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The Solution Control area/options appear on the Nonlinear Parameters form. Access the Nonlinear Parameters form as follows:
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The solution control parameters on the Nonlinear Parameters form differ depending on the type of analysis.
Nonlinear Static Analysis The following parameters apply to the entire analysis case, unless analysis of staged construction has been selected, in which case the parameters apply separately to each stage of construction.
Maximum Total Steps per Stage edit box. The maximum number of steps allowed in the analysis. 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 analysis is approximately proportional to the total number of steps.
Maximum Null (Zero) Steps per Stage 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.
Maximum Iterations per Step edit box. Iteration is used to make sure that equilibrium is achieved at each step of the analysis. Use this parameter to control the number of iterations allowed in a step before the program tries using a smaller substep. The default value of 10 works well in many situations.
Iteration 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.
Tip: To get good results, significantly smaller values of convergence tolerance may be needed for large-displacement problems than for other types of nonlinearity. Try decreasing values until consistent results are obtained.
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.
Maximum Iterations per Substep edit box. Iteration is used to make sure that equilibrium is achieved at each step of the analysis. Use this parameter to control the number of iterations allowed in a step before the program tries using a smaller substep. The default value of 10 works well in many situations.
Iteration 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.
Tip: To get good results, significantly smaller values of convergence tolerance may be needed for large-displacement problems than for other types of nonlinearity. Try decreasing values until consistent results are obtained.
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.