Dear Mr. Loren Brown,
We are facing a client where in their spec required to make sure that Displacement for SUS+OCC case not more than 20 mm in Horizontal and Vertical Direction.

I try to explain to client that we might not want to report the Displacement for such Combinatiojn Load Case, as the result for displacement can't be guarantee true, but yes for Stress because when combine a load case using scalar or Absolute method CAESAR II wil take the stress form each basic load case to calculate the displacement, force and moment.

I have convinced them, but looks like they want more detail explanation because they still questioning if using Scalar or Absolute Method for this combination case CAESAR II gives the correct result for Stress, how come the result for displacement can be uncorrect since this displacement are calculated from the stress itself.

So this is typically is software issue, or the way CAESAR II calculated combined load case and its result. Can you explain a little bit why CAESAR II can gives uncorrect result for displacement that calculated from combine load case, that as we know this displacements are calculated from the stress that are correct in the first place.
Why Algebraic Method can give correct result for stress, force, and restraint load that calculated from the displacement at the first place while Scalar/Absolute method not?

Regards

You have to be careful with combination load cases, depending on the components (what base cases are combined), whether or not the system is non-linear, and the combination method empolyed (scalar, algebraic, absolute) - some results may not be applicable.

If you want to ensure that the combined displacement doesn't exceed 20 mm, why not use an OPE case which includes all SUS and OCC loads (also thermal loads) instead?

(Displacements are not computed from stresses, rather it is the other way around. The FEA system is solved for the displacements. From the displacements the element's forces and moments are determined. From the forces and moments, the Code equations are applied to compute the stresses. To see the differing effects of the combination methods {scalar, versus algebraic, versus absolute}, see the help text in the Static Load Case Editor - there are numeric examples there.)