Slab on grade moment capacity by hand?

[StructureMan44]

Are there any resources that provide guidance on how to determine the moment capacity of a slab on grade without using computer FEM? In the past I have treated slabs (many more than 2.5' thick) as beams spanning between spread footings or increases in slab depth. This is extremely conservative because the slab has some level of support along it's span and because the top and bottom reinforcement often have nearly the same reinforcement resulting in a large decrease in moment capacity when using beam equations.

 

[SteelPE]

What exactly are you trying to do?

I have a reference that gives some equations for calculating post capacities for posts supported by these slabs, but these capacities are usually very low at about 10k-12k. These equations can be found in a book titled Designing Floor Slabs on Grade. If you are ending up with 2.5' thick slabs, it seems like:

a) your load is very large
b) you are actually have a structural slab system (maybe on piles?)

 

[Ron]

In most slab on grade applications, shear controls for loading. Why do you need to determine the bending moment? If you are looking at traffic loads, then you can relate bending to the modulus of rupture of the concrete.

 

[StructureMan44]

SteelPE: In this situation I am attempting to determine the moment capacity of an existing foundation. We will be mounting some large equipment on it which will produce a moment. Again determining the induced moment by treating the slab as a beam is very conservative.

Ron: In a situation like this is shear typically all that is checked?