RIGID

I/O Options Entry

RIGID – Constraint Processing Method

Description

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, and MPC entries.

Format

RIGID = option

Argument

Options

Value

Description

option

<LGELIM, LAGRAN>

No default

Note:

If the RIGID entry is not specified in the model, LAGRAN is the default method.

LGELIM

Activates the Constraint Elimination Method. See comments 3 and 4.

LAGRAN

Activates the Lagrange Multiplier Method.

Comments

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 and Modal Frequency Response and Modal Transient analysis.

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 superior solution properties with regard to speed and stability.

For static analysis, 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). If PCG is used as the solver, the Elimination method is activated by default and the Lagrange Multiplier Method is not available. The RIGID entry is not supported for Geometric Nonlinear Static Analysis. The LAGRAN method is always used for Nonlinear Static Analysis.

For Dynamic Analysis, constraint elimination method (LGELIM) is activated by default if AMLS/AMSES is picked as the eigenvalue solver. The Lagrange Multiplier Method (LAGRAN) is not available for AMLS/AMSES. For Lanczos, the constraint elimination method is not active by default and can be enforced by using RIGID=LGELIM.