What is Spread Footing?
A spread footing is a type of structural component that acts as a base for a building's foundation. These components are constructed from concrete and are often reinforced with rebar or steel to add additional support. Depending on the size and configuration of the building, the footers can be buried just below ground level or several feet below the surface. In cold climates, they are always placed below the frost line to minimize problems with concrete heaving that occurs during freeze/thaw cycles.
Like all footers, a spread footing is used to help support the foundation or piers below a building. While traditional spot footings only have a single point of contact with the foundation, the spread variety extend continuously across the entire building footprint. Grade beams also extend across the entire footprint, but they tend to distribute loads to specific points, while a spread footing transfers the load to the entire ground, not a single point. This makes grade beams the best choice for very unstable soil, while these footers require the soil to be fairly stable all around.
This type of footer design is highly beneficial to builders and homeowners. Since they transfer the weight of the building over a large area, they have little risk of failure compared to spot footers. This design helps to minimize cracks and other signs of damage that occur as a building settles over time. By spreading the weight out in this manner, homeowners can often extend the life of their home while minimizing structural damage. These components can also help make up for spots of loose or unstable soil without the need for extensive excavation and filling.
The size and quantity of spread footers is typically determined by structural engineers or architects, who refer to local building codes for assistance with their design. A geotechnical engineer may be consulted to test the quality of the soil, which can determine if it is stable enough to support the proposed building. Typically, these footings are the same size as the foundation wall in terms of thickness, though they may vary widely in height, depending on soil conditions.
A spread footing may also be combined with spot footers, particularly on larger buildings. In this type of design, the spread component is installed under the entire foundation, which typically runs under only the exterior walls. Spot footers are then placed under individual points under the interior walls to help distribute heavy loads over a larger area.
It's always "footing", never "footer".
It is easy to take the foundation of a building for granted, but this is one of the most important structural components and often one of the biggest obstacles to building bigger taller buildings, or building structures of a certain size in a certain area. We don't often think about the quality of the soil beneath our feet. But for builders and engineers this is a constant concern. Soil varieties can differ greatly, and in some circumstances they make it next to impossible to build structures of scale. Concrete footing featuring concrete reinforcement is one tool that engineers use to overcome these obstacles.
@backdraft - It's true that architects exacerbate the challenges of structural engineers. Most of modern architecture was dominated by the international style. One of the hallmarks of this style was floor level entrances that were set in from the broader perimeter of the building. Think of a large square perched on top of a smaller square. This means that very large buildings often had to be supported by relatively small foundations. In circumstances like these, spread footing the foundation is invaluable.
@johnrss - You are right. Good example. Spread footing is a fairly old principle that has been updated and re imagined in the light of today's engineering and scientific breakthroughs. Builders have long known that structures are more stable if they sit flat across large surfaces of ground. But the ambition of today's architects makes thins principle difficult to put into practice. An engineer can spend an entire career just trying to find ways make buildings sit on the soil beneath them. It is no small feat.
The principle at work here is basically the same as when a person lies on a bed of nails. Imagine that the person is the building and the bed of nails is the foundation. If you have just one nail, the entire weight of the body is forced down onto that one point and the person is sure to be impaled. But if you have dozens and dozens of nails, the persons body weight is spread across these and they can lie on the spike bed with relative comfort.
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