) = the internal pressure coefficient. For fully enclosed buildings (GC pi ) = 0.18, indicating that either positive or negative pressure can occur within the building. 20
The flat roof snow load (ASCE 7-10 S7.3): p f = 0.7C e C t I s p g where • “flat” = roof slope 5 o • C e = the exposure factor which depends on the terrain. • C t = the thermal factor which refers to the average temperature within the building. • I s = the snow importance factor • p g = the ground snow load The minimum roof snow load for low-slope roofs: • Where p g is 20 lb/ft 2 (0.96 kN/m 2 ) or less: p m = I s p g • Where p g exceeds 20 lb/ft 2 (0.96 kN/m 2 ): p m = I s (20) Snow Loads 21
Structural Design Allowable-stress design (ASD) • For ASD method, the elastic stress in the material must not exceed the allowable stress for each of various load combinations. • ASD method include both the material and load uncertainties into a single factor of safety. Strength design or LRFD (Load and resistance factor design) • Material uncertainty is separated from load uncertainty. • For example, to account for the uncertainty of loads, this method uses load factors applied to the combinations of loads. 22
(ASCE 7-10 S2.3) 1. 1.4D 2. 1.2D + 1.6L + 0.5(L r or S or R) 3. 1.2D + 1.6(L r or S or R) + (L or 0.5W) 4. 1.2D + 1.0W + L + 0.5(L r or S or R) 5. 1.2D + 1.0E + L + 0.2S 6. 0.9D + 1.0W 7. 0.9D + 1.0E See exceptions in ASCE 7-10 S2.3.2 where • D = dead loads; L = live loads • L r = roof live loads; S = snow loads • R = rain loads; W = wind loads • E = earthquake loads Load Combinations for Strength Design 23
(ASCE 7-10 S2.4) 1. D 2. D + L 3. D + (L r or S or R) 4. D + 0.75L + 0.75 (L r or S or R) 5. D + (0.6W or 0.7E) 6. (a) D + 0.75L + 0.75(0.6W) + 0.75 (L r or S or R) (b) D + 0.75L + 0.75(0.7E) + 0.75 S 7. 0.6D + 0.6W 8. 0.6D + 0.7E See exceptions in ASCE 7-10 S2.4.1 Load Combinations for Allowable Stress Design 24
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