How Slab-On-Ground Foundations Work

Slab foundations work by acting as a buffer between the supporting soils and the house structure

The purpose of a slab-on-ground foundation is to do two things:

• Transfer loads from the house to the supporting soil so that settlement is minimized. If the house was heavy enough and the soil weak enough, the foundation and the house could settle into the soil. Even poorly designed and constructed foundations are not susceptible to this type of excess settlement.
• Buffer or resist distortions in the supporting soil so that they do not cause structural damage to the house. Moisture conditions in the supporting soil is continually changing. The result is that the supporting soil swells as it absorbs moisture and shrinks as it gets dryer. The result is that the soil continually distorts. Slab-on-ground foundations protect the house from the moisture-induced distortions in the soil by spanning over them.

Slab-on-ground foundations work by bending and tilting

Slab-on-ground foundations are designed to be stiff, which is a structural engineer’s way of saying that they are difficult to bend.

Because they are stiff, they act as a buffer that mitigates the differential distortions between the supporting soil and house supported on the foundation.They resist the moisture-induced distortion of the supporting soil. The stiffness of slab-on-ground foundations and by spanning over moisture-induced distortions in the supporting soil.

If the foundation was not stiff, if it was flexible, any distortions in the supporting soil would be matched by similar distortions in the slab surface.

Slab foundations should remain reasonably level

The foundation is intended to do this while maintaining the surface levelness within permissible levelness tolerances. The levelness tolerances are dependent on the as-constructed levelness of the foundation surface and construction of the house.

The intent of the design protocol is for the foundation surface to distort within a range that:
• Does not result in significant structural damage to the house frame.
• Does not distort the frame structure so that numerous doors and windows cannot function.
• It is extremely important to understand that slab-on-ground foundations are not designed to eliminate the possibility of cosmetic damage or minor door problems

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