Time History Load
A phenomenon that causes loads to vary with time can be applied in CAEPIPE for time history analysis. You may hand-calculate these values or use a simulation program (such as a transient fluid flow program for fluid hammer analysis) to get the variation of forces or moments with time at different points in the piping system. These loads are then input into CAEPIPE as time functions, which are later applied at the corresponding nodes of the CAEPIPE model as “Time Varying Loads.”
Time functions (input under menu Misc> Time Functions) are a series of non-dimensional values versus time, which describe the variation of the forcing function with time. The actual value of the time function at any time is found by linear interpolation between time points.
When you input a “Time Varying Load” at a node, you may apply a scale factor, if necessary, along with the direction of the force or moment.
CAEPIPE will then apply these loads to compute the response of the piping system by performing a time history analysis.
Various parameters for time history analysis are specified in the Time History Analysis Control dialog under menu Loads > Time History.
Time step
The time step (time interval) at which the analysis is performed should be typically no more than 10% (smaller the better) of the period of the highest frequency of interest, i.e., higher the frequency, smaller the time step, e.g., for a 33 Hz maximum frequency, the time step would be about 0.003 seconds {=10% of (1/33)}.
Number of time steps
The time history response is calculated for a total time (in seconds) of Time step × Number of time steps. This is how long you want to study system response. The total time may exceed the range of available data in the time function. The time function is only a forcing function. But, CAEPIPE can compute system response to it well after the forcing function ceases. For example, the effect of a heavy steam hammer could linger on for a minute while your forcing function data could span only 8s (seconds).
Output interval
The output interval is a multiple of time step at which you want CAEPIPE to save (and later display) results. For example, assuming a time step of 0.01s, if you wanted to see results at every 10th time step (i.e., at 0.1s, 0.2s, 0.3s, and so on), enter 10 for output interval. You need to check the “Save Results” checkbox to see these time varying results which are saved in the file modelname.rth. If the “Save Results” checkbox is not checked, only the enveloped results are available but not their variation in time. The .RTH files may be large for big models with many time steps (so ensure that you have adequate storage and permission settings before performing time history analysis with “Save Results” checked).
Note that CAEPIPE calculates the enveloped (maximum) results at every time step and not just at every output interval. The output interval is strictly for display purpose in the results and for saving in the .RTH file. The output interval does not affect the analysis.
Damping
Express the damping factor as a percentage (not as a fraction). Enter 5, not 0.05, for 5% damping.
Input: Time step = 0.01s, Number of time steps: 1000, Output interval: 10, Damping: 5%, Save Results checkbox Checked.
CAEPIPE calculates the solutions at every 0.01s up to 10s. The results are saved every 0.1s in the .rth file. Note however, that CAEPIPE calculates the enveloped (maximum) results at every time step interval and not just at every output interval. The output interval is strictly for display purpose in the results and does not affect analysis.
The Time Varying Load dialog is shown below:
The Time Varying Load at a node is specified by its direction, units, the associated time function and a scale factor. This scale factor is a scalar value, which when multiplied by the non-dimensional time function will give the actual magnitude of the forcing function in the unit selected in the above dialog.