For nonlinear direct-integration time-history load cases, you can choose whether or not to Use Iteration from the Solution Control form. For nonlinear static and staged-construction load cases, iteration is always used. The parameters below apply in either case.
Use Iteration. If Yes, iteration will be used to make sure that equilibrium is achieved at each step of the analysis, whether or not event-to-event stepping is used. Note that iteration will always be used for nonlinear static/staged-construction load cases, and will always be used for nonlinear direct-integration time-history load cases when 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. This can improve the speed of the solution and increase the likelihood of completing the analysis, but the results should be checked to make sure the equilibrium error is acceptable.
Maximum Constant-Stiffness Iterations and Maximum Newton-Raphson Iterations. For each step, constant-stiffness iteration is tried first. If convergence is not achieved, Newton-Raphson (tangent-stiffness) iteration is tried next. If both fail, the step size is reduced, and the process is repeated. Use these parameters to control how many iterations of each type is allowed. Setting either parameter to zero prevents that type of iteration. Setting both to zero causes the program to automatically determine the number of iterations to allow. Constant-stiffness iterations are faster than Newton-Raphson iterations, but the latter are usually more effective, especially for cables and geometric nonlinearity. The default values work well in many situations.
Iteration Convergence Tolerance (Relative). 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.