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Nonlinear static analysis : Modelling implications

Cases​

The principle of superposition is not valid for nonlinear behaviour so unlike linear static analysis, results from different analyses must not be added together. An analysis case can be defined as the summation of factored load cases. This total loading is then analysed. Combination Cases formed from previously analysed Gss, nonlinear or GsRelax results will probably contain invalid results. Combination Cases should only be used for producing envelopes in these cases.

Dynamic Relaxation: Modelling implications​

Limitations​

The following modelling options are not allowed when analysing with the dynamic relaxation nonlinear solver:

Properties​

Materials – To take account of material nonlinearity, Material Properties with a yield stress can be defined using the materials wizard. The user defined materials are assigned to 1D sections in the selection/numeric material field of the Section table. Note that 2D properties require an elastic material.

Elements - Cable elements can behave as Sliding cable elements as well as Normal with the nonlinear analysis dynamic relaxation solver. See Element Types for more information.

The effect of the geometric stiffness can be included in the Element stiffness options.

Cases​

As the principle of superposition is not valid for nonlinear behaviour results from different analyses must not be added together. To mitigate this an analysis can be performed in steps with loads being applied incrementally during the analysis and the intermediate results stored. These incremental load options are offered by the Analysis Wizard.

Dynamic data​

Although the solver uses a method called dynamic relaxation, the nonlinear options cannot be used to study the dynamic behaviour of a structure. Any data relating to dynamic analysis included in the model will be ignored during GsRelax analysis.

Nonlinear static analysis: ties and struts​

Analysis of structures with ties and struts is the simplest nonlinear option. No special action is required by the user to activate this option as it depends only on the types of elements in the model.

Modelling implications of ties and struts​

The following elements cause a solution to be nonlinear:

  • Tie – tension only elements that otherwise resemble bars.
  • Strut – compression only elements that otherwise resemble bars.
  • Cables – similar to tie element but the properties are specified differently.

Nonlinear behaviour is only appropriate for a static analysis and cannot be incorporated into a modal analysis (dynamics or buckling). In these cases these elements behave as bars.

Nonlinear static analysis​

The analysis of ties and struts uses an iterative solution technique, which means that each load case has to be considered separately. The number of iterations required for convergence of each task is shown in the report view.

If the solution fails to converge or the results are insufficiently accurate (for example as indicated by a large residual in the Total Loads and Reactions) then you can adjust the number of iterations and the convergence tolerance in the Advanced dialog in the Analysis Wizard.

Results of nonlinear static analysis​

The same set of results is available from a nonlinear static analysis as for a linear static analysis. However in this case superposition of results is not valid. Details of results available and how they can be viewed are in the Results Display Options section.