Question: AutoPIPE: Dynamic Results InterpretationA. My question is how come the code compliance section does not include both the positive and negative dynamic results (ie GRTP# +/- R#)? Is there a way to do that?
B. Also, I am confused about how to interpret the local and global forces for the combinations which contain the dynamic support load. Why are the local forces for the dynamic load given as +ve?
TR Number: NoneProduct: AutoPIPEVersion: n/aArea: GeneralProblem ID#: 74535Keywords:R1 - R50 Response spectrum cases 1 to 50H1 - H10 Harmonic cases 1 to 10M1 - M50 Time history cases 1 to 50F1 - F10 Force spectrum cases 1 to 10S1 - S10 Seismic Anchor Movement cases 1 to 10
Answer: All dynamic results in AutoPIPE are positive. Since it is the maximum absolute value of the oscillating response.
By reviewing all of AutoPIPE's dynamic results, the following load cases are reported as positive numbers:
...R1 - R50 Response spectrum cases 1 to 50...H1 - H10 Harmonic cases 1 to 10...M1 - M50 Time history cases 1 to 50...F1 - F10 Force spectrum cases 1 to 10...S1 - S10 Seismic Anchor Movement cases 1 to 10
For more details on this subject please see the following AutoPIPE help section:
A. Help > Contents> Contents Tab> Reference Information> Results Interpretation> Select the "Results Interpretation Overview" link, at the bottom of the listing select "Dynamic Support Forces" link, read the entire section for complete details and workaround.B. Help > Contents> Contents Tab> Reference Information> Analysis Considerations> Modal Analysis> Modal Analysis.
Crate additional User combinations using -1.0 Factor with the Dynamic load case. For example R1 is defined as shown below with respective displacement of vertical pipe anchored at the bottom:
Create a new user combination as followed, NOTE: the -1.0 factor with respective displacement of same vertical pipe anchored at the bottom:
My structures have slightly different seismic displacements at different places on the same floor. I am designing a piping system spanning several independent structures which have different phases. It appears that the supports gaps are not recognized by the software. I have created a very simple model that should not fail because the line stop gaps are much larger than the imposed seismic displacements in he axial direction.
1. Are there various options/settings which I need to change to make the analysis work ?
2. Is there a work around ?
AutoPIPE's SAM (Seismic Anchor Movement, S1..S10) analysis is a Dynamic analysis and is subjected to all restrictions of a dynamic analysis primarily being analyzed as a linear analysis.
From the online help:
A linear analysis ignores all defined support gaps and friction coefficients, and assumes a linear soil stiffness response (k2 is ignored). In addition, the initial soil stiffness (k1) is used for the horizontal and longitudinal directions, whereas the average of the up and down initial stiffnesses is used for the vertical direction.
Unfortunately at this time there is no workaround for a S.A.M. analysis to consider a gaped support.
See the extract from the piping model. When we do the modal analysis, the response spectrum for (free) guide supports with NO Side Restraint (or big gaps) seems to be erroneous - showing no sideways movement under sideward force spectrum (and thus generating huge side loads). If the support is modeled as V-Stop , the behavior changes and is consistent with what is expected (moves sideways). . The above problem can be seen from a typical example by comparing results for S50, modeled as a guide without any side restraint and S52, modeled as a V-Stop. S50 behaves like a guide without any gap on side (no side movement in R1 & R2 response spectrum) We noticed the same phenomenon everywhere in the model and only a small extract of the whole model being attached for your check.
The behavior is as expected. In AutoPIPE and most stress analysis programs, dynamic analysis is linear. This means there is only one set of mode shapes and frequencies. When a support lifts off, stiffness and frequencies change. To make the system linear, AutoPIPE set all gaps and friction to zero. A guide as modeled with 3 gaps will be a fully restrained guide when performing dynamic analysis. To model this correctly is to use V-stop or incline support for horizontal restraints, not a guide. Notice that even for V-stop with gaps, there will be no lift-off during dynamic analysis as gaps are set to zero. To model friction, you can add an incline spring and estimate the stiffness based on the expected displacement and friction force. You would need to do a trial and error on this though.
Reports - Results Interpretation