| Applies To | |||
| Product(s): | AutoPIPE | ||
| Version(s): | ALL | ||
| Area: | Report | ||
| Original Author: | Bentley Technical Support Group | ||
| Date Logged & Current Version | June 2015 09.06.02.06 |
Problem:
When reviewing the load on a gaped support, there is a load in the same direction as a large gap. The pipe never closes the gap, so why is there a load on the support in that direction?
Solution:
If you are running a non-linear analysis, 9 times out of 10 the answer is due 1 of 3 reasons: Nonlinear analysis, Friction, or Dynamic analysis rules.
1. Nonlinear analysis
If the model has gaps, friction, or soil settings configured in the model, be sure that the Analysis set is running as a Non-Linear Analysis. If the analysis set is analyzed as a linear analysis, then all gaps. friction, and soil settings are ignored. Please review AutoPIPE help for details on Linear analysis vs Non-linear analysis.
2. Load sequence
Understanding AutoPIPE's "Load Sequencing" concept is necessary in order to comprehend the results for specific load combinations; for more information, visit these WIKI pages:
1. Non-linear Load Sequencing Explained
2. Non-Linear Analysis - Gap Support review in AutoPIPE
As stated in WIKI pages above, strongly advisable to not print all of the individual non-code combinations, instead print only the operational circumstances (GrT1P1, GrE1, GrT1W1, etc.). When attempting to comprehend the Why behind an operating combinations, then print and review the individual load cases that make up a specific operating condition.
Example:
Guide support with the following gap settings:
Stiffness = Rigid
Gap Up = 10 inches
Gap Down = 0.00 inches
Gap Left and Right = 0.5 inches
Friction = 0.0
Results at guide support::
How does AutoPIPE arrive at these results?
Step #1: Confirm analysis set is run as a Linear or Non-Linear analysis
Review output Analysis Summary for the correct Analysis Set.
Step #2: Review Analysis set Load sequence
Again, review output Analysis Summary for the correct Analysis Set.
Sequence is Gr - T1 - P1 - E1
Step #3: With full understanding of load sequencing, apply the loads and review the results in sequence.
Note: the results below will concentrate on the vertical axis, because the gaps in left / right never close,or cause a load in those directions.
a. First load case in the load sequence is GR. As defined above, the support at this point does not have any gap down. Therefore the gap is closed and loading is immediate with any downward loading. From the results report the valve is bearing down on the support with a load of -25268 lbs
Next, Thermal case T1 is applied. Starting point is based on the end point of the previous load case, GR. Thermal 1 cause the pipe move vertically up 7.5024019e-07 inches (value retrieved from results grid after selecting cell with 0.000 value). The slight upward movement reduces the load on the support by 7502 lbs. Note, the loading value is +ve because of the upward movement. Again, with the upward movement the load reduction must be +ve up.
However, at this time, the loading and displacement of the support is:
(note results from Gr (-25268) are added to the results from T1 (7502) = -17766 lbs)
Next load in the sequence is Pressure. Again the starting point of this load case is the endpoint of G+T1. P1 load case causes the pipe to move vertically down -1.0754208e-06 inches. Note, the loading value is -ve because of the downward movement. Again, with the downward movement the load increase must be -ve down.
Once more, the results at this time is cumulative, Gr+T1+P1,
(-25268) + 7502 + (-10754) = -28520 lbs
Next load in the sequence is Earthquake. Should be ingrained by now, but worth repeating again, the starting point of this load case is the endpoint of G+T1+P1. E1 load case causes the pipe to move vertically up +.024 inches. Keep in mind by moving vertically up this distance the pipe is no longer in contact with the support. In order for this to happen the pipe must overcome a vertical force of its starting point loading and displacement.
Note, the loading value is +ve because of the upward movement. Again, with the upward movement the load increase must be +ve up.
Once more !, the results at this time is cumulative, Gr+T1+P1+E1,
(-25268) + 7502 + (-10754) + 28520 = 0.000 lbs
Conclusion, one only needs to review and design with loading / displacement results of operating conditions, ex. GrT1P1E1. The individual load cases should only be reviewed when questioning operating condition results. The individual load case results help the designer understand contributing factors in a particular combination, and vastly increase the length of a complete output report.
3. Friction
Another reason for forces in gapped direction would be directly attributed to Friction. Perform the following test: for each support in question change the Friction to 0.00 for no friction. Now re-run the analysis and check the results. The force should now be removed in the gapped direction. Why you ask? because one or more gaps may have closed. Example: guide support with the following gap settings:
Stiffness = Rigid
Gap Up = 100
Gap Down = 0.00
Gap Left and Right = 0.1
Friction = 0.3
Look at the Support output report for all operating condition (ex. GrT1P1E1)
Force: fx = 944 (axial), fy = 250 (up), fz = 2181 (left)
Movement in local coordinate: dx = 1.63 (axial), dy = 0.94 (up), dz = 0.1 (left)
Because the left gap on the support closed there is a friction force as the pipe is moving up. However, when performing the test above, there is no friction and thus free to move up therefore force in the up direction would then be 0.00 as expected.
4. Dynamic analysis rules
Remember that all dynamic analysis depend on Modal Analysis results and are subject to the rules of performing a Modal Analysis.
Note: if using a guide support where 2 opposite directions have large gap settings. Highly suggest to model using an Incline support instead of a Guide support.
See Also
"Support" sub report using Results> Output Report in AutoPIPE