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TR.37.8 Geometric Nonlinear Analysis

Note: This requires a STAAD.Pro Advanced license.

General Format


The first analysis step must be stable, otherwise use ARC control to prevent instability. The procedure does not use follower loads. Loads are evaluated at the joints before the first step; then those loads translate with the joint but do not rotate with the joint. Equilibrium is computed in the displaced position.

The following table describes the parameters available for nonlinear analysis:

Table 1. Geometric Nonlinear Analysis parameters
Parameter Value Default Value Description
ARC f1 0.0

Displacement control. Value is the absolute displacement limit for the first analysis step. If max. displacement is greater than this limit, ARC will calculate a new step size for the first step and a new value for STEPS. Value should be in current length units.

ARC = 0 indicates no displacement control.

ITERATION i1 100 Max. Number of iterations to achieve equilibrium in the deformed position to the tolerance specified.
TOLERANCE f2 0.0001 inch For convergence, two successive iteration results must have all displacements the same within this tolerance. Value entered is in current units.
STEPS i2 1 Number of load steps. Load is applied in stages if entered. One means that all of the load is applied in the first step.
REBUILD i3 1 Frequency of rebuilds of the tangent K matrix per load step & iteration.
KG i4 0 This parameter controls whether the geometric stiffness, KG, is added to the stiffness matrix, K.
  • KG or KG 1 Use K+KG for the stiffness matrix. (Default)
  • KG 0 (or KG omitted) Do not use KG.
JOINT_TARGET i5 none Joint being monitored in a displacement target analysis.
i6 1 Global degree of freedom (1 through 6):
  1. Global X
  2. Global Y
  3. Global Z
  4. Moment about Global X
  5. Moment about Global Y
  6. Moment about Global Z
DISPL_TARGET f3 none Displacement target value in current length units.
print-spec none

Standard STAAD analysis print options.

See TR.37.1 Linear Elastic Analysis for details.

If a target displacement is used, then both the JOINT_TARGET and DISPL_TARGET must be defined. If the degree of freedom is not specified, then the Global X direction is assumed.

If Joint target and Displacement target are entered and the STEPS parameter is greater than two; then the analysis will proceed step by step until the targeted joint degree of freedom has displaced DISPL_TARGET length units or more.

Nonlinear entities such as tension/compression members, multilinear springs, gaps, etc. are not supported when using a nonlinear analysis. Additionally, nonlinear analysis does not account for post-buckling stiffness of members.

CAUTION: The deprecated NONLINEAR n ANALYSIS command will adopt the new procedure (unless a SET NONLINEAR OLD command is entered before the joint coordinates to invoke the old procedure for backward compatibility). This uses n iterations. It is strongly recommended to use the new procedure.
Note: Due to the mechanisms used to include control/dependent systems, if the reactions on control nodes are not included in a statics check then an out of balance report may result. This can be avoided by adding a short stiff member from a control node to the support.

Temperature loads are not supported for geometric nonlinear analysis.


REBUILD 1 KG joint 11 2 DISPL_TARGET 0.5