Sirs,

I am new to stress and i want to know what is the reason to add oc load to opr load case and sustract is back form opr case and than we add scallar in sustain,

why cant we directly add it to sustain case?

plase guide.
ie
L1=W+P
L2=W+P+T
L3=L2-L1 (EXP)
L4=W+P+T+U (OPR)
L5=L4-L2 (PURE OCC)
L6=L1+L5 (OCC)

WHY CANT WE ADD DIRECTLY
W+P+U ??????

This combination (ope+occ)is to obtain loading on equipment nozzle's and on restraints in normal operating condition.In Load case option, COMB METHOD-Absolute value of forces.

//why cant we directly add it to sustain case?//

you can directly add to the sustain case, but some time will get convergence problem.

Sir,
can you explain convergence.

You can only add occasional loads to the Sustained case if the system is linear (from a boundary condition) point of view. If the system is non-linear, you want the occasional stresses (in the operating condition) added to the sustained stresses.

For a non-linear system, the only way to obtain the occasional stresses (in the operating condition) is to subtract two OPE cases (one with the OCC load and one without).

This has nothing to do with convergence. If you want to learn more about "convergence", use the [Search].