I/O Options and Subcase Information Entry The RIGID command can be used in the I/O Options section to select the constraint processing method for RBAR, RROD, RBE1, RBE2, RBE3, RSPLINE, RSSCON, RBODY and MPC entries.




Argument Options Description
Note: If the RIGID entry is not specified in the model, AUTO is the default method for all supported cases. 8 9
Activates the Constraint Elimination Method. 3
Activates the Lagrange Multiplier Method.
AUTO (Default)
Automatic selection of LGELIM versus LAGRAN. 8


  1. The RIGID entry should be specified above the first subcase.
  2. The RIGID entry can be used to select the constraint processing method for Static, Normal Modes and Buckling Analysis.
  3. The LGELIM method activates constraint processing via the elimination of the dependent degrees of freedom from the equations of the unconstrained system. Traditionally, in OptiStruct, constraints are enforced by augmentation of the system with additional variables (Lagrange Multipliers) using the LAGRAN method. This leads to indefinite systems that in rare scenarios, may cause difficulties to the solver. On the other hand, the elimination of the dependent degrees of freedom usually leads to positive definite systems with better solution properties in terms of stability and often speed.
  4. This option can be used together with the SOLVTYP Entry to select different types of direct solvers with either the Lagrange Multiplier Method (LAGRAN) or Constraint Elimination Method (LGELIM).
  5. In linear static analysis, if PCG is used as the solver, the Elimination method is activated by default and the Lagrange Multiplier Method is not available.
  6. For nonlinear static analysis, the LAGRAN method is the default, while the LGELIM method is not recommended.
  7. For Normal Modes Analysis, the constrain elimination method (LGELIM) is activated by default, if AMLS/AMSES is picked as the eigenvalue solver.
  8. If the RIGID card is missing or AUTO is specified, OptiStruct will select the optimal algorithm to treat constraints on a case by case basis, depending on whether constraint reduction or augmentation is supported and which method is likely to have better performance.
  9. Here, the possible ways to treat constraints available to the user is summarized:
    Constraint Method Linear Static Nonlinear Static Normal Modes Buckling
      /BCS/MUMPS/PARDISO PCG Small Displacement Large Displacement Lanczos AMES/AMLS  
    LGELIM Available Available Available Available Available Available Available
    LAGRAN Available -- Available Available Available -- Available