Applies To Product(s): AutoPIPE, Version(s): 2004, XM, & V8i Environment: N/A Area: Dynamic Analysis Subarea: Original Author: Bentley Technical Support Group Dynamic Analysis FAQ: Modal Response Spectrum Harmonic Time History Force Spectrum General Comments, Questions, and Answers Item #1: There is some confusion on how AutoPIPE closes the gap of a support in a dynamic analysis. It isn’t clear how this makes sense that the program will apply a support at a point where the pipe may not actually be moving enough lose the gap and to reach the support in reality. Any clarification on this? Answer : All AutoPIPE Dynamic analysis require that a modal analysis be performed. From AutoPIPE online help: AutoPIPE lumps the mass of the pipe, components and contents, etc. at the associated node point. This assumption yields a diagonal mass matrix with no mass coupling terms. There are three mass degrees of freedom per node. Rotational mass is ignored, except for points with eccentric weights, specified using the Insert/Xtra data/Weight command). At these points there may be up to three additional rotational masses and thus three additional mass degrees of freedom. It should be noted that for the eccentric weight, the coupling terms between the translation and rotational degrees of freedom are neglected. The structure is assumed to be linear and thus all gaps, friction and soil yielding is ignored. Item #2: Program should block dynamic analysis if static analysis set #1 not analyzed. Answer : This has been logged as a new enhancement under CAE-CR-10998. The internal setup of all the dynamic cases still rests with analysis set #1. If analysis set #1 is not analyzed with dynamic loads specified, your dynamic cases cannot be setup properly. Item #3 For V-Stop type support, like rod hanger, is AutoPIPE capable of deactivating the support in dynamic analysis in the analysis when it is active in static analysis? Answer : No, supports cannot be deactivated for a dynamic analysis. Only work around is to create a model with the support actually removed for dynamic analysis. Item #4 There are two types of global damping factor used in dynamic analysis of steel structures:1) % of critical damping and 2) % of structural damping. The default % value of critical damping used in Caesar II is 3%. Critical damping should be specified if the dynamic analysis is performed based on a combination of modal space (transformed from physical space to modal space through the natural frequencies and mode shapes) within the frequency range of interests. The structural damping coefficient of steel is typically 0.5%. This value is used when dynamic analysis is performed in physical space (the mathematical model represented by lumped mass and stiffness of piping members). We need to find out which type of dynamic analysis is being used. I think it is likely to be performed in modal space. Then I suggest to use 3% of critical damping - the default value used by Caesar II. If it is performed in physical space, then we should use 0.5% for structural damping. These values all stand for relatively small damping resistance from steel structures, which are known to have low damping resistance to dynamic loading. If there are special damping elements to help steel structure against dynamic movement, we should specify the damping coefficient for the specific element, not for the entire steel structure. Please note that we typically don't need to specify damping coefficient for snubbers, which should be considered as a rigid support during the quick pressure relief event. We believe Autopipe uses modal space. Can you, or your Time History Specialist, answer this question. Answer: Yes, a single damping ratio is applied to all modes. It has nothing to do with pipes or beams. Item #5: Dynamic load factor for safety valve transient open force has been given by ASME 31.1 Appendix II. We call it DLF ASME curve. And response force equals to gas exhaust reaction force plus DLF. In CAESAR, after input force-time curve, which is same as our Time History profile. Caesar can generate DLF spectrum. These spectrum are different from DLF ASME curve. Long duration and short duration generate different DLF curve. In AutoPIPE, it seems there is only ASME curve considered, and I can't find any result of those generated load factor with different period case in report. Could you give some instruction about this? Answer: This topic is covered in the official AutoPIPE Admin training class. Please contact your account manager for training options and schedule. Item #6: AutoPIPE's Solutions to Piping Vibrations: Types of Vibration AutoPIPE Solution 1. Mechanical: Harmonic Displacement Analysis A. Machinery Unbalanced Forces and Moments B. Structure Borne Vibrations 2. Pulsation: Harmonic Force Analysis A. Flow induced vibration due to compressors and pumps 3. Pressure Surge / Hydraulic Hammer Force Time History 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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