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This page is devoted to *frequently asked questions* (FAQ) related to [shells|kb:Shell].
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h1. General

General frequently asked questions are discussed as follows:

h2. What are the practical limits on the maximum thickness of shell elementsobjects relative to the shell dimension?

When using the thick plate*Answer:* During [shell|kb:Shell] formulation, both the bendingratio andof theplan sheardimension deformationsto are accurately (withinthickness concerns the assumptionsdeformation ofspan thebetween formulation)inflection accountedpoints, for.and Inot havethe attachedactual verificationplan exampledimension 2-012of inthe which 0shell.1 inShell xthickness 0.1may ineven shellbe elementsgreater withthan 0.5the inactual thicknessplan aredimensions usedso andlong as the solutionprojection obtainedof fromcurvature SAP2000under exactlyplate-bending matchesbehavior independentmeets solution.the deformation-span Asto athickness generalratios rule,which onefollow:
would
expect that thickThick-plate effects would become importantsignificant when the deformation-span to thickness ratio is between aboutapproximately 20:1 toand 10:1,. andThe theformulation adequacyitself of the formulation would be goodis adequate for a ratio of down to about 5:1 or 4:1.

NoteVerification thatexample this2-012 is theattached. spanThis ofexample deformationmodels wea are0.1in talkingx about here0.1in Asshell theobject elementswith are meshed, the elements may actually be thicker than the plan dimension, and that is OK. The important thing to consider is the ration of the span of deformation to thickness.

However, please note that all shell elements are approximate and a0.5in thickness. The solution obtained using [SAP2000|sap2000:home] matches that from the independent solution, validating the formulation and demonstrating the accuracy of shell behavior.

Please note that shell formulation is an approximate and special case of three-dimensional elasticity. Depending on your specific needs, shell elements Shell objects may be appropriate for some applications, while [solid|kb:Solid] objects may be appropriate,more butsuitable for someothers, othersuch typesas of analyses (such aswith the assessment of locallocalized behavior). youSimple maytest obtain more accurate response by using solid elements. You can always set up simple test problemsproblems may always be run to check the differencedifferences between different modeling[modeling|kb:Modeling techniques] approaches.


h2. CouldWhat you explainis the difference between thick shellthin and thinthick shell elementsformulations?

*Answer:* The twoinclusion thicknessof formulationstransverse for area section, availableshear deformation in SAP2000, determine whether or not transverse shearing deformations are included in the plate-bending behavior of a plate or shell element:
* The thick-plate (Mindlin/Reissner) formulation includes the effects of transverse shear deformation
* The thin-plate (Kirchhoff) formulation neglects transverse shearing deformation

Shearing deformations tendplate-bending behavior is the main difference between thin and thick [shell|kb:Shell] formulation. Thin-plate formulation follows a Kirchhoff application, which neglects transverse shear deformation, whereas thick-plate formulation follows Mindlin/Reissner, which does account for shear behavior. Thickness formulation has no effect upon membrane behavior, only plate-bending behavior.

Shear deformation tends to be important when theshell thickness is greater than about one-tenth to one-fifthapproximately {^}1{^}/{~}5{~} to {^}1{^}/{~}10{~} of the span. They can of plate-bending curvature. Shearing may also be quitebecome significant in the vicinitylocations of bending-stress concentrations, suchwhich asoccur near sudden changes in thickness or support conditions, and near holesopenings or re-entrant corners. Even for thinThick-plate bendingformulation problemsis wherebest shearingfor deformationssuch areapplications.
truly negligible, the thick
Thick-plate formulation tendsis toalso berecommended more in general because it tends to be more accurate, althoughthough somewhatslightly stiffer, thaneven thefor thin-plate formulation bending problems where shear deformation is truly negligible. However, the accuracy of the thick-plate formulation is more sensitive to [mesh|kb:Meshing] distortion and large aspect ratios, and meshtherefore distortionshould thannot is the thin-plate formulation.

Itbe used in such cases when shear deformation is generallyknown recommendedto thatbe yousmall.
use
the thick-plate formulation unless you are using a distorted mesh and you know that shearing deformations will be small, or unless you are trying to match a theoretical thin-plate solution. The thickness formulation has no effect upon membrane behavior, only upon plate-bending behavior.

As a general rule, the contribution of shear deformations becomes important when the span to thickness ratio is about 20:1 to 10:1, and the adequacy of the formulation would be good for a ratio of down to about 5:1 or 4:1. Note that this is the span of deformation. As the elements are meshed, the elements may actually be thicker than the plan dimension, and that is OK. The important thing to consider is the ration of the span of deformation to thickness.


h2. I have area object with more than 4 vertices and I have not specified any meshing. However the analysis model shows meshIn general, the contribution of shear deformation becomes significant when ratio between the span of plate-bending curvature and thickness is approximately 20:1 or 10:1. The formulation itself is adequate for ratio down to 5:1 or 4:1. In that this ratio is dependent upon the projected span of curvature, shell thickness may be greater than the actual plan dimensions of a shell object.


h2. How has my area object been meshed before any meshing is specified?

*Extended Question:* I have an area object with more than four vertices for which meshing is not specified. However, this object is meshed in the analysis model. How was this mesh created?

If no auto-meshing has been assigned to an area object that has been drawn using more than 4 nodes, the program will use general meshing tool to mesh such areas created?

*Answer:* {new-tab-link:http://www.csiberkeley.com/}CSI{new-tab-link} Software uses a general [meshing|kb:Meshing] tool to mesh objects with more than four [nodes|kb:Joint] when no auto-meshing has been assigned.


h2. What is the difference between the load assignments "Uniform (Shell)" and "Uniform to Frame (Shell)" loads

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When using the "?

*Answer:* The difference between these two options is as follows:

* *Uniform (Shell)" option, the* -- uniform loadsloading areis applied directly to thea [shell elements and are transferred|kb:Shell] object, then loading transfers to the structure viathrough theshell [joints of the shell element|kb:Joint] which coincide with structural members.

When using the "* *Uniform to FramesFrame (Shell)" option, the* -- uniform loadsloading areis applied directly to the [frame|kb:Frame] elementsobjects definedspecified along the edges of thea shell. underLoad consideration.distribution Youmay can definebe one way or two way load distribution. Please note that you can review how the "Uniform" to Frames loads are distributed to the frame elements in your model by using " two way. Distribution of loading may be reviewed through Display > Show Load Assigns > Area" menu command..

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h2. How canis I modela simply -supported slab modeled using shell elementsobjects?

Please*Answer:* For response, please see the [Modeling simply supported shells|tutorials:Modeling simply supported shells] tutorial.


h2. How can I post-process area output (thethe bridge-diaphragm area-object is usedoutput to model bridge diaphragm) to getobtain meaningful design forces?

*Answer:* YouDiaphragm coulddesign obtainforces diaphragmmay designbe forcesobtained from the [joint|kb:Joint] forces of the shells within the [shell|kb:Shell] objects used to model the bridge diaphragms. These shellThrough post-processing, joint forces may wouldbe needtransformed tointo bebridge transformedresponse toat the location of interest. Since Pleasethis notebasically thatsummarizes the programprocess uses this procedure to obtainfor generating section -cut forces; using section cuts, it may be more productive to obtain the design forces would, therefore, be another alternativethrough [section cuts|kb:Section cut].


h2. How are stresses for the shell elementsstresses calculated?, Canand Ican comparethese thestresses calculatedbe stresses directlycompared to the allowable stress of shell material?

*Answer:* Yes, the [shell|kb:Shell] stresses obtained from SAP2000 can be directly comparedusing [SAP2000|sap2000:home] may be compared directly to the allowable stress of the shell material., Youas canthis displayis the shellessence stressesof byAllowable right-buttonStress click on the shell elements when theDesign (ASD). Shell stresses aremay displayed or by reviewing the stresses in abe reviewed in tabular format viathrough "Display > Show Tables > ANALYSISAnalysis RESULTSResults > Element Output > Area Output > Table: Element Stresses - Area Shells".

The stressesStresses are calculated as athe product of the stress-strain constitutive matrix and the strain vector which is obtained from joint displacements. The procedure is not based on a simple formula, but rather on several techniques described in the following references cited in the {new-tab-link:http://www.csiberkeley.com/}CSI{new-tab-link} [_Analysis Reference Manual_|doc:CSI Analysis Reference Manual][joint|kb:Joint] displacements. The techniques and formulations involved are adopted from the following publications:

* Ibrahimbegovic, A., Wilson, E. (1991). [A Unified Formulation for Triangular and Quadrilateral Flat Shell Finite Elements with Six Nodal Degrees of Freedom |http://onlinelibrary.wiley.com/doi/10.1002/cnm.1630070102/abstract?systemMessage=Wiley+Online+Library+will+be+disrupted+on+26+May+from+10%3A00-12%3A00+BST+%2805%3A00-07%3A00+EDT%29+for+essential+maintenance]. _Communications in Applied Numerical Methods, 7_(1), 1-9.

* Taylor, R., Simo, J. (1985). Bending and Membrane Elements for Analysis of Thick and Thin Shells. _Proceedings of the NUMEETA 1985 Conference_, Swansea, Wales.

A comprehensive description can also be found in Dr. Edwardin LDr. Wilson’s booktextbook:

* Wilson, Dr. Edward L. _Static & Dynamic analysis of Structures_. 4th ed. Berkeley: Computers and Structures, Inc., 2004. Print.
Available for purchase on the {new-tab-link:http://www.csiberkeley.com/}CSI{new-tab-link} Products > {link-window:href=http://orders.csiberkeley.com/SearchResults.asp?Cat=2}Books {link-window} page

Additional details may be found Seein pagethe [Shell element formulation|kb:Shell element formulation DRAFT] for additional detailsarticle.


h2. How does the programsoftware apply stiffness modifiers forto shell elementsobjects?

*Answer:* When [stiffness modifiers|kb:stiffnessProperty modifiermodifiers] are applied, the corresponding terms in the [stiffness matrix] of the shell elementstiffness willmatrix getare modified. YouThis couldis alsocomparable lookto atmodifying, thisfor asa reducing (or increasing)given direction, the [Young's|kb:Young's modulus] Modulus or the shear moduli of the material for the given directions. The structural stiffness matrix for the entire structure is then assembled, and theglobal equilibrium equations are solved to find theto unknowngenerate displacementsdeflections. TheThese displacements and arerotations then usedrelate to strain fields calculatewhich theproduce internal member forces and stresses.


h1. Layered Shell Element shell object

Frequently asked questions associated with a [layered shell|kb:Layered shell] object are discussed as follows:

h2. CanFor youunsymmetrical explainlayering, thehow coupling ofare membrane and plate behavior for nonsymmetrical layeringbehaviors coupled?

*Extended Question:* The CSI Analysis Reference Manual states: that "Unlessunless the layering is fully symmetrical in the thickness direction, membrane and plate behavior will be coupled.". If the element is defined such that planes remain plane in the thickness direction, then even for a symmetric definition of membrane layers, would not the layers would contribute to the out -of -plane bending stiffness and seedemonstrate stresses when there is rotations in the plate element. Am I missing something herewith plate behavior?

*Answer:* YouThis areis correct that the membrane layers contribute to bending stiffness. The statement here regardsconcerns whether or not transverse loading generates membrane forces/deformation, and likewise whether or not in-plane loading generates bending. The membrane forces and deformation, and likewise, whether or not in-plane loading generates bending behavior. As stated, in-plane deflection will not produce plate bending when a [layered shell|kb:Layered shell] is symmetric through its thickness.

This coupling effect could be demonstrated by applying a uniform temperature load to the [shell|kb:Shell] elementobject. For symmetrical layering, the temperature load will cause only in -plane deformations. For nonsymmetrical layering, out-of-plane deformations will be generated. ThisAs illustrated isin similarthe tofollowing bi-metalWikipedia striparticle, illustratedthis inis thissimilar Wikipedia article: [to {new-tab-link:http://en.wikipedia.org/wiki/Bi-metallic_strip]


h2}bimetallic strip{new-tab-link} behavior.
Could
you
pleaseh2. explainHow how thedoes material orientation is relatedrelate to the elementobject orientation?

*Extended Question:* ItHow is not clear how the material orientation and the S11, S22, and S12 orientation are defined for theshell elements.objects? Are the 1, 2, and 3 directions the same as the local axes forof the shell? element,A thatmaterial canangle beof seen90 asdegrees red, white, and blue colors on the screen? In the "Nonlinear Shear Wall" movie, is used for longitudinal bar elements, ain materialthe angleNonlinear ofShear 90 degrees is used and then the Wall {new-tab-link:http://www.csiberkeley.com/support/watch-and-learn}Watch & Learn{new-tab-link} video, then nonlinearity is defined in the S11 direction, which is the horizontal direction for the element. Is Thisthis suggestscorrect that internally, the programsoftware is rotatingrotates the local axis, for that layer, by 90 degrees relative to the element local axis. Is that the right interpretation? Would it have the same effect to have the material at zero degrees, but define the nonlinearity in the S22 direction? On the other hand, if the angle issimply just referringrefers to material orientation, should not the nonlinearity be defined in the S22 direction, which is the nonlinear direction?

*Answer:* As shown in our the figures below (the first one was taken from our manual, while the
second
one was taken from the video)*Answer:* As shown in Figure 1, the material orientation inof a given layer is defined relative to the material local coordinate system. Material byangle specifyingis ameasured materialcounterclockwise angle.from Usinglocal materialmember angleorientation. ofUsing 90a degrees90° andmaterial specifyingangle thewith nonlinearity inalong S11 direction has the same effect as using material angle of 0 degrees and specifying the a 0° material angle with nonlinearity inalong S22.
direction.
For such uniaxial materials likeas rebar, whosewhere behavior is only defined in the material 1 direction, you must use the former specification a 90° material angle with nonlinearity along S11 must be specified. For fiber-wrapped composites, youit may be wantbest to specify material behavior atalong other someanother angle, sayperhaps 45at degrees,45° tofrom the elementmember local axes.

Please Notenote that while material properties are specified in material coordinates, loading and the output forces/ and stresses will always be ininput theand elementreported axes,in eventerms thoughof youmember may specify material properties in the material coordinateslocal axes.

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!Shell_section_material_angle.png|align=center,border=0!

{center-text}Figure 1 - Shell-section material angle{center-text}

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!Shell_section_layer_definition.png|align=center,border=0!

{center-text}Figure 2 - Shell-section layer definition{center-text}


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* [27953: Explanation of shell forces and stresses convention|$4161679]
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* {email:date=1/27/2011|from=am|to=qn|subject=Good explanation of thin, thick and layered shell element formulation|comment=|id=6881428}
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