What if buildings had different foundations?

Buildings are very heavy. They stand on a strong base in the ground called a foundation.

What if the foundation were tiny? Could the ground still hold the building up?

Every building rests on a foundation, the part underneath that sits on the ground and carries the building's weight down to it. You usually can't see it, but it is one of the most important parts of a building.

Foundations come in different sizes. In this lab, a 200-tonne building stands on sandy ground. Its foundation, or footing, touches 40 square meters of ground. What happens if you make the footing much smaller, just 8 square meters, without changing the building at all?

A shallow foundation (a footing) transfers a structure's load to the soil directly beneath it. The soil can safely carry only a limited pressure before it compresses too much or shears, so the footing's area must be matched to the load and to the ground.

This lab keeps a 200 t building on sand and changes only the footing area, from 40 m² to 8 m². It compares the resulting bearing pressure with the presumptive allowable pressure for sand used in the model.

Our building has a wide foundation. It spreads the weight over lots of ground.

The sand can hold it easily. The building stands straight and safe.

Press play and watch the building go up, floor by floor. As it gets heavier, the pressure on the ground grows.

With a 40-square-meter footing, the finished building presses on the sand at about 49 kilopascals. The model says sand can safely carry about 96, so the building is using only about half of the safe limit. Safe. The model also says any footing bigger than about 20.5 square meters would be safe here.

The 200 t building weighs 1.96 MN; spread over 40 m² that is 49.05 kPa. The model's allowable pressure for sand is 95.8 kPa (2,000 pounds per square foot, from the presumptive table in the Residential Code of Ohio, based on the International Residential Code), so the footing uses 51% of it.

The smallest safe footing is A = mg ÷ q_allowable = 20.5 m². The model's verdict is safe.

Now Olly makes the foundation much smaller. The same heavy building presses on a tiny bit of ground.

The sand can't hold that much push! In the model, the building sinks and could crack or tilt.

Shrink the footing to 8 square meters, one fifth of the area. The building weighs exactly the same, but now its weight is squeezed onto a fifth as much ground, so the pressure is five times as high: about 245 kilopascals.

That's about two and a half times what the sand can safely carry. The model says the ground would squash, so the building would sink and might crack or tilt. Watch the building go up: the pressure goes past the safe limit when the building is only about two fifths finished.

On 8 m² the bearing pressure is 245.3 kPa, 256% of the allowable 95.8 kPa. The model treats up to three times the allowable value as excessive settlement (cracking, tilting) and beyond that as bearing failure, so this case is rated 'settles'. During the five-second build, q passes the allowable value at 39% of the final mass.

The same 8 m² footing would be safe on crystalline bedrock (43% of 575 kPa), excessive on gravel (171%) and sedimentary rock (128%), and a bearing failure on clay (341%).

Push your hand flat on soft sand. Then push with one finger, just as hard.

The finger sinks in more! A small foundation is like the finger.

Pressure is how hard something pushes on each bit of area. It is the force divided by the area it is spread over. The same weight on a smaller area means more pressure on every square meter. That's why the sharp end of a needle can break the skin when a fingertip pushing just as hard can't.

Each kind of ground can only take so much pressure. Our model is simple: it uses one safe value for each type of ground from a building rulebook, and treats the weight as spread evenly. Its sinking and tilting drawings show the idea; it doesn't calculate how far the building sinks.

Pressure is P = F ÷ A, so at fixed load it is inversely proportional to area: one fifth of the area gives five times the pressure. Each soil class's allowable value is a presumptive one from a building code's table. The model treats it as a cautious limit, well below the pressure that would break the ground, which is why it has a band between 'safe' and 'gives way'.

Model limits: uniform pressure under a shallow footing; one value per soil class; no change with depth, groundwater, layering, footing size or depth; no settlement calculation; no deep foundations (piles). The Residential Code of Ohio uses its presumptive values only in place of a full geotechnical evaluation, and a building official can require a soil test where the soil is questionable.

In Italy, there is a famous tower that leans to one side.

It leans because its foundation sank unevenly into soft ground.

The Leaning Tower of Pisa in Italy shows what can happen when the ground under a building gives a little. Building started in 1173, and the tower was already leaning after its first three floors were finished. Britannica explains that the lean came from its foundations settling unevenly in the soft ground.

Engineers began a big project to steady it in 1990. They carefully removed earth from underneath the foundations, and the lean was reduced by about 44 centimeters. The tower reopened to visitors in 2001.

Britannica describes the Leaning Tower of Pisa, begun in 1173, as famous for the settling of its foundations: the lean became apparent in the late 1170s after three of eight planned stories, caused by uneven settling in soft ground. Later builders made new stories taller on the short side, but the extra masonry made the structure sink further.

A stabilization project begun in 1990 siphoned earth from beneath the foundations, reducing the lean by 17 inches (44 cm) to 13.5 feet (4.1 m) from the perpendicular; the work was completed in 2001 and the tower reopened. It is a real case of the settlement this model only describes in words.