Response Control phase

In this type of loading/solution scheme, it is not the load vector that is controlled, as in the load control case, but rather the response of a particular node in the structure. Indeed, when setting a response control phase, the user is requested to define the node and corresponding degree-of-freedom that is to be controlled by the algorithm, together with the target displacement at which the analysis is to be terminated. Moreover, the number of increments, in which the target displacement is to be subdivided into for incremental application, should be specified.

The load factor , therefore, is not directly controlled by the user but is instead automatically calculated by the program so that the applied load vector at a particular increment i corresponds to the attainment of the target displacement at the controlled node at that increment. When the solution at a particular step fails to converge, the initial displacement increment is reduced until convergence is reached, after which it tries to return to its initial value (refer to automatic step adjustment for further details). The phase finishes when the target displacement is reached or when structural or numerical collapse occurs.

With this loading strategy, it is possible to (i) capture irregular response features (e.g. soft-storey), (ii) capture the softening post-peak branch of the response and (iii) obtain an even distribution of force-displacement curve points. For these reasons, this type of loading/solution phase usually constitutes the best option for carrying out non-adaptive pushover analysis.

Notes

1. Response control can be employed also in conjunction with displacement incremental loads.
2. Response control does not allow the modelling of snap-back and snap-through response types [e.g. Crisfield, 1991], observed in structures subjected to levels of deformation large enough to cause a shift in their mechanism of deformation and response. For such extreme cases, the employment of Automatic Response Control is required.
3. The program will automatically adjust the value of the first increment so that the latter added to the gravity loads-induced displacement equals the initially envisaged target displacement value at the end of the first increment. In other words, if the user wanted, for instance, to impose a 200 mm top floor displacement applied in 100 increments, and if the gravity loads would cause a horizontal displacement of 0.04mm, then the displacement load increments would be 1.96, 2.0, 2.0, ..., 2.0. This adjustment will, however, occur only in those cases where the gravity loads-induced displacement is lower than the envisaged first horizontal loads increment; if this condition that does hold (e.g. disp_gravt=2.07, in the example above), then the displacement increments will all be identical and equal to (200-2.07)/100=1.9793 (clearly a much less "elegant" figure).