What is the difference between Plate element , Shell element and Surface?
Plate element and shell element
Both terms represent the same thing in the STAAD context, which is, a 3-noded (triangular) or a 4-noded (quadrilateral) element to which a thickness has to be assigned as a property.
In STAAD, this element has both attributes - membrane (in-plane effect) and bending (out-of-plane effect). The bending effect can be shut off by declaring it as ELEMENT PLANE STRESS. The in-plane effect can't be shut off.
Plate Element and Surface
If you want to model a structure which contains a wall, slab or panel type component, you have two choices in STAAD :
a) Model that panel using a collection of individual elements. This is called a finite element mesh. This is an assembly of the 2d triangular and/or quadrilateral elements described above.
b) Model that as a single physical object called a Surface.
Option (a) is achieved using the mesh generation facilities in STAAD.
In option (b), (surface object), what happens under the hood is that, during the analysis, STAAD transforms the surface into a finite element mesh. The type of mesh (number of elements, type of elements, size of elements, etc.) that is generated from the surface is based on the parameters that you provide at the time of defining the surface.The details of the mesh thus generated are to a large extent, masked from the user. Results are presented for that surface, not for the individual elements that it is made up of.
In other words, a surface is merely an object that represents a collection of elements. When the program goes through the analysis phase, it subdivides the surface into a plate elements . From analysis point of view, both plate and surface are the same thing . The difference is in the interpretation of results . For plates , the stresses are reported while for Surface the force is reported. Another significant difference is that STAAD.Pro reports the in-plane moment MZ for surfaces but the same is not reported for plates.
In general it is recommended to use plates for modeling foundation slabs, elevated slabs, diaphragms and walls. Surface elements should only be used for modeling shear walls.