SPECIFICATION FOR THE DESIGN OF - Transcon Steel

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82 Commentary on the Prescriptive Method for One and Two Family Dwellings - 2004 C3.1.2.1 Lateral Buckling Strength M n = S c F c Φ b = 0.90 (Eq. C3.1.2.1-1) By calculating the effective section modulus based on mid-span compression flange bracing, the resulting un-factored nominal moment is: M n = 12,921 in-lb (iterative calculation) C3.1.3 "Beams Having One Flange Through-Fastened to Deck or Sheathing" Does not apply. The factored moment is the smallest nominal moment calculated per AISI Specification Sections C3.1.1, C3.1.2, and C3.1.3 multiplied by the appropriate resistant factor, Φ. wL M u = Φ M n = 8 2 L L = 121.2” L = 10’-2” F1.5 Shear Capacity Φ M n = 8 b w Design shear strength = Φ v V = 744.13 lbs, L = 8(0.90)(12,921) 76 12 w = uniform load (plf) = 76 plf Use the design shear strength to calculate the maximum unsupported span length. wL 2V 2x744.13 V = L = L = = 19’-7” 2 w 76 F1.6 Deflection Limit Deflection due to total loads: δ = 5 4 wL (simply supported single span deflection equation) 384EI W = (5 psf + 20 psf)(24”/12”) = 50 plf (service load) 384(29,500,000)(1.4506) L = = 12'-5" 3 5(240)(50) 12 Deflection due to live load: W = (20 psf)(24”/12”) = 40 plf (service load) 384(29,500,000)(1.4506) L = = 11'-8" 3 5(360)(40) 12 The resulting joist span is 10’-2” (controlled by lateral buckling strength)
Commentary on the Prescriptive Method for One and Two Family Dwellings - 2004 83 F2 Rafter Design Determine if an unbraced 800S162-43 steel rafter (6:12 slope) is adequate for a 32-foot building subjected to 90 mph (Exposure Category B) wind speed and 30 psf ground snow load. Assume ceiling joist is 43 mil thick. F2.1 Design Assumptions Rafter Spacing = 24” Ceiling Joist Bearing Length = 3.5” <strong>Steel</strong> Yield Strength = 33 ksi Roof Dead Load = 7 psf Ceiling Dead Load = 5 psf Total Load Deflection Limit = L/180 F2.2 Design Methodology The rafter span table was designed based primarily on gravity loads, hence the rafter spans are reported on the horizontal projection of the rafter, regardless of the slope. The gravity loads consist of a 7 psf dead load and the greater of a minimum 16 psf live load or the applied roof snow load (applied snow loads are calculated by multiplying the ground snow load by 0.7 and no further reductions or increases are made for special cases). Wind load effects are correlated to equivalent snow loads wind pressures were calculated using the ASCE 7-98 components and cladding coefficients. Wind loads acting perpendicular to the plane of the rafter were adjusted to represent loads acting orthogonal to the horizontal projection of the rafter (as shown in Figure I4.1). Wind loads were examined for both uplift and downward loads and the worst case was correlated to a corresponding snow load. F2.3 Design Loads Dead Load = (7 psf)( cos 26.56°)(24”/12”) = 15.65 psf Roof Live Load is the maximum of Roof Snow Load and Roof Minimum Live Load: Roof Snow Load = 0.7(30 psf)(24”/12”) = 42 plf Roof Live Load = 16 psf (24”/12”) = 32 plf Both uplift and inward acting wind loads must be examined and the worst case converted to an equivalent snow load effect. ASCE 7 (ASCE, 1998) components and cladding pressure coefficients are used to calculate the wind load. ρ = q h [(GC p ) – (GC pi )] lb/ft 2 (Eq. 6-18) q h = 0.00256K z K zt K d V 2 I (Eq. 6-13) K z = 0.70 for exposure B (Table 6-5) K zt = 0.72 (maximum value used) (Figure 6-2) K d = 0.85 (Table 6-6) GC p = -1.4 (Tributary area = 100 ft 2 ) (Figure 6-5B) GC p = +0.3 (Tributary area = 100 ft 2 ) (Figure 6-5B) GC pi = ± 0.18 (Table 6-7) Downward load (inward): q = 0.00256(0.70)(0.72)(0.85)(90) 2 [0.3 – (-0.18)] = 4.26 psf Upward load (uplift or outward): q = 0.00256(0.70)(0.72)(0.85)(90) 2 [-1.4 - (0.18)] = -14 psf
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SPECIFICATION FOR THE DESIGN OF - Transcon Steel

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