AutoPIPE Wiki 03. Beam Structure modeling in AutoPIPE

03. Beam Structure modeling in AutoPIPE

Overview

The mass and flexibility of structural supports can have a profound impact on the piping stress analysis. In many instances, if the support structure deflects only a fraction of an inch, it can affect piping moments to the extent where hangers can be eliminated, and/or piping loads on equipment nozzles and supports can be significantly altered. In addition, consideration of structural support flexibilities changes the natural frequencies of the piping system.

Modeling pipe/structure interaction as part of piping stress analysis

Despite the fact that structural support flexibilities can have a significant impact on piping loads and stresses, most piping engineers do not consider these flexibilities in their piping stress, analysis. Typically, structural supports are modeled as simple supports such as rigid guides, vertical stops, line stops, etc. with no consideration given to the flexibility of the support itself. Although AutoPIPE allows engineers to input a stiffness as part of the support element, the piping engineer often does not have convenient access to structural support details needed to calculate support stiffness'. As a result, the support is typically treated as rigid.

As an alternative to using simple supports, AutoPIPE enables engineers to easily build the support structure into the piping model in order to consider pipe/structure interaction. The structural modeling is done within the piping model itself and takes advantage of AutoPIPE's graphical user interface, Using AutoPIPE's ability to graphically copy & paste between separate jobs or within the same job, users can now create libraries of common support structures and paste them into their new model in seconds. We will explain later AutoPIPE's unique abilities in defining the relationship between piping and structural steel.

AutoPIPE users are not burdened with going back and forth between separate programs or batch files to model support structures. Currently, AutoPIPE supports the American Institute of Steel Construction cross, sections as well as a material properties library for common structural steel materials. Once a cross section has been interactively selected, the user immediately sees the cross sectional properties with moments of inertia from the AutoPIPE structural database as well as the steel material properties. Nonstandard cross sections or cross sectional properties for International steel shapes can be entered as nonstandard or saved in the AutoPIPE database. AutoPIPE's structural steel analysis provides beta angle options, moment releases for pinned joints, and rigid end length options. Users see the structural model graphically as it is being built.

Once the support structure has been created, users can define the connection between piping and structural steel using one of AutoPIPE's 2 point supports such as a guide or a vertical stop. A user can, for example, define the support connection between node B10 on his piping system and node 21 on his support frame by connecting them with a guide from B10 to 21. From there, the user can place gaps in any direction in the event that the pipe would lift off or move, and can also input friction. The flexibility of the support structure, including friction loads, will then be considered as part of the piping analysis. In addition to piping analysis results, users can also review beam deflections, beam forces and moments.

AutoPIPE incorporates Timoshenko beam theory for calculating the stiffness matrices. The assumptions associated with using these type of beam element are:

i. Plane section remain plane but not necessarily perpendicular to the centerline of the beam
ii. Effects of shear deformation are included
iii. Only valid for small deformations (the cross section or properties of the beam are not affected by the deformations)
iv. Only valid for stresses under elastic range (no plastic deformation)
v. Modulus is considered the same in tension and compression

The accuracy of an analysis with combined beam and piping will depend on the assumptions made for the small displacement theory and whether these assumptions would cause an error which is with acceptable limits:

a. Displacement is small in comparison to the smallest dimension of the structure
b. Rotations are small and square of rotations is much smaller than unity
c. The strain is small and square of strain can be neglected
d. The equations written for the un-deformed configuration of the structure can be assumed to be valid for the deformed configuration

Comparison of the results for the simplest form of the problem (something like a cantilever beam), with results from another analysis incorporating more refined analysis method. If the error between the analysis results are within an acceptable limits, the results may be assumed to be within the limitations or within a range for which the results are acceptable.

Some specific technical references / resources discussing the above topics which are relevant to how AutoPIPE has specifically implemented its beam element are available in AutoPIPE's Help> Reference Information section>. SAP IV technical manual is available online, which acted as the basis for AutoPIPE analysis engine. The general implementation for the stiffness matrix for the Timoshenko beam theory is available in text books, consider online search for these texts..

Limitations:

For a complete list of beam limitations in a specific version of AutoPIPE, see the following AutoPIPE help section:

Help > Contents> Contents Tab> Bentley AutoPIPE> Command Reference> Insert command> Structure Group> Adding, modifying, and deleting beams

Note: for all other loading not listed under "Beam Members CANNOT" will be applied to beams structures. Also, recall that for some loads such as Static Seismic and Response spectrum, these loads are applied to Support and Anchor components. Therefore, like pipe, Beams and Beam structures will be affected by these loads through Support and Anchor locations.