Automatic Response Control phase

This type of loading/solution scheme, adapted from the work of Trueb [1983] and Izzuddin [1991], differs from the response control type only in the fact that it is the program that automatically chooses which nodal degree-of-freedom to control during the analysis and the displacement increment to apply at each analysis step, depending on the convergence characteristics at each analysis step. The user, on the other hand, is asked to define the node, degree-of-freedom and respective target displacement at which the analysis will be completed.

The program uses the "target degree-of-freedom" as the first control entity for the analysis, changing it whenever another nodal degree-of-freedom with a higher rate of nominal tangential translational response (i.e. larger displacement variation between two consecutive steps) is found. In this manner, it results not only possible for highly geometrically nonlinear snap-back and snap-through responses [e.g. Crisfield, 1991] to be accurately predicted, but also to obtain analyses' solution in the minimum amount of time, rendering this type of loading/solution phase the preferred option for obtaining expeditious and accurate estimations of the force and displacement capacity of structures.

Notes

1. When carrying out automatic response control pushover analysis on non-symmetric models, it may happen that the program starts applying the load in the 'negative' direction, effectively pulling the structure backwards, rather than pushing it forwards. This occurs when the non-symmetric structure being analysed proves to be more flexible/deformable in 'pulling’ rather than ‘pushing’, a feature that the automatic response algorithm cannot overlook. If users do wish to force the structure to deform in a different direction, then they should start the pushover analysis with load or response control phases, to initiate the deformation in the desired direction, after which they might change to automatic response control, since the already displaced degrees-of-freedom will be inevitably selected as the control ones.
2. The automatic reduction and increase of the loading step may, on occasions, cause the force-displacement curve points to result very uneven, for which reason the pushover response curve may not always be visually 'adequate'.