PIPESTRESS - Dynamic and time history analyses


Two types of time history loads can be analyzed: applied forces and support movements.

PIPESTRESS uses the modal superposition method. The natural modes of vibration up to the cut-off frequency are extracted. The contributions of  the individual modes are then calculated and combined by algebraic addition.


Rigid modes

The contribution to the total system response of the higher (rigid) uncalculated modes is evaluated with the "left-out-force" method. DST has rigorous test problems validating this method.

The PIPESTRESS utility program SPECT assists the engineer in determining the rigid mode cut-off frequency of the applied load. This frequency is the frequency above which the piping system modes act rigidly for the applied load. If modes are calculated up to the cut-off frequency, then the left-out-force correction very accurately defines the contribution of the higher modes.


Special features

Calculation of time history cases using strain energy weighted composite modal damping defined in ASME N-1233.

Calculation of multi-support level time history support acceleration analysis for up to 99 support levels. The multi-level participation factors are calculated by a stiffness-weighted method rather than the standard mass-weighted method. This represents a useful simplification.

Option to calculate time history cases using the "shift" option. This option is similar to the method of "peak broadening" used for floor response spectrum analysis.

The utility program THIST is a PIPESTRESS post-processor. This program is used to obtain more detailed results, including time history plots for selected degrees-of-freedom. THIST generates reports, plots, and files that can be entered into Excel.





In the plot below produced by SPECT, the user was able to determine the cut-off frequency for the dynamic load as 60 Hz.


The dynamic calculation consisted of the superposition of modal components up to 60 Hz plus the left-out-force correction. Although structural modes were calculated only to 60 Hz, the total solution was within a few percent of the "all modes" solution


Time History Z Force point ID=101 "Low-frequency modes without LOF"


Time History Z Force point ID=101 "Low-frequency modes with LOF"