Applies To Product(s): AutoPIPE, Version(s): 2004, XM, & V8i Environment: N/A Area: Subarea: Original Author: Bentley Technical Support Group Comments, Questions, and Answers: Question #1: Output report, subsection - Frequency found the Participation Factor-X =-0, Captured Modal Mass-X = 0 and Cumulative Modal Mass-X = 0, for all calculated natural frequencies, but in anchors there are existing forces greater than 0, FX(Point 10) = 350.87 N and FX(Point 100) = 360.91 N. How can this situation be explained? Answer The model was found to be a single segment, 100m long on the Global X axis. The pipe has no bends, expansion loops, or is modeled on any other axis except Global X. As a result the participation factors were reported in the output report as 0.00 for the same pipe axis (Global X), however the actual value may not be 0.00. AutoPIPE is only able to report a number with 2 significant digits. use the following procedure to validate the actual value calculated: 1. Open the model 2. Analyze the model 3. Select Results Grids 4. Select Frequency Tab in the grid 5. Select any cell in the Particip. Factor X column, note that the value is not 0.00 but much smaller, ex: -5.2860372e-011. AutoPIPE output report is not able to show such small values. Suggestion, confirm actual value calculated using the results grid. Question #2 Given a model with a single pipe, modeled between 2 anchors on the Global X axis, the calculated inertial forces and moments in the TEST1 model are FX, FY, FZ (where, FZ=2258.35 N), MX, MY (where, MY=5588.1 N-m) & MZ, for both anchor points 10 and 100, are significantly lower than the inertial forces and moments FX, FY (where, FY=3030.55) FZ, MX, MY (where, MY=8585.51N-m) & MZ calculated in TEST2 model, accordingly. The only difference between these 2 models is that modal analysis cut of freq = 66 hz for TEST1 & 120 hz to TEST2. Thus, even if more frequencies and their associated modal masses are obtained in the analysis TEST2 model (Fr eq = 120 Hz and 12 modes obtained), the lesser inertial forces and moments values are obtained in anchor points 10 and 100, com paring with TEST1 (Freq = 60.2 Hz and 6 modes obtained), respectively. So, how can this situation be explained? Answer : One issue was that more than a different value for modal analysis was found between the 2 original models. To be absolutely sure that the models are identical, open the 1st model, saved as 2nd model name, and make the changes as required, ex: change modal cut-off frequency from 33hz to 120 hz. To answer the question, might find that the anchor reaction is more for the model with higher cut-off frequency as it has higher level of discretization and thus able to capture higher modes better. Question #3 Analyzing the 2 models TEST1 and TEST2, we observe that for the first 6 vibration modes (Mode No 1 to Mode No 6) each calculation model gives different values for the first 6 calculated frequencies, as well as for the first 6 Participation Factors (in t he X, Y and Z directions) and for the first 6 Captured Modal Mass (in the X, Y and Z directions) respectively. Obviously, the first 6 Cumulated Modal Masses (in the X, Y and Z directions) are also different in the 2 models (TEST1 and TEST2). Furthermore, the first 6 Shape Modes from each model are different. How is this possible, given the identical conditions in the calculation models - except for the different Cutoff Frequencies (66Hz and 120 Hz, respectively)? Answer : One issue was that more than a different value for modal analysis was found between the 2 original models. To be absolutely sure that the models are identical, open the 1st model, saved as 2nd model name, and make the changes as required, ex: change modal cut-off frequency from 33hz to 120 hz. Knowing the files are now identical with exception to changes made, performed a model analysis and Static analysis on both models. Changing the cut-off frequency when you have Tools Model Options Edit Mass points per span = A option enabled will lead to different level of discretization in the model. That should explain the differences in the frequencies. Another words, not expect the first 7 frequencies to be same between the two models as they are no longer "equivalent" - because the level of discretization is not same. The higher the cutoff frequency, the higher the level of discretization. Question #4 I am confused on the interaction of the Cutoff Frequency and Maximum Number of modes? It seems the Cutoff Frequency under the Model Options is always superseded by the values declared under the Dynamic Analysis? When/Where does the Cutoff Frequency under the Model Options control? Answer: Edit Model options .. this cut off frequency is used to calculate the optimal mass span length only From Online help: Question #3 Are there any (practical )maximum cut-off frequency values, I tried 100,000 and the following occurred:? Answer: See the following for suggestions to this error message : click here Question #5 Estimate the cutoff frequency for my dynamic water hammer analysis? Answer: The maximum frequency cutoff can be estimated from: SQRT (E/p)/L, Where: E is the modulus of elasticity of the pipe material, p is the density of the pipe material L is the length of a single pipe element in the primary run that is to have accurate stresses computed due to the passing of the water hammer originated acoustic stress wave. Example: Calculation of the maximum cutoff frequency for between 2 elbows (node point 45 & 75) with 20-foot pipe lengths is given as follows: When performing a modal analysis on the piping system, impulse loading such as water hammer may have high excitation frequencies even as high as 200-300 Hz. For small piping systems, the extraction of high frequency modes is relatively fast and will more accurately predict local dynamic responses than the static correction methods. Question #5 Comments from Mitch Sklar training class on Modal Analysis: --- The captured modal mass percentage tells how much of the response is attributed to a particular mode and also tells on the mode orientation (X,Y or Z). The sum of modal masses should be 100% if all modes are counted. But since many modes are not counted, the sum is less than 100 and hence the importance of the ZPA and missing mass options for dynamic analysis. --- Modal analysis advantage is that it can give good results by analyzing few modes. But that is not true for all systems. Earthquake loading does not have frequencies above 50 Hz and so setting cut-off frequency to 50 or 33 Hz and including missing mass or ZPA will usually give satisfactory results. Some people like to make sure the captured modal mass is at least 75% or even 90%. Water hammer is usually a high frequency load and required analyzing modes above 150 Hz. The frequency range of the modes should always cover the frequency content of the loading function and in some cases you need to analyze for all the modes by including a large cut-off frequency (e.g. 1.E20). This is usually done to capture support reactions in the absence of using ZPA or missing mass correction to correct for these unused higher modes. --- The usual procedure for determining how many modes are sufficient is to extract a certain number of modes and review the results; then to repeat the analysis while extracting 5 to 10 additional modes, and comparing the new results to the old. If there is a significant change between the results, a new analysis is made, again extracting 5 to 10 more modes above those that were extracted for the second analysis. This iterative process continues until the results taper off, becoming asymptotic. The fact you are getting different results for different cut-off frequency indicates that your loading have higher frequencies. See Also Bentley AutoPIPE External Links Bentley Technical Support KnowledgeBase Bentley LEARN Server Comments or Corrections? Bentley's Technical Support Group requests that you please submit any comments you have on this Wiki article to the "Comments" area below. THANK YOU!
↧