Spread footings

A wall delivers its load in a strip as thin as the wall itself. Soil cannot take that: pressed that hard it squeezes out sideways or compresses until the wall sinks. A spread footing is the widening at the bottom of the wall that fixes this. It is the lowest part of a shallow foundation, and its whole job is to spread the same load over enough ground that the pressure falls to what the soil can safely bear.

Every soil has an allowable bearing pressure: the most it may be loaded per unit of area. Divide the load by it and you have the area the footing needs. For a strip footing under a wall, work per foot of wall and the area is simply a width:

B = w ÷ q_aB footing width (ft) · w load per foot of wall (lb/ft) · q_a allowable bearing pressure (psf)

In the figure a wall carrying 3,000 lb per foot, plus about 580 lb of its own concrete, stands on clay allowed 2,000 psf. It needs a footing at least 1.79 ft wide, which rounds up to 22 in (560). Put the same wall on sand and the arithmetic asks for under 7 in, so a minimum proportion takes over instead. The weight of the wall and footing counts: in cold climates the wall runs deep and can add a quarter to the load.

These are presumptive values, conservative figures a code allows when nobody has tested the ground. Codes publish their own tables, often lower than these; on anything bigger than a house, a geotechnical engineer digs test pits or borings and gives a value for the site. Peat and other organic soils have no safe value at all: they are dug out, or the building is carried down through them on piles or caissons.

Light walls hardly need the formula. For light-frame buildings on stable, granular soil with loads under about 2,000 lb per foot (29 kN/m), the footing can simply be proportioned from the wall:

Wall thickness T

8 in (205) at least, for concrete or masonry carrying up to two floors.

Footing thickness

T, so 8 in (205).

Projection

T/2 each side of the wall, so 4 in (100).

Footing width

2T, so 16 in (405).

Push the footing wider and each projecting toe behaves like a short cantilever, pressed upward by the soil and bending about the face of the wall. Once a toe projects more than half the wall’s thickness, plain concrete is not enough: steel bars run across the bottom of the footing to take the tension. Longitudinal bars run along it in any case, against shrinkage and temperature cracks, with 3 in (75) of concrete between steel and earth. The figure thickens wide footings with their projection as a stand-in for the engineer’s check on shear; real sizes come from that calculation, with concrete of at least 2,500 psi (17 MPa).

Water in the ground expands as it freezes, and in silty, wet soil it gathers into lenses of ice that lift whatever rests above. A footing caught in that zone rises in winter and drops in the thaw. So the bottom of every footing is placed below the depth that frost reaches, the frost line, which ranges from nothing in the far south of the United States to five feet and more in the north. This teaching model carries the footing 12 in (305) below that line; a project needs its own foundation and frost-protection design.

Depth alone is not enough. The footing must bear on undisturbed soil, free of roots and organic matter. Where the excavation runs too deep, the ground is made up with engineered fill, compacted in layers of 8 to 12 in (205 to 305) at a controlled moisture content, never with loose spoil.

The lines under the footing are pressure bulbs: lines along which the extra vertical stress in the soil is the same fraction of the pressure under the footing, here 80, 60, 40 and 20 percent. The stress spreads sideways as it goes down and thins out. Under the middle of a strip footing it falls to about 55 percent at a depth equal to the footing’s width, 31 percent at twice the width, and 20 percent at three times.

The bulbs scale with the footing. Make the footing twice as wide at the same pressure and it stresses soil twice as deep. That is why a wide footing over a soft layer can settle more than a narrow one, even though its contact pressure is the same. As the soil under load consolidates the building settles: quickly and slightly on dense sand and gravel, slowly and further on clay. What matters most is that it settles evenly, so footings are sized to put the same pressure on the ground everywhere. Uneven, differential settlement is what cracks walls.

Foundation wall

Cast concrete or concrete block, 8 in (205) at least, standing 6 in (150) or more above finish grade under wood construction. It also resists the push of the soil against it.

Key and dowels

A keyway cast into the footing and steel dowels lock the wall down and resist sliding.

Sill and anchors

A treated 2×6 sill on a strip of sill sealer, held by ½ in (13) bolts set 7 in (180) into the concrete, no more than 6 ft (1830) apart and within 12 in (305) of each end of a sill piece.

Dampproofing

A bituminous coating from 6 in (150) above grade down to the footing; waterproofing where groundwater presses.

Footing drain

A perforated pipe at least 4 in (100) across, bedded in crushed stone with 6 in (150) over it and filter fabric on top, laid to fall to a storm sewer, dry well or open ground.

Finish grade

Sloped away from the wall at 5 percent or more, so surface water never reaches the drain at all.

Crawl space

The ground inside covered with a vapour retarder, the space ventilated to keep the wood dry.

A strip footing under a full-height basement wall is one answer among several. Where frost is shallow the same footing carries a short crawl-space wall or the thickened edge of a slab; where it is deep, insulation laid around a shallow footing can keep the frost out instead of digging below it; and where the soil near the surface cannot carry the house at any width, piers or piles reach down to ground that can. The foundation types figure puts the reference house’s wall on each of them and says when each is the right one, and excavation and backfill builds this footing in its hole, in order.