a) 2D- Tria, Quard (single, or 2nd order)
b)3D- Tetra, Brick (single or 2nd order)
a). Shell Structural:- SHELL 63, SHELL 93 (93 does not support CTOP & CBOT features)
2. Solid Structural:- SOLID 45 (single order), solid92 (2nd order)
When third dimension of component is fairly high as compare to other two dimension (I section beam), meshing with a shell element is a best option. For meshing with shell element we required to take mid surfaces. Some times either outside surface/inner surface itself is sufficient depends upon the connectivity you needed with other element. Than you just mesh it with shell element and provide thickness to it.