One Step Submodeling

The field SUBID specifies the identification number of the subcase that contains the global structure definition (via SUBMODEL) and the field SID specifies the set of grid points in the local structure that defines the displacement cut surface. The displacements from the global structure are applied to this set of grid points.

An example input file for Global-Local Analysis is:

$ Subcase Information Section 
SUBCASE 1
$ Submodel specific SPCs and LOADs can be defined here.
SUBMODEL, 11
SUBCASE 2
$ Submodel specific SPCs and LOADs can be defined here.
SUBMODEL, 12
GLOBSUB, 1, 15
BEGIN BULK
SET, 11, ELEM     $ defines the global structure, for example, the full building 
		  $ in Figure 2 
SET, 12, ELEM     $ defines the local structure, for example, the pillar-roof 
		  $ joint in Figure 2
SET, 15, GRID     $ defines the cut surface, for example, the interface grids of 
		  $ the pillar-roof joint in Figure 2 at which the displacements are                  $ interpolated. 
. . .

The save/restart functionality of the one step submodeling allows you to save results of one or more global subcases. Additionally, you may choose to modify the input deck of local subcases and/or add more local subcases to be solved, and rerun the model without solving the global subcase multiple times. The global subcase results can be recovered from the previously saved restart file. This is only supported for Linear Static Analysis.

Insert the following entry in the control section of the subcase to be saved:

ASSIGN, SAVEDISP, <Filename>

Insert the following entry in the control section of the subcase to be recovered:

ASSIGN, GETDISP, <Filename>

1. Solution transfer between local and global substructures uses 2D and 3D elements in the global model to interpolate the displacements and rotations at each GRID location belonging to the cut surface set SID. It is recommended that there are no other elements (beams, rods, or rigids) near the cut surface in the global structure as the solution of these elements will not affect the local model. The cut surface should be selected such that in the global structure there are no GRID’s which are connected only to 1D or rigid elements.
2. The solution in the elements, near the cut surface in the local model, may lead to some noise resulting from interpolation from the global substructure and should be discarded from the results. The actual zone of interest (for example, stress concentration) should be separated from the cut surface grids by a minimum of two layers of elements in the local model.
3. The cut surface should consist only of grids on the cuts (sections) through the structure. Usually, free edges/surfaces without any loading or support conditions should not be part of the cut surface.
4. There is no requirement that the cut surface grids exist only on the boundary of the local model. For shell submodels, it may be beneficial to include all grids in the outer layer of elements, as it will improve the transfer of rotations between subcases.
5. When certain loading and/or support conditions of the global submodel remain in the local submodel, it is recommended to represent them as loads that are consistent with the local model. For instance, point loading of the global shell structure should be replaced with the equivalent of distributed loading on the local solid substructure. Alternatively, users could decide not to apply the loading/support conditions to the local submodel. In such cases, the grids in the originally loaded/supported area(s) should be included in the cut surface so that the displacements from the global submodel are interpolated and applied to the local structure. However, this technique may significantly reduce accuracy of the local solution.
6. The cut surface may represent single or multiple cuts (sections) through the structure. Multiple cuts should not be closer than the element size of the global model.
7. For each grid in the cut surface, OptiStruct selects the nearest elements (shell or solid elements from the global model, based on the center location of each element) and uses a solution from all the grids of these elements to interpolate to the cut surface grid location. When the cut surface contains two disconnected substructures, for example, two almost touching shells, care should be taken that this interpolation will not contain nodes from the other substructure. In such cases, PARAM,GLOBEXPT,0 can be used to reduce the number of elements used for interpolation (it will use only grids located close to centers of the elements in the global model).
8. The GLOBSUB entry should always reference the subcase ID of a global subcase that is defined above its corresponding local subcase.
9. This functionality is currently only available for Linear Static Analysis. All optimization types with responses from Linear Static Analysis are supported.
   • SPCFORCE/residual force responses are not supported.
   • Topology must lie outside the local part(s) and any design variables affecting the local submodel should also be mapped to the global model.