Line Search

For nonlinear static, nonlinear staged construction, and nonlinear direct integration time history load cases, you can choose whether or not to Use Line Search from the Solution Control form. This option is only available when iteration is used.

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. This increases the computation time for each iteration, but often results in fewer iterations and better convergence behavior, with a net gain in efficiency. For nonlinear static/staged-construction load cases, line search will not be used when event-to-event stepping is used, but this restriction does not apply to nonlinear direct-integration time-history load cases.
Maximum Line Searches per Iteration. Use this parameter to limit the number of line searches allowed per iteration, usually in the range of 5 to 40.
Line-Search Acceptance Tolerance (Relative). The solution increment is successively reduced or increased until the minimum error is found, the maximum number of line searches is reached, or the relative change in error from the previous trial is less than the acceptance tolerance set here. The practical range is about 0.02 to 0.50. This value should not be too small, since the goal is only to improve the next iteration.
Line-Search Step Factor. For each line-search trial, the solution increment is successively reduced by the step factor specified here until the minimum error is found, the maximum number of line searches is reached, or the acceptance tolerance is satisfied. If reducing the increment does not decrease the error, the increment is instead increased by the step factor following the same procedure. This factor must be greater than 1.0, and should usually be no more than 2.0.

The default values are recommended as the starting point. For stiffening systems that are slow to converge, try increasing the maximum number of line searches per iteration, decreasing the acceptance tolerance, and reducing the step factor.

The line search option is one of the Solution Control area/options that appear on the Nonlinear Parameters form. Access the Nonlinear Parameters form as follows:

Click the Define menu > Load Cases command to display the Define Load Cases form.
Click the Add New Load Case button or highlight a previously defined load case and click the Add Copy of Load Case or the Modify/Show Load Case button to display the Load Case Data form.
Then do one of the following:
- Select Static from the Load Case Type drop-down list and the Nonlinear or Nonlinear Staged Construction option for the Analysis Type.
- Select Time History from the Load Case Type drop-down list and the Nonlinear and Direct Integration options for the Analysis Type and Time History Type.
Click the "Nonlinear Parameters" Modify/Show button to display the Nonlinear Parameters form.