Every structure transmits its weight to the ground. The soil's ability to support that load without excessive settlement or shear failure is called its bearing capacity. Get it wrong, and you get the Leaning Tower of Pisa — or worse, a catastrophic foundation failure.

What determines bearing capacity? Three main factors:

• Soil type: Rock can handle 100+ tons/ft². Dense gravel: 4–6 tons/ft². Stiff clay: 2–4 tons/ft². Soft clay or loose sand: as low as 0.5 tons/ft². Organic soils and peat are essentially unbuildable without remediation.
• Depth of footing: Deeper footings engage more soil in resisting shear failure. Terzaghi's bearing capacity equation accounts for this — capacity increases roughly linearly with depth for cohesionless soils.
• Width of footing: Wider footings spread load over more area, but the allowable bearing pressure doesn't increase proportionally because the failure zone deepens too.

The fundamental design check is simple: the pressure under your footing must stay below the allowable bearing capacity.

q = P / A

where q is the contact pressure, P is the column load, and A is the footing area. You need q ≤ q_allowable.

Worked example: A steel column carries 60,000 lbs. Your geotechnical report says allowable bearing pressure is 3,000 psf (pounds per square foot). Required footing area = 60,000 / 3,000 = 20 ft². A 4.5 ft × 4.5 ft square footing (20.25 ft²) works. In practice, you'd round to convenient dimensions and verify the concrete thickness handles punching shear — but the soil check comes first.

Two failure modes matter:

• General shear failure: Dense or stiff soil bulges up beside the footing and fails suddenly. This is the classical bearing capacity problem.
• Settlement: Even below the ultimate capacity, soil compresses under load. Building codes typically limit settlement to about 1 inch for isolated footings. On clay, settlement can take years as water slowly squeezes out (consolidation).

Why software engineers should care: If you're sizing mounting pads for outdoor equipment, designing bases for server room raised floors on grade, or reviewing structural drawings for a data center build, you'll encounter bearing capacity values in geotechnical reports. Knowing that "2,000 psf allowable" means you need 1 ft² of footing per ton of load gives you an immediate sanity check.

Rule of thumb: For preliminary sizing on typical suburban soil (medium-stiff clay or medium-dense sand), assume 2,000–3,000 psf allowable bearing capacity. Always verify with a geotechnical investigation before final design.