Analysis cases

Analysis cases are one of the basic concepts that needs to be understood for anything other than a linear static analysis. It is important to understand the difference between a load case and analysis case. This is best achieved by using an example.

Consider setting up a model with two load cases, one for dead load and one for live load. It can be useful to keep these in separate load cases so let us assume the dead load is load case 1 and the live load is load case 2. If we want to look at the deflections and forces from both of these loads, there are two approaches: either carry out an analysis with the load in load case 1 and an analysis with the loads in load case 2 and then combine the results post-analysis, or carry out an analysis with the loads in both load cases 1 and 2. Provided the analysis is linear these will give the same results but for a nonlinear analysis they will be different. Using the prefix L for load case and A for analysis case we can summarise the different approaches. In the first case we construct two analysis cases one corresponding to each load case:

Analysis case	Load case
A1	L1
A2	L2

and in the second, one analysis case corresponding to the combined loading in cases 1 and 2

Analysis case	Load cases
A1	L1 + L2

The description of the analysis case for static loading has the general form

a1L1 + a2L2 + …

The concept of analysis cases becomes more useful when applied to modal dynamic analysis where the results relate to mode shapes of the structure and not to loading. Thus for a modal analysis we may have ten analysis cases corresponding to modes 1 to 10 and these have nothing to do with loading.

Analysis cases cannot exist in isolation but are defined with respect to analysis tasks.

The types of analysis case are:

Load: Cases that relate to static loading e.g. 1.4L1 + 1.2L2
Mode: Modal results from dynamic or buckling analysis e.g. M1
Response: Response spectrum results e.g. R1
Envelope: e.g. C1 or C2