| Applies To | |||
| Product(s): | AutoPIPE | ||
| Version(s): | All | ||
| Area: | Reports | ||
| Original Author: | Bentley Technical Support Group |
See 2 examples below on how a flange affects the Forces and Moments sub-report
Problem #1:
The results from the following 3 reports (Restraint, Global Force & Moments, and Reference) are all the same but when a flange is added they are different.
Given the following arrangement:
R E S T R A I N T R E A C T I O N S
Point Load FORCES (N )
name combination X Y Z Result
------ ----------------------- ------ ------ ------ ------
A00 Anchor Tag No.: <None>
Gravity{1} -160 -1024 127 1044
G L O B A L F O R C E S & M O M E N T S
Point Load FORCES (N )
name combination X Y Z Result
------ ---------------------- ------ ------ ------ ------
*** Segment A begin *** Line # 250VM-41676-FA01
A00 Gravity{1} 160 1024 -754 1282
R E F E R E N C E P O I N T L O A D S
Load FORCES (N )
Combination Actual Allow. Ratio
---------------------- -------- -------- ------
Tag No.: <None>
Ref. ID: A00-1 After A00, Seg A, loads applied towards A00]
Gravity{1} FX -160 5500 0.03
FY -1024 5500 0.19
FZ 754 2850 0.26
FR 1282 None
Problem: Restraint reaction does not equal the Global Forces & Moments report and Reference Point Load report, why.
Solution#1:
As can be clearly see the Reference Point Load and Force s & Moments report the same value. No issue.
The only issue of concern, why the Restraint loads do not match the other reports. The answer is because the flange weight has been added to the anchor point and not the pipe, 754 N - 127 N = 627 N (weight of flange + bolting).
(42.64 kg flange + 21.32 kg bolt) x 9.81 = 627 N
If you expect the pipe to take the flange weight, please insert a new node point a short distance offset from the anchor and move the flange to the new node point instead of placing it exactly on the anchor. Now all 3 reports will match.
Problem #2:
Why are the forces and moments for this report acting in the +ve vertical direction (Dy = positive Up).
Mainly that:
F00 = Force Y-axis = +180
F01 - = Force Y-axis = +150
F01 += Force Y-axis = +29
F00 = Force Y-axis = 0
How can this be possible?
Solution #2:
First refer to the following AutoPIPE help section:
Help > Contents> Contents Tab> Reference Information> Results Interpretation> Pipe Forces and Moments: Global Option
At any point in the segment, the cross section for pipe force output is obtained by removing all of the segment up to that point as shown below. The pipe forces and moments act on the exposed cross section. Continue reading further down on this page to understand why there maybe 2 different Forces and Moments reported about a single node point, specifically -ve and +ve faces.
Resolve Forces at F00:
Since F00 is the origin point of the segment there is only a +ve face to this node point and nothing would be removed as suggested in help above. Next step would be to create a COG report to see the total vertical weight of the system acting on this node point:
From the 1st COG report the total weight acting on Node point F00 = 179.64 (rounded up to 180). Therefore the free-body diagram at F00 would need a +ve force pushing up on the exposed cross section with a force equal to 180.
A reported value in Global Forces and Moments output report of +180 would be correct.
Resolve the forces on F01 -ve side:
Following the same procedure as performed at F00 above,
a. Remove everything before -ve F01
i. Delete pipe run F00 to F01
ii. For analysis purpose only, add an anchor at F01
b. Recreate another COG report to see the total vertical weight of the system acting on this node point
Compare the 2nd COG report to the 1st COG report, the only difference between these reports was due to pipe weight. The pipe weight was reduced by 1/2 the original amount. This would be expected because both pipe runs (i.e F00 to F01 and F01 to F02) were of equal length and had the same pipe properties. Drawing a free-body diagram on the exposed cross section of F01 -ve would need a force pushing up with a force equal to 150.32 (rounded to 150).
A reported value in Global Forces and Moments output report of +150 would be correct.
Resolve the forces on F01 +ve side:
Following the same procedure as performed at F01 -ve side above,
a. Remove everything before +ve F01
i. Delete pipe run F00 to F01
ii. Delete both flanges at node point F01 ().
iii. For analysis purpose only, add an anchor at F01
b. Recreate another COG report to see the total vertical weight of the system acting on this node point
Compare the 3rd COG report to the 2nd COG report the only difference between these reports was due to flange weight. This would be as expected because flanges are currently modeled as a single point load and both of them would have been inserted on the same -ve face of the node point. Again, as instructed, delete everything before +ve F01. Drawing a free-body diagram one the exposed cross section of F01 +ve face would need a force pushing up with a force equal to 29.32 (rounded to 29).
A reported value in Global Forces and Moments output report of +29 would be correct.
Resolve the forces at F02:
Following the same procedure as performed at F01 +ve side above,
a. Remove everything before F02
i. Delete pipe run F00 to F01
II. Delete pip run F01 to F02
iii. For analysis purpose only, add an anchor at F02
Performing these steps above would leave a model with one node point and no support which when trying to run an analysis would display the following error message:
---------------------------
Error
---------------------------
E516-11: Analysis terminated: model must have more than one point
---------------------------
OK
---------------------------
Also, no COG report can be created in a model that does not have at least 1 component entered. Therefore, a reported value in Global Forces and Moments output report of +0 would be correct.
Conclusion:
Again, AutoPIPE uses a consistent procedure to define the cross section. This procedure requires that all pipe segments have a segment direction. At any point in the segment, the cross section for pipe force output is obtained by removing all of the segment up to that point as shown below. The pipe forces and moments act on the exposed cross section with the remaining segment intact. Any difference between the +ve and -ve side of a node point may be due to:
- Pressure change
- Temperature change
- Pipe identifier change
- Flange points
- Supports
- Additional weight
- Concentrated force
- Tee point
- Uniform distributed load
Therefor, when +ve and -ve forces and moments are reported about a node point the summation of these 2 values may not equal 0.00.
For node points at a junction, the reported values in the "Forces and Moments" output report again may not sum to 0.00. However, by updating the model with the addition of reference points to isolate the junction. The report for gravity case should then sum to 0.00.