4)Sketch an isometric diagram of the beam shown to the right with the annotated

Question

4)Sketch an isometric diagram of the beam shown to the right with the annotated loads for each of the following load combinations

for strength design (LRFD) from ASCE7-10: 2, 3, 4, and 6. Assume that the unit live load is less than 100 psf and that the beam supports either a garage nor place of public assembly

Dead Load = 580plf

Live Load 1 = 400 plf

Live Load 2 = 600 plf

Distributed Snow Load = 620 plf

Concentrated Snow Load = 6kips

Wind Load = 240 plf

Would you expect to get a more economical

design using ASD, LRFD, or would they both

be the same?

Explain

Explanation / Answer

As per ASCE 7-10 the load combinations are given by -

2) 1.2 (D) + 1.6 (L) + 0.5 (Lr or S or R)

3) 1.2 (D) + 1.6 (Lr or S or R) + (L or 0.5W)

4) 1.2 (D) + 1.0 (W) + L + 0.5 (Lr or S or R)

6) 0.9 (D) + 1.0 (W)

Here D= Dead Load, W = Wind Load, L = Live load, Lr = roof live load, S = snow load, R = rain load.

As per ASCE code the live load for garage = 40 psf

As ASD the load combinations will be -

2) D + L

3) D + (Lr or S or R)

4) D +0.75(L) + 0.75 (Lr or S or R)

6) D +0.75(0.6W) +0.75 (L) + 0.75 (Lr or S or R)

As per the above load combinations the load calculated using LRFD will be higher compared to ASD load combinations so the moment calculated will be high from LRFD and hence the section will be at higher side compared to ASD method.

So by ASD we will get more economical section

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