Combination Cases

Combination cases are similar to analysis cases but differ in two respects:

• Results for combination cases are inferred from analysis case results and not calculated explicitly.
• Combination cases can be enveloping cases, as described in Enveloping in GSA.

The syntax of combination cases is similar to that of analysis cases, except that they are based on analysis cases rather that load cases, thus

a1A1 + a2A2 + …

However, the syntax of combination case descriptions is extended to allow specification of enveloping cases.

As combination cases are linear combinations of analysis cases the results of a combination are only valid if all the analysis cases are linear. This means that combination cases should not, in general, be used with P-delta, non-linear analysis cases or when tie or strut elements are present in the model; in this circumstance the desired effect can be achieved by specifying a combination of load cases in an analysis case. In addition modal results represent mode shapes rather than a specific solution so combinations of modal results are unlikely to be helpful.