f. advanced nonlinear analysis - linear vs

6/29/2015 f.AdvancedNonlinearAnalysisLinearvs.NonLinearanalysisinAutoPIPEPipeStressandVesselDesignWikiPipeStressandVesselDesi

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f. Advanced Nonlinear Analysis -Linear vs. Non-Linear analysis inAutoPIPE

Applies To

Product(s): AutoPIPE

Version(s): 2004, XM, V8i

Environment: N/A

Area:

Subarea:

Original Author: Bentley Technical Support Group

Date Logged& Current Version

June 201509.06.02.06



Lets first introduce the topic of linear and nonlinear analyses.

Linear analysis denitionPipe properties are constantDeflections are smallElastic deformation (pipe goes back to original shape)

So for example for every pound of force you put in, you get 1 inch of displacement

Nonlinear analysis denitionPipe properties change over timeLarge deflectionsPlastic deformation can occur (pipe is permanently deformed)

So the relationship between how much force causes a set amount of displacement changesand that is why it is called nonlinear.



AutoPIPEs nonlinear analysis engine uses two methods to handle piping models. Thetangent stiffness strategy and the secant stiffness strategy which are both used by FEAprograms like ANSYS & ABAQUS.

The tangent stiffness strategy handles all the supports with their gaps. The secantstiffness strategy handles friction.

Friction:The total load approach (like most pipe stress software uses) doesnt understandboundary conditions and how the bearing force can change with the direction of thepipe movement which can possibly double the stress and load range.



For example the magnitude of the friction force as the pipe returns to ambientconditions after an operating condition will be the same magnitude but in the reversedirection and this could possibly double the stress and load range.

It is preferable to perform an incremental analysis (as is done in AutoPIPE) to ensureno extreme load is overlooked.

The incremental analysis approach is also the preferred approach of which follows theASME code philosophy, discussed later

In 2009 ASME published the book Pipe Stress Engineering by L.C. Peng & Alvin Pengwhich stated that the analysis method used by many pipe stress software packageswas incorrect and that the only valid method was used by programs like AutoPIPE.

This book mentions that there are 3 approaches to handling nonlinear analysis:General straight forward approach that many general purpose FEA programs use

Algebraic subtraction which our competitors like Caesar useOperating condition approach which AutoPIPE uses



The authors only recommend the operating condition method like AutoPIPEsbecause it meets the code philosophy and requirements where the other twomethods have flaws.

Why is Load Sequencing so Important?In a nonlinear analysis, the sequence of the loads is important

The principle of superposition does not apply in a nonlinear analysis, therefore, the starting pointfor a load case is important

The following is the default load sequencing in AutoPIPE

Gravity (GR) is analyzed with no initial stateThermal load cases are analyzed with GR as the initial load casePressure load cases are analyzed with the corresponding thermal load case as the initial load caseOccasional load cases are analyzed with GR as the initial load case

Load Sequencing Example 1 ResultsLets take a look at the thermal case as it hits the support. This figure shows the pipemovement from the gravity case to the thermal case. As we can see the pipe moves 0.2in the negative Z direction and hits the support.

Now we will take a look at the seismic case. This figure shows the pipe movement fromthe gravity case to the seismic case. As we can see the pipe moves 0.306 in the positiveZ direction.



And now we will look at the correct load sequencing load case which shows the pipestarting with gravity, (click) applies thermal case which hits the support gap, (click)then with the earthquake load moves to its final position which is 0.106 in the positiveZ direction.

See AlsoBentley AutoPIPE

External LinksBentley Technical Support KnowledgeBase

Bentley LEARN Server



Load Sequence AutoPIPE Non-linear Analysis

Comments or Corrections?Bentley's Technical Support Group requests that you please submit any comments you have onthis Wiki article tothe "Comments" area below. THANK YOU!

f. advanced nonlinear analysis - linear vs

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