What does a foundation actually do?

01

The idea

Soil is weak. Spread the load.

A column would punch straight into the ground. A wide footing spreads the same load over more soil.

02

Play with it

Make the footing wider or narrower, and change the load from the building above.

Try this · 0/3Stop the soil giving wayHold the heaviest loadFind the smallest footing for the starting load

03

The rule

Square footing

p = \dfrac{\force{N}}{\len{B}^2} \le q_{\text{soil}}

So the width you need

\len{B} \ge \sqrt{\dfrac{\force{N}}{q_{\text{soil}}}}

Real footings also check settlement, the footing's own weight and the concrete itself. This is the first check, and the one that sets the size.

04

Your turn

▲ Build1/3

To do

The soil takes 200 kPa. Pick a square footing that holds without wasting concrete.

= Solve2/3

To do

A square footing $\len{1.5}\,\mathrm{m}$ wide carries $\force{450}\,\mathrm{kN}$ . What pressure does it put on the soil?

A square footing $\len{6}\,\mathrm{ft}$ wide carries $\force{144}\,\mathrm{kip}$ . What pressure does it put on the soil?

◆ Pick3/3

To do

05 · Takeaway

Foundations spread the load.

p = \dfrac{\force{N}}{\len{B}^2}