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Shallow Footing w Structural Slab vs Thickened Edge

Shallow Footing w Structural Slab vs Thickened Edge

Shallow Footing w Structural Slab vs Thickened Edge


Designing a frost protected shallow foundation home and for various reasons a shallow strip footing with stem wall and an internal structural slab is being considered next to a thickened edge slab. As a non-engineer - I am curious as to why the standard detail for a thickened edge slab has compacted granular beneath whereas a structural slab has void form underneath? Surely, the thickened edge slab is just as susceptible to clay swell, no? The subject site is in climate zone 6 and has a water table about 3'-0" below grade. Getting this right is imperative.

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RE: Shallow Footing w Structural Slab vs Thickened Edge

Geotech engineer not structural so I don't claim to be an expert in the actual design of the slab but:

A structural slab is designed and reinforced for the void beneath it. Loads are transferred to the footings or piles which are below the frost depth or zone of influence for shrink swell; the slab and the foundation are protected from the uplift forces by the void form which simply compresses as the clay swells and doesn't transfer any load to the footings.

Thickened edge slabs usually refer to slabs that are not structural (i.e. the slab is not spanning the void beneath it) so I suppose the granular base is simply an attempt at providing a uniform base for the slab. I suppose it's possible that you could design a 'thickened slab' and the structure it supports to be stiff enough to just sort of float around on the soil as it heaves and shrinks - if the heave was uniform enough and the service connections to the building flexible enough.

RE: Shallow Footing w Structural Slab vs Thickened Edge

They can be used for structural purposes, but not normally highly loaded, but can be. The big thing is that the supported object has to be free to move; if movement is not permitted, a 'real' foundation should be used.

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