How to perform a Force Spectrum Analysis in AutoPIPE?
A force spectrum analysis (alternate to time history analysis) is used to analyze the effect of short sudden or impulsive loads such as water hammer, or relief valve blow down. This shock or impulsive loading is usually represented as a piece-wise, linear time history in which the force magnitude is plotted against time. A series of single DOF oscillators are subjected to the force time history. The maximum response of these oscillators is extracted from the solution obtained using the Duhamel integral approach. A family of these maxima obtained from oscillators with different frequency represents the force spectrum of the time history.
This procedure is very complex, time consuming, and used to take many hours to complete. The Force Spectrum command available from the Load menu can be used to generate a force spectrum data file for a given
AutoPIPE’s force spectrum analysis begins by combining the intermodal results using the SRSS combination method. Then the interspatial spectrums are applied using the absolute summation method, in order to create the F1, F2, and F3 load case results. The analysis assumes all forces are acting at the same time and the results are stored in the file SYSNAME.FSP.
The Force Spectrum option in the Analyze menu enables the user to perform a force spectrum analysis using previously stored Modal Analysis results and Force Spectrum files to calculate forces and moments due to short duration loads. Up to ten force spectrum load cases (F1 – F10) may be defined. However, each time a force spectrum analysis is performed, the results from any previous force spectrum analysis will be overwritten.
The force spectrum input data can be accessed under the Load> Dynamic Analysis> Force Spectrum tab. In versions prior to 9.2 and earlier, the Force Spectrum dialog was displayed when selecting Analyze> Force Spectrum command.
Figure A: While AutoPIPE permits both Missing Mass and ZPA static corrections to be included at the same time, it is not recommended.
The following example and subsequent screen shots provide direction in performing a successful Force Spectrum Analysis:
Summary of procedure:
1. Open model2. Create Time History files3. Convert Time History loads to Force Spectrum loads (*.SPC) 4. Apply Force Spectrum load (*.SPC) to model node point C07 in the +Z dir5. Run Analyze All (includes Static Analysis, Modal Analysis , & Force Spectrum Analysis)6. Review results
Open existing AutoPIPE example model APHAM1.DAT (Version 9.2 and higher: located in Example> folder)
Fig 1-1: APHAM1.DAT example. A pump is located between segments A and C (Seg A pump suction and Seg C, pump discharge).
a. Select Load > Time history Profile. b. Enter the file name (ex. Test1)c. Press the Tab key, select Force from drop down.d. Press Tab key , enter the following data in the fields below as required to represent a force function profile:
Fig 2-1: Time History Profile.
e. Select OK button ( Test1.TIH file will be created in same folder as AutoPIPE.dat file).
a. Select Load> Convert to Force Spectrum. The following dialog will be displayed.
Figure 3-1: Convert to Force Spectrum dialog
b. Enter the following data into the fields above:
i. Force Spectrum name = Force1 ii. Time history name = Test1.TIH (magnitude of force (lbf) plotted against time (sec), select TIH file created from previous step).iii. Maximum Frequency = 10,000 Hziv. Number of (data pair) points = 30; (the maximum number of data pairs allowed is 50.)v. Damping Ratio = 0.0; (This is the damping ratio which will be applied to all modes during the generation of the force spectrum file. AutoPIPE assigns the default (linear-linear) interpolation method and a damping ratio of zero to the resulting force spectrum data file.
Fig 3-2: Convert to Force Spectrum dialogvi. Select OK button ( Force1.SPC file will be created in same folder as AutoPIPE.dat file).
Press OK button to continue
c. The contents of the Force1.SPC can be displayed within AutoPIPE’s Response Force Spectrum or through Windows Notepad.
i. Within AutoPIPE, Select Load> Response/Force Spectrum and the following empty dialog is displayed:
Fig 3-3: Response/Force Spectrum
ii. In the Spectrum Filename Dropdown, Select Load Force1 and the following dialog will be display:
Fig 3-4: Response/Force Spectrum Force.SPC file with the data pair points displayed:
Press OK button to continue
iii. Alternately one can open the Force1.SPC file using any word editing program (ex. NotePad ++, Ultra edit, MS Word, etc..) :
Fig 3-6: Response/Force Spectrum Force1.SPC file displayed with Windows Notepad
a. Select Load> Dynamic Analysis> Force Spectrum tab
b. Select New button to create new force spectrum load case 1
Fig 4-1: Force Spectrum Input Tab
c. Select Define button in order to add point of force spectrum application, force spectrum data, scale factor, and load direction.
Fig 4-2: Force Spectrum Point & Force Application Input Tab
d. Select New button again to Add spectrums on specific points for analysis.
e. Enter the following data in the fields above:
i. Point = C07 (Note: cannot apply Force Spectrum load case to elbow TTR point, use Near, Mid, or far bend point names).ii. Select Spectrum file (Force1.SPC ) from dropdown list iii. Enter Scale factor of force spectrum = 1.0 (default).iv. Enter directional cosines = Dx: 0.0 / Dy: 0.0 / Dz: 1.0
v. Select Apply button, and then Select OK button to close dialog screen, and go back to Force Spectrum tab. (Force spectrum file, Force1.SPC, is now assigned to node point C07 in the +Z direction with a scale factor of 1.0.)
f. Add check mark for both ZPA and Missing Mass (optional)g. Add Description (Optional)
Fig 4-4: Force Spectrum Input Tab
h. Select Apply button and then select OK
The following screen should be displayed, select YES button
Fig 5-1: Analysis Confirmation Tab
Fig 6-1: Analysis Selection Tab
Press OK button to continue.
a. Select Tools> Combinations> Select each of the Cases/Combinations tabs to verify the correct Load Cases and Combinations been generated and available for review
Fig 7-1: Load Case Tab
Fig 7-2: Code Combination Tab
Fig 7-3: Non Code Combination Tab
b. Reviewing the displacement of point C07 before and after the force spectrum application:
Fig 7-4: point C07 before force spectrum application
Fig 7-5: point C07 after force spectrum application
The process above demonstrates how to perform a Force Spectrum analysis using AutoPIPE CONNECT 10.01.xx.xx . This technique uses the time history profile converted into a Force Spectrum file and then applied it to a single point in a specified direction.
"Response / Force Spectrum Loads" - AutoPIPE Load Case
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