Subcase Specific Modeling

Subcase specific modeling allows analyzing multiple structures in a single solver run.

The structures can be completely independent; can represent different regions of the same model, or different assemblies sharing common parts. In a traditional modeling scenario, the entire model will be solved for each solver run, regardless of the boundary conditions and multiple models will have to be created to solve structures with variable sections. This method allows specific sections of the model to be solved independently without affecting the rest of the structure.


There are various advantages to subcase specific modeling for both analysis and optimization. The primary motivation for subcase specific modeling is the ability to solve independent structures with common parts. For example, in the case of a pickup truck, various cabin shapes can be solved for, by allowing the bed and the chassis to remain unchanged. This is accomplished by assigning elements belonging to different cabin types to different sets. The constant chassis and bed can be a defined as another element set. These element sets can now be combined to allow various structural subcase specific modeling to be modeled without having to repeat the common parts of the structure for each solution.

Figure 1. Application of Subcase Specific Modeling

In Figure 1, each cabin body can be defined as a specific element set and the common parts will make up one element set. These can be independently combined under three different subcases with different boundary conditions and solved in one single solver run.


The subcase specific modeling functionality is realized with the use of element sets and the subcase selector entry SUBMODEL. The SUBMODEL entry can be used within a specific subcase in the Subcase Information section to select a certain element set for the solution.

SUBMODEL Subcase Information Entry

The full model may consist of several common or shared sections which remain constant while other sections are changed to find the best fit for a particular application. In such cases, the SUBMODEL entry can be used to choose a set of elements for selective solution. The element set identification number is the only input required for this entry.


The fields "SID" and "SID_r" specify the identification numbers of the element and rigid element sets, respectively defining the subcase specific model that is solved within this subcase. The "SID_r" field is optional. Refer to the OptiStruct Reference Guide for detailed information.

Subcase Specific Modeling - Example OptiStruct Input Deck

$ Subcase Information Section


$ Model specific SPC's and LOAD's can be defined here.



$ Model specific SPC's and LOAD's can be defined here.


$ Bulk Data Section

SET, 1, ELEM $ defines the shared/common part, for example, the chassis, bed and wheels in Figure 1

SET, 2, ELEM $ defines the individual part, for example, cabin body 1 in Figure 1

SET, 3, ELEM $ defines the individual part, for example, cabin body 2 in Figure 1

SET, 11, ELEM, OR, 1, 2 $ defines the full truck model with cabin body 1 and the common parts

SET, 12, ELEM, OR, 1, 3 $ defines the full truck model with cabin body 2 and the common parts


  1. Single Point Constraints (SPC), Loads (LOAD), Multi Point Constraints (MPC) and other similar subcase selectors should define attributes only applicable to the specific submodel or substructure. These attributes should apply exclusively to the subcase-specific model defined via SUBMODEL. The SUBMODEL entry does not trim the specified attributes (loads, constraints and so on) to the defined subcase.
  2. This functionality is currently available for linear static analysis only. All optimization types with responses from Linear Static analysis are supported (except SPCFORCE/residual force responses).
  3. There is no restriction on the use of GRID's. If two submodels have a GRID at the same location, you can use the shared GRID entry or two separate GRID entries.
  4. For subcase specific configuration, definition of all rigids and CONNECT entries are processed with INSTANCE-based relocations.