:
⎛ cos(36.86°
)
Wind Uplift Load =
⎟ ⎞
10 .3
⎜
= 13.73 psf (perpendicular to header)
⎝ sin(36.86°
) ⎠
Applying this wind pressure over a 21-foot member length, the uplift load is:
Wind Uplift Load = 13.73 psf x 21 ft = 288.3 lbs/ft
E5.2 Load Combinations:
1. 0.9D – 1.6W = 0.9(158) – 1.6(288.3) = -319 plf controls
2. 1.2D – 1.6W + 0.5(L r or S) = 1.2(158) – 1.6(288.3) + 0.5(490) = 26.7 plf
3. 1.2D – 0.8W + 1.6(L r or S) = 1.2(158) – 0.8(288.3) + 1.6(490) = 743 plf
E5.3 Member Properties
L 1 = 8.0”
L 2 = 1.5”
t = 0.0566”
S xtop = 0.8564 in 3
E5.4 Bending Capacity
M nu = R M ng
Long leg of angle
Short leg of angle
Design thickness
Section modulus for one angle
(Eq. B3.1.2-1)
where
M ng = gravity moment capacity determined by Eq. B3.1.1-1
R = uplift reduction factor
= 0.25 for L h /t ≤ 150
= 0.20 for L h /t ≥ 170
= use linear interpolation for 150 < L h /t < 170
L h = vertical leg dimension of the angle
t = design thickness
For LRFD, the design moment capacity for uplift shall be determined as follows:
M u = ΦM nu
Φ = 0.80
Eq. B3.1.3-4
M ng = (1.7128)(33,000) = 56,519 lb-in (for two angles)
L h /t = 8.0/0.0566 = 141.3 ≤ 150, Therefore, R = 0.25
M nu = 0.25(56,519) = 14,130 in-lb
2
wL
M
Φ M = L = 8Φ
8
w
8(0.8)(14,130)
L =
= 4’-10”
319
12