Applies To Product(s): AutoPIPE, Version(s): 2004, XM, & V8i Environment: N/A Area: Frequency Results Subarea: Original Author: Bentley Technical Support Group Comments, Questions, and Answers with AutoPIPE's Output "Mode Shape" Sub Report: Item #1: Natural frequency and mode shapes are property of the structure and depend on the mass and elasticity. They describe the tendency of the structure to vibrate when subjected to dynamic loading. Number of frequencies and mode shapes with which a structure can vibrate depends on the number of mass degrees of freedom in the structure. Mode shapes describe relative displacement of the structure (mass normalized). Number of mode shapes and frequencies required depends on the frequency of the applied load. Mode shape #1 occurs at a frequency, that is the fundamental frequency or the 1st natural frequency. Each mode shape after that frequency up to the cut-off frequency are additional natural frequencies of the system. When looking at the frequency output report, the last column on the far right (Percent Average) provides the user with how much of the mode shape is contributing to the total captured mass of the system. As an iterative approach the modal analysis settings "cut-of frequency" in the model can be incrementally increased to achieve a suggested total of between 75% - 100% participation. As higher cut-off frequencies are entered there will be a point where the higher modes do not contribute as much to the total vs the lower modes. Therefore, no need to analyze with any higher modes as they are not contributing to the results. Item #2: What options are available to increase the participation factors in a model with multiple disconnected systems? Answer: The participation factors can be increased by increasing number of mass points so that missing mass at the supports is minimized. However, that may not be effective for all models. Another way to have more mass is to add more modes up to the limit of 999. Change the Modal analysis setting for number of modes to 999, before analyzing the model In large models with multiple disconnected systems (segments that represent other systems of piping not all connected in one group, check groups of segments by pressing Select Connected Segment provides a list of segment groups or disconnected segments) another more effective way is to reduce the model size, so that more modes can be analyzed in a smaller model. If needed, create a new model for each disconnect found in the original model. If some of these disconnected piping has a much larger frequency, then you will not get a single mode for that subsystem and the analysis may not be correct even if there model achieves 95% modal participation. For ZPA option, this options will essentially impose a static force just like static earthquake analysis and would apply to subsystems with no frequencies. Subdividing the model is more important for shock or water hammer type of loading since the loading have a higher frequency. Item #3: I have built a very simple cantilever model for verification of the calculation of natural frequency of a piping system. To verify the accuracy of AutoPIPE v8i frequency calculations, I used the following model: The pipe section chosen was 4" STD (sch 40) pipe with OD 4.5", ID 4.026", with no corrosion allowance and a density of 0 for the contents. The material chosen was API5L?B at a room temperature of 70 deg F. The model was built using a single element, with one end rigidly fixed in all directions and the other end free. The modal run resulted in a natural frequency of 32.6473 Hz. Please see attached output from AutoPIPE. Hand Calculation This simple model can be hand calculated using standard published formulas in closed form. The hand calculated natural frequency is 47.12 Hz as follows: C is a constant that is 3.52 for the 1st mode. E = 29.5x106 psi. I = 7.23259 in4, w = 10.78 lb/ft = 0.898 lb/in. g = 386 in/sec2. L = 60". ANSYS FEA Comparison I built a model of the same system with ANSYS 11.0. With this model, I get 47.22 Hz using a single element model as well that is identical as far as I can tell to the AutoPIPE model. Discussion I would expect the AutoPIPE natural frequencies to be lower than using an ANSYS beam model for common systems due to the flexibility of elbows since I don't believe that ANSYS models the elbow flexibility well. However, I would see no reason why the simple cantilever system doesn't match hand calculations Answer: The natural frequency formula assume a uniformly distributed weight. In AutoPIPE the weight is lumped at the nodes. So ½ the weight is placed on each end. In order to simulate the natural frequency, additional mass points can be added. Either actual or automatic points can be added. When the automatic mass point option is used (Tools Model Options Edit) with cut-off frequency set at 200 Hz, the first mode frequency will be 44.5. When the cut-off frequency is higher at 2000 Hz, the frequency becomes 46.4 Hz. 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!
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