Dynamic Analysis FAQ:
E. Force Spectrum
Note:
1. AutoPIPE's Dynamic analysis load cases including: Modal, Response Spectrum, Harmonic, Time History and Force Spectrum.
2. See more information about AutoPIPE's Dynamic Data Storage files - (*.SPC, *.TIH, *.THL, *.FS, *.HMF)
3. The only periodic force in AutoPIPE is harmonic analysis. To perform harmonic analysis, you would need to perform a Fourier series of the time function and compute harmonic amplitudes and phase angles. You would then enter these as harmonic force file HMF. Or import harmonic load from PULS.
4. All dynamic load are +ve (positive), how to consider -ve Dynamic Loads?
General Comments, Questions, and Answers
Item #1:
There is some confusion on how / why AutoPIPE closes the gap of all supports (gaps =0.00), Friction = 0.00, and soil properties are ignored when performing a dynamic analysis. why?
Answer: All AutoPIPE Dynamic analysis require that a modal analysis be performed. From AutoPIPE help-
Help> Reference Information> Analysis Considerations> Modal Analysis Theory
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. *
Furthermore, read introduction section of "Treatment of Support Friction in Pipe Stress Analysis" paper by Liang Chuan Peng. Mr. Peng wrote:
Support friction in a piping system can prevent the pipe from free expansion, thus creating a higher stress in the pipe and a higher load on the connecting equipment. However, in certain instances the friction can help stabilize the system and reduce damage. Even in dealing with pure thermal expansion, the friction can serve as guides thus preventing a large load from being transmitted to the rotating equipment. Therefore, there is no rule of thumb as to whether it is non-conservative to ignore the friction. In general, when dealing with the dynamic load, the friction tends to reduce the magnitude of both the pipe stress and the equipment load. In this case, the omission of the friction is conservative. However, there is no general rule governing the static load, in this case, the effect of the friction need to be investigated to simulate as closely as possible the real situation.
As mentioned, it is conservative to ignore friction for dynamic analysis.
* Note, an enhancement has been logged (867829) to add a feature that will allow Gaps / Friction / Soil settings will betaken into consideration during a dynamic analysis
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.
Fixed in AutoPIPE V8i 09.06.01.10 and higher
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 |
References:
'Dynamics of Structures' by Clough & Penizen is a good reference book
Goodling, E. C. 1989 Simplified Analysis of Steam hammer Pipe Support Loads.