The solution control parameters on the Nonlinear Parameters form differ depending on the type of analysis.
Note: When an item is clicked on this form, a description of that item displays in the display area at the bottom of the form.
Nonlinear Static Analysis The following parameters apply to the entire load case, unless analysis of staged construction has been selected, in which case the parameters apply separately to each stage of construction.
Maximum Total Steps edit box. The maximum number of steps allowed in the load case. It may include saved steps as well as intermediate substeps whose results are not saved. Setting this value controls how long the analysis will run. Start with a smaller value to get a feel for the time the analysis will take. If an analysis does not reach its target load or displacement before reaching the maximum number of steps, re-run the analysis after increasing the maximum number of saved steps. The length of time it takes to run a nonlinear static load case is approximately proportional to the total number of steps.
Maximum Null Steps edit box. Null (zero) steps occur during the nonlinear solution procedure when:
A frame hinge is trying to unload.
An event (yielding, unloading, and so forth) triggers another event.
Iteration does not converge and a smaller step size is attempted.
An excessive number of null steps may indicate that the solution is stalled because of catastrophic failure or numerical sensitivity. If a model is having trouble converging, set the Maximum Null (Zero) Steps so that the solution will terminate early. Set this value equal to the Maximum Total Steps to prevent the analysis from terminating because of null steps.
Use Event-To-Event Stepping. If you select Yes, load steps will automatically be subdivided where significant changes occur in the stiffness of nonlinear elements. This causes a series of nearly linear steps to be taken, which can minimize deviation from equilibrium. For larger models, however, this may result in an excessive number of steps. See Event-to-Event Stepping for additional information and parameters. Note that Line Search will not be used if Event-to-Event Stepping is used.
Use Iteration. Iteration is always used for nonlinear static analysis to make sure that equilibrium is achieved at each step of the analysis, whether or not Event-To-Event Stepping is used. See Nonlinear Iteration for additional information and parameters.
Use Line Search. If you select Yes, a line-search algorithm is used to increase the efficiency of equilibrium iterations by scaling the solution increment in a trial-and-error fashion to find the smallest unbalance. See Line Search for additional information and parameters. Note that Line Search will not be used if Event-to-Event Stepping is used, and is not used for displacement-controlled load cases.
Material Nonlinearity Parameters. The data in this area of the form is informational only. For nonlinear modal time-history (FNA) analysis, only link-object material nonlinearity is considered. For nonlinear static and nonlinear direct-integration time-history analysis, all link- and frame-object nonlinearity is considered.
Maximum Substep Size edit box. The analysis will always stop at every output time step, and at every time step where one of the input time-history functions is defined. In addition, an upper limit on the step size used for integration may be set. For example, suppose the output time step size was 0.005, and the input functions were also defined at 0.005 second. If the Maximum Substep Size is set to 0.001, the program will internally take five integration substeps for every saved output time step. The program may use smaller substeps if necessary to achieve convergence when iterating.
Tip: The accuracy of direct-integration methods is very sensitive to integration time step, especially for stiff (high-frequency) response. Try decreasing the maximum substep size until consistent results are obtained. Keep the output time step size fixed to prevent storing excessive amounts of data.
Note: The default value of zero means no limit, i.e., use the output time-step size.
Minimum Substep Size edit box. When the nonlinear iteration cannot converge within the specified maximum number of iterations, the program automatically reduces the current step size and tries again. Use this parameter to limit the smallest substep size the program will use. If the program tries to reduce the step size below this limit, it will stop the analysis and indicate that convergence has failed.
Use Event-To-Event Stepping. If you select Yes, load steps will automatically be subdivided where significant changes occur in the stiffness of nonlinear elements. This causes a series of nearly linear steps to be taken, which can minimize deviation from equilibrium. For larger models, however, this may result in an excessive number of steps. See Event-to-Event Stepping for additional information and parameters.
Use Iteration. If you select Yes, iteration will be used to make sure that equilibrium is achieved at the end of each time steps, whether or not event-to-event stepping is used. See Nonlinear Iteration for additional information and parameters. Note that iteration will always be used if Event-to-Event Stepping is not used. If you use Event-To-Event Stepping without Iteration, equilibrium is not checked at the end of each time step. Choosing not to iterate can improve the speed of the solution and increase the likelihood of completing the analysis when convergence is difficult, but the results should be checked to make sure that the equilibrium error is acceptable
Use Line Search. If you select Yes, a line-search algorithm is used to increase the efficiency of equilibrium iterations by scaling the solution increment in a trial-and-error fashion to find the smallest unbalance. See Line Search for additional information and parameters. Note that Line Search is only available when Iteration is used.
Material Nonlinearity Parameters. The data in this area of the form is informational only. For nonlinear modal time-history (FNA) analysis, only link-object material nonlinearity is considered. For nonlinear static and nonlinear direct-integration time-history analysis, all link- and frame-object nonlinearity is considered
Static Period edit box. Normally all modes are treated as being dynamic. Optionally this parameter may be used to specify that high-frequency (short period) modes be treated as static, so that they follow the load without any transient response. This may be useful for certain quasi-static analyses. Usually, however, the iteration is more stable if dynamic effects are included.
Tip: It is unlikely that this parameter will ever need to be changed from its default value.
Maximum Substep Size edit box. The analysis will always stop at every output time step, and at every time step where one of the input time-history functions is defined. In addition, an upper limit on the step size used for integration may be set. For example, suppose the output time step size was 0.005, and the input functions were also defined at 0.005 second. If the Maximum Substep Size is set to 0.001, the program will internally take five integration substeps for every saved output time step. The program may use smaller substeps if necessary to achieve convergence when iterating.
Tip: The accuracy of modal methods is NOT very sensitive to integration time step. The main reason for limiting the maximum substep size is for comparison with other analyses that have used such limits.
Note: The default value of zero means no limit, i.e., use the output time-step size.
Minimum Substep Size edit box. When the nonlinear iteration cannot converge within the specified maximum number of iterations, the program automatically reduces the current step size and tries again. Use this parameter to limit the smallest substep size the program will use. If the program tries to reduce the step size below this limit, it will stop the analysis and indicate that convergence had failed.
Force Convergence Tolerance (Relative) edit box. Iteration is used to make sure that equilibrium is achieved at each step of the analysis. Use this parameter to set the relative convergence tolerance that is used to compare the magnitude of force error with the magnitude of the force acting on the structure.
Energy Convergence Tolerance (Relative) edit box. If force convergence occurs within the permitted number of iterations, the work accomplished by the nonlinear forces is compared with the work accomplished by all other force terms in the modal equilibrium equations. If the difference, expressed as a fraction of the total work completed is greater than the energy tolerance, the substep size is reduced and the iteration is tried again. This tolerance is not intended to check for equilibrium, but it is intended to limit the amount of nonlinearity permitted in any substep.
Iteration Limits edit box. Iteration is used to make sure that equilibrium is achieved at each step of the analysis. The number of iterations permitted for force iteration varies between the Maximum and Minimum Iteration Limits. The actual number permitted for a given substep is chosen automatically by the program to achieve a balance between iteration and substepping. The number of iterations permitted tends to be larger for smaller substeps.
Tip: It is unlikely that this parameter will ever need to be changed from its default value, unless a convergence factor less than one is being used.
Convergence Factor edit box. Under-relaxation of the force iteration may be used by setting the convergence factor to a value much less than unity (i.e., 0.1 to 0.01). Smaller values increase the stability of the iteration, but require more iterations to achieve convergence.
Tip: It is unlikely that this parameter will ever need to be changed from its default value.