The Occasional loads are primary loads and are treated as primary loads.

Stresses created by occasional loads are primary stresses.

How you combine these?

As you saw, the code require that the combination method is scalar.

Why?

I'm afraid that I will not give you a satisfactory answer, but I will try.

I was searching for this answer and the only reference that I found was that the occasional effects are treated similar to the AISC practice.


Regards,