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Could you explain the difference between thick shell and thin shell elements?
The two thickness formulations for area section, available 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 tend to be important when the thickness is greater than about one-tenth to one-fifth of the span. They can also be quite significant in the vicinity of bending-stress concentrations, such as near sudden changes in thickness or support conditions, and near holes or re-entrant corners. Even for thin-plate bending problems where shearing deformations are truly negligible, the thick-plate formulation tends to be more accurate, although somewhat stiffer, than the thin-plate formulation. However, the accuracy of the thick-plate formulation is more sensitive to large aspect ratios and mesh distortion than is the thin-plate formulation.
It is generally recommended that you 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 is recommended to use when the shear deformations become significant. The thick plate formulation captures both shear and bending deformations, whereas the thin plate formulation is based only on bending deformations and neglects shear deformations.
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.
I have area object with more than 4 vertices and I have not specified any meshing. However the analysis model shows mesh. How was this mesh created?
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It 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.
What is the difference between "Uniform (Shell)" and "Uniform to Frame (Shell)" loads
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