C-04 — foundations

where the structure meets the earth.

footing sizer

the soil decides the size.

The footing must spread the column load over enough area that the soil can handle it — weak soil means a bigger footing, or you go deep.

foundation type
A square pad under a single column. Area = P / q_allow.
column load
P (kips)200
soil capacity
q (ksf)3.00
column / footing
col (in)18
depth (in)24
—
footing size
—
live equation
adjust load and soil capacity to see foundation sizing
q = Contact stress between footing and soil: P/A
qa = Maximum soil pressure before failure
P = Total factored load on the footing
A = Required area: A ≥ P/qa
explained
The footing spreads a concentrated column load over enough soil area to keep bearing pressure below the allowable capacity. Weak soil requires a larger footing; strong soil or rock allows a smaller one. When shallow soils cannot support the load, deep foundations (piles, piers) transfer it to a bearing stratum below.
key concepts
overviewTransferring column loads into the ground

A foundation must transfer the column load into the soil without exceeding the soil's allowable bearing capacity. Weak soil means a bigger footing. When even a large footing isn't enough, you go deep.

bearing capacitySoil bearing capacity governs footing size

Soil can only support so much pressure before it shears or settles. This limit ranges from 1,500 psf for soft clay to 12,000+ psf for rock. A footing must be large enough that the applied pressure stays below this limit.

shallow vs. deepSpread footings near surface, piles to reach strong soil

Spread footings work when competent soil is near the surface. When it's not, you need deep foundations: piles driven to a hard stratum, or drilled shafts socketed into rock.