Grades 3–5 · Machines
How can the ground hold up a whole building?
A building rests on more than the patch of floor we can see. Its weight travels through walls and columns to a foundation underneath. The foundation transfers that load into soil or rock, and the ground provides support in return.
In the diagramA foundation passes a building’s load to the groundBuildingFootingGround2× contact areaTwice the area: half the average push per areaIdeal average only; actual ground support varies.
One kind of foundation has a broad base called a spread footing. Picture the same downward push spread over a small patch or a large patch. The larger patch has less average push on each equal piece of ground. Twice the contact area means half the average pressure for the same load. The footing does not make the building lighter; it spreads the load.
Spreading is useful only when the ground can provide suitable support. Some sites need deep piles or shafts that transfer loads farther down, through their sides and ends. Engineers study the ground and how much it may move. The drawing shows the spreading idea, not a prediction that a particular building will stay still.
About this explanation
Starting knowledge: Understand weight, support and comparing areas.
Sources
- Federal Highway Administration — Foundation Engineering Project: Spread Footings
- Federal Highway Administration — Structural Foundations