Load Control phase

In this type of loading/solution scheme, the user defines the target load multiplier (the factor by which all nominal loads, defined in the Applied Loading module, are multiplied to get the target loads) and the number of increments in which the target load vector is to be subdivided into, for incremental application.

The load factor , therefore, varies between 0 and the target load multiplier value, with an initial step increment that is equal to the ratio between the target load multiplier and the number of increments. The value of is changed only when the solution at a particular step fails to converge, in which case the load factor 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 loading is reached or when structural or numerical collapse occurs.

If the user defined the incremental loads as forces, then a force-controlled pushover is carried out, with the load factor being used to scale directly the applied force vector, until the point of peak capacity. If the user wishes also to capture the post-peak softening behaviour of the structure, then a response or automatic response phase needs to be added to the load control one (the program will automatically switch from one phase to the other). This type of loading/solution strategy is employed when the user needs to control directly the manner in which the force vector is incremented and applied to the structure.

If, on the other hand, the user defined the loads as displacements, then a displacement-controlled pushover is considered instead, with a displacement load vector incrementally applied to the structure. This loading/response strategy is employed when the user wishes to have direct control over the deformed shape of the structure at each stage of the analysis. Its application, however, is usually not recommended, since constraining the deformation of a structure to a predefined shape may conceal its true response characteristics (e.g. soft-storey), unless the more advanced adaptive pushover analysis type is employed.

Notes

1. When one force-based load control phase (+ one response control phase) is employed, the distribution of force-displacement curve points usually results uneven, with higher density in the pre-peak part, where to relatively large force increments correspond to small displacement steps, and lower point concentration in the post-peak range, where to very small force variations may correspond large deformation jumps. To solve or mitigate such problem a response control phase should be used.
2. When the applied incremental loads are displacements, 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 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.