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Displacement Boundary Conditions

Theory

Displacement boundary conditions allow you to specify arbitrary time-varying boundary conditions at specific nodes of the FE model, where the time history of motion is read from an ASCII data file. The position of the node is then calculated by adding this displacement to the initial position of the node. Flexcom uses cubic spline interpolation to find displacement values at time points intermediate to those specified in the data file (for this reason the analysis solution times do not need to match those in the data file). For analysis times before the earliest time specified in the data file, Flexcom uses the values at that earliest time until the analysis time exceeds this value. Similarly, for analysis times after the latest time in the data file, Flexcom uses the values at that latest time.

The format that this data file takes depends on the number of boundary conditions that are specified in the file, but in general the file contains one column of data for the time and an additional column for each boundary condition. Comment lines, denoted by a capital ’C’ in the first column, are permitted, while lines that are completely blank are ignored. An example data file is shown below.

 

 

The first column of data must always contain time values. Subsequent columns then contain the boundary condition timetrace data. Flexcom uses cubic spline interpolation to find displacement values at time points intermediate to those specified in the data file (for this reason the analysis solution times do not need to match those in the data file).

Note that the order in which the boundary conditions are specified in the Timetrace Boundary – Displacement table must correspond to the order in which the relevant data appears in the data file. For example, the three boundary conditions in the file shown above would be specified as shown below.

Columns of data in the ASCII data file should be separated by blank spaces or tabs. You can specify as many boundary conditions as you wish with a single data file – simply leave the first column of the Timetrace Boundary – Displacement table blank for the second and subsequent boundary conditions. The only limitation on this is that each line in the ASCII data file should not exceed 200 characters in length.

Relevant Keywords

•*BOUNDARY is used to define boundary conditions. Specifically, the TYPE=FILE input is used to identify the nodes on the structure whose time-varying displacements are to be read from an ASCII data file.