The Leaning Tower of Pisa (Peterborough)

Public summary: 

Explore this famous Italian Monument while learning about centre of mass, moments and building foundations.

Looking at leaning towers, moments and centres of mass.
Useful information
Kit List: 

Sponge with small holes in one side and large holes in the other.
(the best way to make this is to soak in water, freeze it then drill, clamping between waste wood also could work)
Weighted tower segments
Small dowel segments

Packing Away: 

has a mini blue box


This experiment was inspired by the Institute for Continuing Education (ICE) Peterborough.
Firstly we'll try and place the tower segments and see what happens, you should find it starts to lean after a few segments are placed. Why might this be a concern? We'd rather our buildings didn't fall over! What might we do to try and prevent this?
- Think about where we're building
- Examine the soil
- Build supports (Where?)
- Plan out the weight distribution of the building
Looking at this set up which side do you think has the better 'soil'. The big holes have a lot less material there to support the tower so we'd suspect ground like this to be less suitable for building on. However if we avoided all patches of ground like this we wouldn't be able to build in many places.
When we continue building up the tower leans more and more, but for a while it doesn't fall over. To see why we need to think about moments and centres of mass. Gravity is going to be the force acting on the building, we can imagine gravity pulling straight down through a single point which we call the 'centre of mass'. However we might not mean literally the centre, a seesaw gets pulled down when there are unequal weights on it so gravity is pulling through a point on the heavier side. This point takes into account where the heavy parts of an object are and is biased towards here. Most of the tower pieces have a centre of mass slightly biased towards the bottom because that's where the weights were added however very close to the centre of the circle.
So we now know where the force is acting but what about falling over. The tower is going to pivot as it tips over (like a seesaw) The weight on the seesaw is gravity acting through the centre of mass and the pivot is going to be a point on tower underneath the lean. To decide if it's going to tip we need to check which side the weight (centre of mass) is on. This means the tower can lean without falling over until the centre of mass passes beyond it's base.
How can we try and keep it with in the base. There are lots of options.
- make it heavy at the bottom and lighter at the top (this brings the centre of mass closer to the ground and you can show this allows more lean. We could do this in lots of ways, one might be to make it thinner as it gets higher.
- increase the size of the base.
- move the centre of mass from the centre of the circle and away from the side it's leaning.

The leaning tower actually has uneven weight distribution as it was expected to be a problem when it was originally built. There are many other factors which caused it to work, one was that a large break was taken in the construction, three tiers were finished in 1178 however tier four was only started in 1272. This long break gave the ground underneath time to compress and settle under the weight of the building.

The tower was also built into a banana shape which means it's centre of mass falls off centre.

In 1964 the Italian Government requested international support in stabilising the tower. Ideas included using an 800 tonne lead counterweight in the base of the tower, in the end they opted to remove soil from under the raised end and straighten the tower. The weight of the tower then compressed the gaps in the soil reverting the tilt. During this they used cables to support the tower which is another option (although it was felt it would affect the appearance of the tower).

Another thing that has been done is strengthening of the ground under the leaning side. We can do this in our model by putting in the wooden dowels, this passes some of the force down through the soil layer to the harder rock layer bellow (in our case the table/tupperware). They've also drained water from the clay soil previously.

There are lots of leaning towers about in the world, not all of them are intentional. The Guinness World Record for the "Farthest Leaning Man-made building" belongs to the Capital Gate building in Abu Dhabi was intentionally built with an 18 degree lean using a variety of techniques. It has a large central core (made of steel and cement) which lent in the opposite direction and is pulled vertical by the floors of the building. The core has 15000m^3 of concrete and 10000 tons of steel. There's also a total of 490 pilings, 287 are 1m in diameter, and 20–30 meters deep. The remaining 193 are 60cm in diameter and 20 meters deep. All 490 piles are capped together using a concrete mat footing nearly 2 meters deep. It's also 150m tall, compared to the leaning towers 56m. The leaning tower of Pisa leans to 3.97 degrees post stabilisation.

You could try and talk about the soil geology that causes the problem but this an engineering experiment and on asking Helen (Geology Coordinator 2017-18) she said "The geology underneath is rather boring".

Can talk about the difference between service limit state (SLS) and ultimate limit state (ULS). ULS is the point at which the structure breaks while SLS is the point at which people start feeling uncomfortable, e.g. from visible deflections, cracks, leaning.

Risk Assessment
Date risk assesment last checked: 
Sun, 29/12/2019
Risk assesment checked by: 
Käthe-Marie White
Date risk assesment double checked: 
Fri, 24/01/2020
Risk assesment double-checked by: 
Polly Hooton
Risk Assessment: 
Hazard Risk Affected Person(s) Likelihood Severity Overall Mitigation Likelihood Severity Overall
Dowels Choking hazard if ingested Public 2 5 10 Don’t allow children to play with them unsupervised. Keep track of the number of dowels. Use minimum number of free dowels.
In the event of choking, call a first aider who may perform the Heimlich manoeuvre if qualified to do so.
1 5 5
Falling tower pieces Possible injuries if tower pieces land on feet from a sufficient height. All 2 3 6 Get people to catch around the tower, demonstration to be done in centre of table.
Watch the tower at all times, and don't let kids build it unsupervised.
Call first aider if required.
1 3 3