| Applies To | |
| Product(s): | AutoPIPE |
| Version(s): | ALL; |
| Area: | Report |
| Date Logged & Current Version | Dec. 2017 11.02.00.08 |
Problem:
Why does the stress results change after adding just 1 extra node point on a bend or run of pipe?
Example:
When using the mid-point option on a bend noticed a significant stress difference. Why?
or
Adding node point(s) in pipe run
Solution:
Yes, the results may change by adding one or more node points to the model.
One should remember general information of AutoPIPE (see WIKI here):
Bentley AutoPIPE like all other stress programs uses 1-D finite elements which use simple Beam element center-line based theory, also known as the stiffness or displacement method, to mathematically model the piping system in three dimensional space. The system is formulated into a set of linear equations, describing the system characteristics at each degree of freedom (DOF). However, AutoPIPE differs from other applications because each point (e.g., piping, soil, beam, etc.) in an AutoPIPE modeled system has six (6) degrees of freedom. The exact formulation of these equations and their solution depends on the type of loading being investigated. For Example, AutoPIPE uses a form of Hook's law to calculate displacements and reaction forces based on pipe/anchor stiffness and the applied load
See example model below.
AutoPIPE will analyze the entire model and report results at node points. By Adding / removing node points to the model will calculate a new set of results, of which the maximum could now be higher than originally reported. Example, after looking at the displacement animation of this model imaged above, most stress engineers would surmise that stresses would be higher than currently reported between A02 and A03. Correct, after adding a node point at the center between A02 and A03, the reported stresses was found to be 138112 psi
Reasons why adding/removing Node points affects results:
a. All components modeled have their Mass lumped at node points, thus adding / removing node points will change the stiffness matrix of the model and ultimately the results.
b. For Dynamic Analysis, mass discretization will evenly distribute mass between node points instead of lumping. Consider using "Mass points per span" setting option on the Edit model options dialog (for complete details see help on this feature).
c. If soil properties are applied over the length where node points are to be added, these additional soil spring will aid in supporting the pipe which directly affect pipe displacement and stresses.
d. Non-Linear analysis is an iterative approach that relies heavily on Displacement, Force, and Friction tolerance settings to calculate results. Any changes in the model, like adding / removing node point may affect which iterative point was suitable to satisfy the equation will generate different results than the previous calculated values.
Conclusion, yes, adding/removing node points in a model can change the results.
See Also
"Code Compliance" sub-report using Results> Output Report