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ACI 318 Load Combinations

ACI 318 Load Combinations

I have some doubts concerning load combining in ACI.

From ACI318 and its comment it is not quite clear are the following combinations possible:

0.9 x D + 1.7 x L
(when dead load reduces effects of live load)

1.4 x D + 1.7 x L1 + 0.0 x L2
(it seems reasnoble that if there are several live loads, some of them could have favourable character)

0.75 x (1.4 x D + 1.7 x W)
(combination with wind load, but with zero live load)

0.9 x D + ??? x L + 1.3 x W
(dead load has a favourable character so it is taken with reduced factor. Why is wind factor 1.3 and not 1.28. Can live load "join" this combination and with which factor ???).

0.9 x D + 1.4 x F + 1.7 x L
(F - fluid pressure)

Could fluid load (F), inpact load (I), effects from differential settlement (T) be taken into seismic combination (wind load couldn't)? If could, what will be the right treatment for fluid load or T - load?

I also checked SAP2000 manual. It considers only small part of all possible combinations. Why is that?

thanks in advance

RE: ACI 318 Load Combinations

I'll try to answer a couple of these:

0.9 x D + ??? x L + 1.3 x W

If the live load has a favorable character, it should not be included. If the live load has an unfavorable character, I would use a load factor of 1.3. (or 1.28 if you prefer.)

The 1.3 factor is just a simplification of 0.75*1.7. (Remember - these combinations preceded the widespread use of computers.)

0.9 x D + 1.4 x F + 1.7 x L

The code would not require that fluid load or differential settlement be considered simultaneously with seismic effects. Generally, I would feel like the probability of all those effects occurring simultaneously to be extremely low, and not worth considering. In your particular structure, however, you may feel that there is a real chance of them occurring concurrently. If so, you should develop your own load case.

John M. Hochwalt, PE

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