Explicit dynamic analysis

When the model is nonlinear the only option for dynamic analysis is a time-history analysis which steps through the solution one time step at a time. There are two basic schemes for this time stepping: implicit and explicit. The time-history option implemented in GSA is explicit.

With an explicit time history solution the analysis time step depend on the mesh frequency, so small, stiff elements which have a high frequency will require a small time step that can control the overall solution computational effort.

Modelling implications

An explicit time-history solution proceeds from a knowledge of the structure at a given time, being used to calculate an acceleration which is then used to predict the state at the next time step. With an explicit time history solution the analysis time step depends on the mesh frequency, so small, stiff elements (which have a high frequency) will require a small time step. This limiting time step controls the stability of the analysis, and so can control the overall solution computational effort.

An explicit solution can require a large number (1000s) of iterations, so results are only output at intervals requested by the user. Unlike other analysis options an explicit solution allows two types of output – results for the whole model and results for selected nodes and elements. The whole model results form a (relatively small) set of analysis cases where results such as displacements, forces, etc. are stored for all nodes and elements. The selected node and element results are normally output at a much small time step so that significant results can be tracked as a function of time. These are stored as task results, rather than analysis case results and are normally viewed as charts.

Analysis

An explicit time-history analysis is set up using the Analysis Wizard from the Analysis > New analysis task menu command. Select the Explicit time history option. The following page allow the user to specify the task details, including the spatial and temporal aspects of the load, and the distribution of the structure mass and specification of the damping.

Results

The results for an explicit time history analysis fall into two sets – whole model results and time history results. The whole model results are similar to those for a static analysis, but with the option of velocity and acceleration results for nodes. These display the state of the model at that snapshot in time.

The selected node and element results give the variation with time and they can be viewed charts.

Details of results available and how they can be viewed are in the Results display options section.