A flat roof is subject to the following uniformly distributed loads: a dead load of 21 psf (pounds per square foot of roof surface), a roof live load of 12 psf, a snow load of 13.5 psf, and a wind load of 22 psf upward. (Although the wind itself is in a horizontal direction, the force that it exerts on this roof is upward. It will be upward regardless of wind direction. The dead, live, and snow loads are gravity loads and act downward.). If load and resistance factor design is used, compute the factored load (required strength) in pounds per square foot. Which AISC load com­bination controls?There are 5 load combinations below:Combination 1: 1.4DCombination 2: 1.2D+1.6L+0.5(Lr or S or R)Combination 3: 1.2D+1.6(Lr or S or R) +(0.5L or 0.5W)Combination 4: 1.2D+1.0W+0.5(Lr or S or R)Combination 5: 0.9D+1.0WHow to deal with the wind load? If the wind load W=-22 psf, then the result of combination 5 will be -3.1 psf, which is negative.  And the tensile strength of materials and compressive strength is not the same. How to answer this question?

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Asked Sep 1, 2019
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  • A flat roof is subject to the following uniformly distributed loads: a dead load of 21 psf (pounds per square foot of roof surface), a roof live load of 12 psf, a snow load of 13.5 psf, and a wind load of 22 psf upward. (Although the wind itself is in a horizontal direction, the force that it exerts on this roof is upward. It will be upward regardless of wind direction. The dead, live, and snow loads are gravity loads and act downward.). If load and resistance factor design is used, compute the factored load (required strength) in pounds per square foot. Which AISC load com­bination controls?
  • There are 5 load combinations below:
  • Combination 1: 1.4D
    Combination 2: 1.2D+1.6L+0.5(Lr or S or R)
    Combination 3: 1.2D+1.6(Lr or S or R) +(0.5L or 0.5W)
    Combination 4: 1.2D+1.0W+0.5(Lr or S or R)
    Combination 5: 0.9D+1.0W
  • How to deal with the wind load? If the wind load W=-22 psf, then the result of combination 5 will be -3.1 psf, which is negative.  And the tensile strength of materials and compressive strength is not the same. How to answer this question?
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Expert Answer

Step 1

Given:     Dead load (D)=21psf

                Roof Live load (Lr)=12psf

                 Snow load(S)=13.5psf

    Wind load(W)=22psf (upwards)

Step 2

Calculate the loads by LRFD method:

 

For load Combination 1: P1 =1.4D

 Put D= 21psf

 

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P 1.4x21 29.4psf

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Step 3

 For load Combination 2: 1.2D+1.6L...

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P 1.2x21 1.6x0+0.5x 13.5 =31.95psf (Because S is larger than Ls, hnd R =0 ,evaluate P2 using S)

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