Random Vibration PSD Load

Starting Version 12.20, CAEPIPE has the ability to perform Random Vibration Power Spectral Density (PSD) Load Analysis on piping systems. Refer to the Section titled Dynamic Analysis in CAEPIPE Technical Reference Manual for further details and limitations of this analysis.

Users can simulate three (3) PSD load cases, one in each Global X, Y and Z directions. The inputs required for this analysis are directional PSD load, Method, Scale factor, Mode sum, and Fatigue calculation parameters as detailed below:

PSD Load

Select a PSD Data profile from the drop-down combo box, which should have been input in the PSD Load table for each global direction. The PSD Data tables can be input in Layout Window Menu: Misc > PSD Data.

Damping (%)

Input the damping factor as a percentage (not as a fraction). Enter 5, for 5% damping and not 0.05.

Method

Select among the two Normal Mode Methods to calculate the response of the system to PSD Loads. The Normal Mode Method (Approximate) is suitable for white noise vibration or its assumption as white noise near natural modes. Refer to the Section titled Dynamic Analysis in CAEPIPE Technical Reference Manual.

Number of Integration Intervals. In case the user selects Normal Mode Method (Standard), user needs to specify the number of integration time intervals. The lowest and highest frequency specified in the PSD data table is the lower and upper bounds of integration.

Scale factor (Probability)

Select the scale factor as 1 Sigma / 2 Sigma / 3 Sigma, where Sigma is a standard deviation across the zero-mean normal (Gaussian). The probability that the response lies between ± 1 Sigma is 68.3%, the probability that it lies between ± 2 Sigma is 95.4%, and that for ± 3 Sigma is 99.7%. The resulting displacements, element forces, support loads and stresses will be scaled accordingly.

Note: The results for fatigue damage factor and fatigue life are not affected as those are calculated from Steinberg 3-band method, which considers cumulative fatigue damage.

Mode Sum

Select one of the four choices, SRSS (square root of sum of squares), Closely spaced (CSM), Absolute (ABS) and Naval Research Lab (NRL). Refer to the Section titled Dynamic Analysis in CAEPIPE Technical Reference Manual for further details.

Exposure Time (Unit: Hours)

Exposure time calculates the damage factor for piping exposure to random excitation for specified duration of time in hours. This input is optional. If left blank, the damage factor will not be computed and the corresponding column in the sorted stresses will be left BLANK and the results are sorted based on the stress intensity. On the other hand, when the exposure time is input, then the sorted stresses results are sorted in descending order of the damage factor.

Constant, R

The constant "R" represents cumulative loading cycles and is initially set to 1.0 in CAEPIPE, such that if R exceeds 1.0, CAEPIPE will flag the results with red color. Users have the option to input R after applying a safety factor, example 0.7. For further details, refer to the section on Dynamic Analysis in the CAEPIPE Technical Reference Manual.