Soil Supported Slab Floor with Structural Footing
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Soil Supported Slab Floor with Structural Footing

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Soil Supported Slab Floor with Structural Footing.

This type of foundation is a hybrid between a soil-supported slab and piers or strip footing that support the main structural load. Therefore, it could be either a deep foundation with a shallow component for the slab or a shallow foundation.

This type of construction is frequently used for shopping centers, warehouses, and light structures and is sometimes used for residential construction.

For residential construction it works best for non-expansive clay sites. In expansive soil, because the foundation elements are disconnected, a plane surface across the floor of the house is difficult to maintain and stiffened or uniform-thickness mat may be more appropriate.

- When to use a soil-supported slab floor with structural footing.

Used in light commercial or industrial construction may utilize this procedure, sometimes in expansive soils. In expansive soil the main structural load, such as drilled piers, and the slab is soil-supported with consideration given to the amount of differential heave that may take place because of the clay soils.

The heave can be reduced by the upper clays being removed and replace with an inert fill material. In some cases, we have good bearing soils, such as a weathered rock, is available only a few meters below the surface, which may or may not have swelling clay above it. This condition would be a good candidate for this type of construction.

- Design considerations.

Engineer should know the use of the structure before designing this type of foundation. For example, in warehouse structure, the loading condition have high-pressured came from trucks or forklifts tire, and the floor slab must be designed as if it were a concrete pavement to avoid being damaged by wheel pressures.

The floor slab is typically placed on a layer of compacted fill. For warehouse or manufacturing buildings this may be 4 to 5 feet thick to provide a truck dock heigh construction. Because this is a thin, flat slab, it must rely entirely on the fill of support, and the fill should be well compacted and tested for density. The fill must be not consisting of expansive clays and to be inert. 

Since this type of construction is typically used for large areas, such as shopping centres, warehouses, or industrial complex, joint spacing or control of slab cracking due to concrete shrinkage is important. In addition, isolation joints around pier or footing tops or “plinths” should be supplied with expansion joint material providing for a slight bit of movement between the more rigidly establish footing top and the floor slab, which may tend to move a little bit. 

Concrete during shrinkage requires joint spacing ranging from 12 to 15 feet and is a function both shrinkage characteristics of the concrete and the amount of reinforcing steel in the slab, which must be determined by the foundation designer.

A rule of thump is that concrete slabs shrink due to drying about 1/8 inch per 20 horizontal feet. The contraction of the slab is resisted by subgrade friction, setting up tensile stresses. If these stresses exceed the concrete tensile strength aided by reinforcing steel, the concrete cracks.

- Installation.

Crack control joints must be tooled in during finishing, formed with a plastic strip during concrete placement, or swan with a diamond- bladed saw. Joints are formed from 1/8 to ¼ inch wide and extend about 10 to 25 percent of the slab thickness.

This “control” the shrinkage cracking to straight line. Joints are typically filled with an appropriate joint filler. If sawing is used it should commence very quickly, within 4 to 12 hours of the concrete finishing, or as soon as the green concrete can support the equipment and sawing will not ravel the concrete.

The concrete has already begun to shrink as it is losing water by evaporation, and it is not uncommon in this type of construction to see a shrinkage crack occurring one to two inches away from and parallel to the control joint because the joint was swan too late. This is a tricky problem for contractors and is frequently not done properly.

A construction joint is needed where a large slab concrete placement is interrupted, either by design or construction expediency. An expansion joint is one that permits the concrete to expand or contract and is commonly used in concrete pavement and bridge decks where temperature changes are large. Floors inside building exist in a controlled environment where temperatures do not fluctuate much, reducing the need for expansion joint.


Control of cracking and joint construction is important, especially if the slab is to be the finished floor surface, which may be stained or patterned concrete. Unsightly random shrinkage cracks will greatly detract from the architectural intent as well as provide a fractured area that can enlarge from wheeled traffic. This consideration is frequently overlooked, and it is difficult to find skilled contractors or designers who can produce this work properly.

For some industrial or warehouse applications, a hardener may be specified over the concrete surface to resist abrasion from high-pressure tire traffic or movement of heavy, skid-mounted loads. The manufacture’s recommendations should be carefully followed for the use of hardener. 


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