IDA parameters

In Incremental Dynamic Analysis (IDA), structures are subjected to a succession of transient loads, which usually consist of acceleration time-histories of increasing intensity, as described in here. Therefore, users who are interested in using this type of analysis, are strongly advised to first consult the Time-history Curves section, where the loading application procedure for dynamic time-history analysis is described. The latter is fully applicable to IDA cases, noting however that a number of additional parameters, included in the IDA Parameters module, needs to be defined. These parameters are:

Scaling factors
Each time-history run of an IDA is carried out for a given input motion intensity, defined by the product of the Scaling Factors with the accelerogram introduced by the user. Usually, the input motion is incrementally scaled from a low elastic response value up to a large value, corresponding to the attainment of a pre-defined post-yield target limit state.

Fixed and/or variable scaling patterns can be used, either in isolation or in combination. With fixed patterns (Start-End-Step), the user defines the start scaling factor, corresponding to the first time-history run, the end scaling factor, corresponding to the last time-history analysis to be carried out, and a scaling factor step which is used to define the evenly spaced intermediate time-history levels. With a variable scaling pattern (Distinct Scaling Factors), on the other hand, non-evenly spaced sequences of scaling factors can be used, with the user being required to explicitly define all scaling factors to be considered during the incremental dynamic analysis (unless used in combination with a fixed scaling pattern, in which case only odd non-sequential factors may need to be specified).

Dynamic Pushover Curve
When carrying out Incremental Dynamic Analysis, the user is often interested in obtaining the so-called Dynamic Pushover Curve (or IDA envelope), which consists of a plot of peak values of base shear versus maximum values of top, or other, displacement, as obtained in each of the dynamic runs. It is therefore possible to explicitly define which nodes are to be considered in the computation of the maximum relative displacement (difference between the absolute displacement values of the two user-defined nodes, the second of which usually refers to a support node) at each dynamic run.

The degree-of-freedom of interest is also explicitly defined by the user, as is the time-window around the maximum drift value within which to find the corresponding peak base shear value (or vice-versa), in case the user is interested in obtaining a curve of corresponding displacement and shear peak values, instead of a curve of not-necessarily correlated pairs of peak displacement and shear values.

Note: Usually, the behaviour of structures within their elastic response range can be represented through the use of 2-3 pairs of shear-displacement points, fairly well spaced. In the post-yield region, on the other hand, a finer representation of the dynamic pushover curve may be required. In such cases, users might find useful to employ a combination of both fixed and variable scaling patterns, whereby 2-3 distinct scaling factors are used for the elastic region and then start-end-step range of values is employed for the post-yield response phase.