ACI 318 (Load Combinations) 1

[Struct-007]

Clause 5.3.2 of ACI 318-14 states that: "the effect of one or more loads not acting simultaneously must be investigated".

What does that mean? Does this mean we need to investigate Dead Load Alone, Live Load Alone, EQ Load Alone and Wind Load Alone? I thought we need to use the Load Combinations in Section 5.3.1 of ACI 318-14.

Please help me understand the meaning of this Clause.

Thanks.

 

[r13]

I think it is meant to consider pattern load, or uneven live load, that might lead to critical effects, such as stress reversal.

 

[Struct-007]

Thank you. Yes, it can be explained this way but still not clear for me.

Like for example, in the following load combo:
1.2D + 1.0E + 1.0L + 0.2S
Do we need to derive the following load combos from it (considering that dead load must always be applied)?
1.2D + 1.0E
1.2D + 1.0E + 1.0L
1.2D + 1.0E + 0.2S
1.2D + 1.0L
1.2D + 1.0L + 0.2S
1.2D + 0.2S

 

[r13]

No. You shall use these load combinations, but add load cases for the live load.

For instance:
1) 1.2D + 1.0E + 1.0L + 0.2S (L - all spans loaded); 1a) 1.2D + 1.0E + 1.0L + 0.2S (L - alternate span loaded only); 1b) 1.2D + 1.0E + 1.0L + 0.2S (L - two adjacent spans loaded only).... so on so forth. Note the basic combination remains the same, but the different loading conditions of the live load (pattern load). You can see, depending on the number of spans, the number of cases to be investigated will be more or less than presented here. The logic behind this investigation is to capture the worst possible live load condition for the frame members, otherwise might not be caught if only the fully loaded condition is considered in the analysis. Once you are gaining sufficient experience, you maybe able to pin point the worst cases for certain members by a quick inspection on your model, rather than run all cases.

 

[Struct-007]

Thank you for your comprehensive answer, I appreciate it!

So, it seems Clause 5.3.2 of ACI 318-14 is applicable to live loads (and to some extent snow load) as we have a choice on where to place it. I don't believe the same can apply to dead load, seismic load, wind load, and other environmental loads.

 

[r13]

Exactly. Any load that could be non-continuous at times. Judgement is required when altering uniform snow load, as the unbalanced condition is already covered by code, such as accumulation over parapet and roof obstructions. Alternate span load pattern can be both an unnecessary over kill, and unrealistic.
If equipment is present, you have ask yourself what might cause "if it is not there", unless it is rigidly build into the structure, which is rare.

 

[JoshPlumSE]

It's usually meant for live load, storage loads and such. I've done it for roof live load too. Not so much for rain or snow. That's handled more through ponding and snow drift concepts. But, you're free to do it for them too. I just don't see it as being directly applicable.

 

[MSUK90]

r13, JoshPlumSE, If its a simple frame or system, applying patterned loads seems comfortable but how do you incorporate patterned loading for uneven spans and columns randomly placed in framing(I generally deal with projects on flat slabs so questioning considering that in mind).