Lessons · Engineering · What the ground will hold
What the ground underneath will actually hold
Soil has a bearing capacity: a pressure above which it fails or settles unacceptably. A foundation spreads a column's load over enough area to bring the pressure below it. Required area is the load divided by the allowable bearing pressure.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
Structures are rarely limited by the concrete in the pad. They are limited by the ground, which nobody designed and which varies across a site -- and the only way to know is to test it.
How to think about it
Take the load at the base of the column and the allowable bearing pressure the problem gives, divide to get the area, then choose plan dimensions that provide it. Keep the pad square unless something stops you.
Worked example
Column load 900 kN, allowable bearing 150 kPa.Both given.
Area = 900 / 150 = 6 square metres.Load over pressure.
Square pad: side = square root of 6 = 2.45 m.Round up to a buildable size.
Allowable pressure comes from investigation, not assumption.It varies across one site.
Your turn
A column carries 600 kN onto soil with 200 kPa allowable bearing. Write the required pad area in square metres.
600 / 200 = square metres
Solve one, graded on the server
The trap
Using an allowable pressure from memory or from another site. It is a measured property of this ground and it changes within a single plot.