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Reference Guide
This manual provides a detailed list and usage information regarding input entries, output entries, and parameters available in OptiStruct.
Legacy Data
Previously Supported Input
A list of previously supported input keywords.
PARAM, GAPOFFST
Bulk Data Entry

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What's New
View new features for OptiStruct 2025.1.
Overview
OptiStruct is a proven, modern structural solver with comprehensive, accurate and scalable solutions for linear and nonlinear analyses across statics and dynamics, vibrations, acoustics, fatigue, heat transfer, and multiphysics disciplines.
Tutorials
Discover OptiStruct functionality with interactive tutorials.
User Guide
This manual provides detailed information regarding the features, functionality, and simulation methods available in OptiStruct.
Reference Guide
This manual provides a detailed list and usage information regarding input entries, output entries, and parameters available in OptiStruct.
- Input Data
- Output Data
- Legacy Data
  - Previous (OS3.5) Input Format
    OptiStruct will continue to support the old input format from version 3.5. The new optimization capabilities will not be available if the old format is used for the set up of the optimization problem, that is, if matfrac, mini, maxi, ubcon or lbcon are present. This section is intended to be used for the purpose of debugging or re-running older decks.
  - Deck Setup in OptiStruct 5.0 and higher with OptiStruct 3.5 Objectives and Constraints
    Setting up an optimization was simpler in OptiStruct 3.5, but it was also very limited. Starting with OptiStruct 5.0, a lot more flexibility has been added to the way objectives and constraints are set up, but the problem setup is more complex.
  - Previously Supported Input
    A list of previously supported input keywords.
    - DENSRES
      I/O Options EntryThe DENSRES command can be used in the I/O Options section to control the frequency of output of design results (density, shape, or thickness).
    - FORMAT
      I/O Options EntryThe FORMAT command can be used in the I/O Options section to indicate the format in which results are to be output.
    - GPFOS
      I/O Options and Subcase Information Entry Used to request fatigue factor of safety, hydrostatic pressure, and microscopic shear stress output for grid-based FATDEF for all subcases or individual subcases, respectively.
    - RESULTS
      I/O Options EntryThe RESULTS command can be used in the I/O Options section to determine the frequency of output of analysis results for all subcases or for individual subcases, respectively.
    - SET/PSET
      I/O Options and Subcase Information Entry The SET and PSET commands can be used in the I/O Options or Subcase Information section input deck to define sets of grids, elements, properties, or frequencies.
    - SHRES
      I/O Options EntryThe SHRES command can be used in the I/O Options section to control the frequency of output of the state files (.#.sh file and the .#.grid file).
    - BEAD
      Bulk Data Entry Defines parameters for the generation of topography design variables.
    - FLSPOUT
      I/O Options and Subcase Information Entry The FLSPOUT command can be used in the I/O Options section to control output of modal participation factors for coupled fluid-structure models.
    - DOPTPRM, LATLB
      Bulk Data Entry This parameter is used to control the Automatic Reanalysis process at the end of Phase 1 in Lattice Optimization.
    - DOPTPRM, LATTICE
      Bulk Data Entry For Lattice Optimization, the application of the Stress NORM method for stress constraint calculation, the beam cleaning process, and the application of Euler Buckling constraints in the second phase are controlled by this parameter.
    - DOPTPRM, MINMETH
      Bulk Data Entry This parameter specifies the method of minimum member size control.
    - DOPTPRM, POROSITY
      Bulk Data Entry Controls the amount of intermediate densities in the model in the first phase of the Lattice Structure Optimization run.
    - INFILE
      I/O Options EntryThe INFILE command is used in the I/O Options section to identify the file containing the Bulk Data Entries.
    - MASSTOH3D
      I/O Options EntryThe MASSTOH3D command can be used in the I/O Options section to request the output of the mass matrix to the .h3d file.
    - MODELMPC
      Subcase Information Entry The MODELMPC command can be used in the Subcase Information section while including h3d DMIG in residual runs. This card converts interior DOFs of the DMIG to exterior DOFs.
    - PARAM, AMLSASPC
      Bulk Data Entry This parameter is used to indicate when to automatically constrain degrees-of-freedom with no stiffness for AMLS run.
    - PARAM, CMFTINIT
      Bulk Data Entry Defines lower threshold for *.HM.comp.cmf and *.HM.ent.cmf HyperMesh command files.
    - PARAM, CMFTSTEP
      Bulk Data Entry Defines step or interval value for *.HM.comp.cmf and *.HM.ent.cmf HyperMesh command.
    - PARAM, GAPOFFST
      Bulk Data Entry
    - PARAM, I64SLV
      Bulk Data Entry Enforces the activation of internal long (64-bit) integer sparse direct solver.
    - PARAM, INRGAP
      Bulk Data Entry PARAM, INRGAP can be used during nonlinear gap analysis to improve the performance for some models.
    - PARAM, OSTTSPRT
      Bulk Data Entry One-Step Transient Thermal Stress Analysis solution and convergence information is output in the .out file. Nonlinear OSTTS is performed by generating internal static subcases at each output time step.
    - PARAM, PRESUBNL
      Bulk Data Entry Forces OptiStruct to run models in which Linear Buckling Analysis or Preloaded Analysis is defined, in conjunction with nonlinear materials (MATS1, MATHE, or MGASK) or Large Displacement Nonlinear Analysis.
    - PARAM, PRTRENUM
      Bulk Data Entry
    - PARAM, SH4NRP
      Bulk Data Entry This parameter controls the full projection of 4-node shell elements in NLGEOM implicit analysis.
    - ROTATION
      I/O Options EntryThe ROTATION command can be used in the I/O Options or Subcase Information sections to request the output of rotation information for all subcases or individual subcases, respectively.
    - UPDATE
      I/O Options and Subcase Information Entry The UPDATE command controls the behavior of ASSIGN,UPDATE,<filename>.
Example Guide
The OptiStruct Example Guide is a collection of solved examples for various solution sequences and optimization types and provides you with examples of the real-world applications and capabilities of OptiStruct.
Verification Problems
This manual presents solved verification models including NAFEMS problems.
Frequently Asked Questions
This section provides quick responses to typical and frequently asked questions regarding OptiStruct.

PARAM, GAPOFFST

Bulk Data Entry


Parameter	Values	Description
`GAPOFFST`	YES, NO	YES (Default) Frictional offset for nonlinear gap analysis is active. NO Frictional offset for nonlinear gap analysis in inactive. For more details, refer to the PGAP description.