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[ramu_se]

Dear All,

In the case of load combinations, we add a limit state of collapse load combinations as well as a limit state of serviceability load combinations and also we create envelope load combinations for the limit state of collapse & limit state of serviceability in any of the software like Etabs or Staad.

While interpreting the results, like deflections, lateral sway can use envelope load combinations or do we need to select the load combinations individually?

These envelope load combinations can be used for the design of the structural elements?

Kindly provide your guidance.

Thanks & Regards,
Ramu.

[s.bhushanraj]

Envelope gives you the maximum and minimum of the forces or displacements for a group of load combinations. You can group all collapse or serviceability combinations together and find the envelope.

If you are just interested in one of the quantities, say max. positive bending moment, you can find it in the envelope. But, say if you are designing a column for biaxial moments (Mxx, Myy) and axial force (P), you need to take the corresponding forces in the combination and element where you got the max value. So there will be three cases here,
1. Max. Mxx, corresponding Myy, and P,
2. Max. Myy, corresponding Mxx, and P,
3. Max. P, corresponding Mxx and Myy

But it is not necessary that these 3 sets only control design. There could be some load combinations, where the P, Mxx and Myy are not maximum. But they are all higher enough to govern your column design due to their interaction. So, if you do not want to miss any of these, you need to consider all load combinations. Envelopes are very useful for certain cases of design and has to be used with caution. I have seen people designing for Max P, Max Myy, Max Mxx. But this never occurs in reality, therefore resulting in a highly uneconomical design.

The column example I gave is just one case where there is a high chance of misuse of envelopes. Any part of design that involves interaction of two or more force/displacement components are potential candidates for this misuse of envelopes. Whenever you are interested in only 1 force/displacement quantity you can use envelopes for quick results extraction.

[rahul.leslie]

Sri. Bhushanraj has pointed out a very important aspect of load envelope... on a simpler note, i suppose,  the attached picture gives a simple view on the gist of load envelopes.