Spread Footings
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This leads us to the configuration of footings. Houses may have spread footings (strip footings) that support the perimeter walls. These footings are wide pads that are continuous around the perimeter of the house. In some cases, the pads may be widened and/or thickened to accommodate concentrated loads from fireplaces, pilasters, etc.
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Pilasters
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A pilaster is a thickening of a foundation wall. It may be thickened to receive the concentrated load of a beam resting on top of the pilaster, or it may be acting as a stiffener to prevent the foundation wall from bowing inward.
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Pad Footings
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Pad footings are similar to continuous footings except they are usually under a single pier or column. Pad footings spread the load out, usually in a square, with the column or pier sitting in the middle of the square. It’s common for houses to have strip footings around the perimeter and pad footings on the building interior under columns.
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Piles
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Piles are used instead of footings, typically where the soil quality is poor. They are, generally speaking, more expensive to install and have to be driven into the ground with specialized equipment. They can work one of two ways:
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Endbearing
Friction
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1. Piles can be driven down to a point where they bear on bedrock or other sound substrate.
2. Piles can be driven into soil far enough that the friction of the soil against the sides of the pile is enough to resist any downward movement.
Incidentally, if a house is supported on piles, they probably won’t be visible and you may not know it.
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Piers
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Piers are columns that may be completely concealed in the soil, or may project above. Most of you will be familiar with the piers that are commonly used to build exterior wood decks and porches. These piers may be poured concrete, often with the concrete poured into a cardboard cylinder in a hole dug in the ground. Piers usually, but not always, have footings. Piers can either be thought of as posts or columns, or can be thought of as short piles that bear on their ends.
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Grade Beams
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Grade beams are usually concrete beams that are supported on footings, piles or piers and are located at grade. In some cases, they extend below grade, and usually extend only slightly above grade. Grade beams transfer the loads from the building down to the footings or piles.
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Caissons
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Caissons are foundation systems created by drilling holes and filling them with concrete. A caisson pile is a cast-in-place pile that has a hollow tube driven into the ground. The earth is excavated from the tube, and concrete is poured into the tube. Some caisson piles are flared out at the bottom to create a larger bearing surface. These are sometimes called bell caissons.
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Grade Beams
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Grade beams are usually concrete beams that are supported on footings, piles or piers and are located at grade. In some cases, they extend below grade, and usually extend only slightly above grade. Grade beams transfer the loads from the building down to the footings or piles.
By now it should be clear that footings and foundations are –
- Important to the stability of the house
- Expensive
- Mostly out of sight
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Materials
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Footings and foundations should be strong so they can transfer loads and durable with respect to exposure from air, water, soil and insect attack. Most modern footings are concrete (sometimes reinforced.) Footings on older buildings may be brick or stone. While we won’t talk much about preserved wood foundation systems, these systems sometimes employ a wood footing.
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Foundations
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Foundations may be concrete, concrete block, cinder block, brick, hollow clay tile (terra cotta), stone (either dry laid or laid in mortar) or wood. Wood was common on very old buildings and has become common again to the extent that preserved wood foundations are used.
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Piles
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Piles are typically concrete, steel or wood. Again, you likely won’t see these.
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Piers
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Piers might be wood, concrete, concrete block, brick or stone.
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