Dynamic Relaxation Convergence & Damping

Monitoring the Iterative Process

During the analysis, the program tests the out-of-balance forces and moments (residuals) at each free node against target values (or Control Parameters). The frequency of this test can be adjusted. To save computing time this should not be done every cycle.

During GsRelax analysis, analysis progress information will be shown in the progress dialog box. This allows users to assess the progress of the run. In particular they can check that the simulated damped vibration has not become unstable and that the residuals are decreasing. There are three options for progress information: full progress information, summary progress information and no progress information. From the progress dialog box it is possible to reset the residual target, dummy mass factor etc or even to stop the analysis by clicking the relevant button

Summary of the analysis progress information includes the current cycles, maximum cycles, current residuals and residual targets.

Full analysis progress information includes the above plus, the maximum displacement and rotations.

Also a graphics view will be periodically updated to show the deflected shape as the analysis progresses. The frequency of updating the progress dialog box and updating the graphic view can be set prior to the analysis. The analysis speed is slightly affected by the frequency of updating the progress information, but the speed is significantly affected by the frequency of updating the graphic view especially for large structures. It is recommended not to update the graphic view too frequently for medium and large size structures.

Interrupting the Analysis

GsRelax analysis can be interrupted at any time during the analysis by clicking the stop button on the progress dialog box. When the stop button is clicked, there are three options. To continue, abandon or terminate the analysis. Terminating the analysis will mean that the current deformed shape and internal forces of the structure will be saved as result, even though the specified residual targets have not been met. The terminate option should be used cautiously and make sure that the non-converged results are not misused since they will be inaccurate. Alternatively adjust the target residual values to accept a less accurate solution.

Optimising Convergence

By default automatic adjustment of dummy mass factors, dummy inertia factors, and damping occurs. However if convergence problems occur, the user can override these values and disable the automatic adjustment. This can be done before analysis in the Convergence control box accessed by selecting “Advanced” in the Analysis wizard. It can also be done from the progress dialog box. More details on Optimising Convergence are given below.

Termination

The user can set single target values for force and moment residuals. Alternatively the user can specify a target Residual Percentage. These are set as GsRelax Control Parameters.

It is important not to set too rigorous target values and to monitor results. The target values may be less than the errors that are generated by the computer. In such cases the structure does not shift significantly as cycles repeat and the residuals do not decrease. The run should be terminated since the best possible accuracy has been achieved.

If the analysis does not converge it will be stopped after the number of cycles, or a length of time specified as termination controls. Non convergence should be investigated as it is possible to improve convergence by adjusting damping, or modifying a badly conditioned model. It is also possible that an acceptable solution can be achieved with higher out of balance forces and that the convergence criteria should be relaxed by adjusting the residual controls.

In addition, limits can be set to control the automatic load increment option. With this option GsRelax is searching for the “capacity” of the model by successively increasing the load factor. The capacity can be defined as the load at which analysis ceases to converge, or, the load at which excessive deflections or rotations occur. This can also be used to investigate the load factor at which certain serviceability criteria are met.

For most structures there is a load factor beyond which the capacity becomes unimportant, as the loading will never be achieved, even considering normal safety factors. With the automatic load increment option it is important to set the max load factor at which the program should cease to search for the model capacity. Use this to avoid unnecessary analysis.