AIRL
AIRL, NRB,
RIGID_CALC
Specifies that automatic inertia relief calculations are to be performed.
NRBNumber of rigid body modes in the model:
AUTO
—
Activate automatic inertia relief (default). The program automatically determines the number of rigid body modes.
n—
Activate automatic inertia relief with the assumption of
nrigid body modes (1≤n≤ 6).0
—
Deactivate automatic inertia relief.
RIGID_CALCMethod for computing the rigid body modes:
0
—
Use an eigensolver to compute rigid body modes (default).
1
—
Use the geometry to compute rigid body modes (valid only when
NRB= AUTO). The geometric rigid body vectors are calculated according to Equation 15–171 in Participation Factors and Mode Coefficients in the Theory Reference.
Notes
The AIRL command activates automatic inertia relief for models having up to six rigid body modes. This method is only valid for linear static analyses (ANTYPE,STATIC).
By default (NRB = AUTO), the number of rigid body modes in the
structure is automatically determined. The static solution is altered so that the inertial
effects factor into counterbalancing the external loads. This method relies on calculation of the
rigid body modes using either an eigensolver (RIGID_CALC = 0, which is
the default) or the model geometry (RIGID_CALC = 1).
For the NRB = AUTO option, no supports should be defined.
For a model that is partially constrained by design, you must set
NRB to the number of rigid body modes present in the structure and
set RIGID_CALC = 0. The use of geometry
(RIGID_CALC = 1) to compute the rigid-body modes of a partially
constrained model is not supported.
Loads may be input as usual. Displacements and stresses are calculated as usual.
Use IRLIST to print inertia relief calculation results. The mass and moment of inertia summary printed before the solution is accurate (because of the additional pre-calculations required for inertia relief). See Inertia Relief in the Theory Reference for calculation details. See Including Inertia Relief Calculations in the Basic Analysis Guide for procedural details, including requirements and limitations.
When RIGID_CALC = 0, inertia relief
calculation results are available only when the node chosen by the algorithm as the virtual
reference point (mentioned in Linear Static Analysis Without Supports) has six
degrees of freedom.
This command is also valid in PREP7.
Example Usage
Example command input for including automatic inertia relief calculations in a linear static analysis is found in Including Inertia Relief Calculations in the Basic Analysis Guide.