I am making a simple concrete box model in SAP2000. It is an underground concrete vault and sketch below shows elevation view. The top portion shows the actual dimensions and the bottom portion is what I think to make an analysis model. Is it the proper and best way to model to obtain proper member forces? The loadings are soil pressures from outside the vault.
Setting up the model according to your sketch would be reasonable. All area objects could be also modeled at their midplanes with a slight overlap at the corners. Both approaches would be adequate for obtaining global response, but it is important to remember that shell elements (by their very nature) are approximate and special case of three-dimensional elasticity.
If the wall thickness becomes large relative to the overall dimensions, or if you are interested in detailed results at the corners, using three-dimensional solid elements may provide more accurate results. It may however require some additional effort to setup the model and to interpret the analysis results when solid elements are used.