How does the dome of St. Paul’s Cathedral support itself? The dome of St Paul’s Cathedral in London looks like one massive dome, but it is actually two domes with a cone of bricks between them to give it strength and structural support.
St Paul’s Cathedral is one of the main landmarks in London. It is most famous for its dome. The cathedral is 111m high and it was the tallest building in London until 1963. There is even a law in London called the St Paul’s Heights policy. In order to keep views of the Cathedral and other monuments clear from the South Bank of the Thames, the Thames bridges, and along some streets, the maximum heights of buildings were strictly enforced. This is why there are only tall buildings in some areas of London and not others. The main body of the cathedral is a cross shape, as with most churches, and there is a round tower of columns on the center of the cross. On top of the tower is a huge dome and a stone lantern on the top of that.
The cathedral was designed by the famous architect Sir Christopher Wren. There was possibly a church on the site from Roman times, shown by the fact that it is unusual to name a church after St Paul in Britain. There were a few churches on the spot until Old St Paul’s was built in 1087. This was expanded until it was cathedral size, but it was burned to the ground in the Great Fire of London in 1666. After the fire, a lot of London needed to be rebuilt, and Sir Christopher Wren was chosen as the architect of many of the buildings, including St Paul’s Cathedral. It took 31 years to build the whole cathedral. It was finished in 1708.
The most famous part of the cathedral is probably its dome. It looks to be a free-standing dome with a lantern on the top. On top of the lantern is a golden ball and cross. In order to make this dome, Wren had to employ several different methods and tricks. The first technique is how he worked out the size and the shape of the dome. He used a method that has been known about for a while and that is known as “catenary”, which comes from Latin and means “chain”. If you hang a chain or a line between two points, the middle of the chain will sag down. That shape and that curve, when turned upside down, will support itself because all of the tension running through the chain is the same as the compression that will run through it when it is stood up. Wren used this mathematical idea to work out the size of the dome.
The second thing he did is to realize that one dome wouldn’t work. He wanted the dome of the cathedral to be the tallest thing in the city and visible from far away. After looking at the catenary, he knew that to make the dome high enough, he would have to increase the height of the columns that it was standing on. That would also mean the inside of the tower was very high and it would have messed up the proportions he was going for. He didn’t want to do that, so he came up with a workaround. The dome of St Paul’s Cathedral is actually two domes, one inside the other. The inner dome is smaller and less steep. It is the roof that we look up at from inside the cathedral. The outer dome is taller and steeper. There is a large space between the two domes.
The third thing he did was to build another dome between the inner and the outer domes for support. This support dome is not so much as dome as a brick cone. The brick cone is 45 cm thick and it rises from the top of the building at the base of the dome. The inner dome is inside the brick cone and the outer dome is above it. The lantern that is on the top of the outer dome is actually the point of the brick cone. This was the only way the lantern could be supported because the dome itself would not be able to support the weight. Inside the dome are chains and scaffolds that connect the two domes to the brick cone and give them strength and support.
The last thing Wren did is to have the top of the inner dome cut away and painted so that it appears as though you are looking up through the lantern. There is no open air over the inner dome, just the brick cone. Wren had windows cut into the brick dome and it is open at the top so the sunlight comes down through the inner dome. He then had the top of the inner dome painted to look like the inside of the lantern. It is a fascinating piece of architecture. And this is what I learned today.
Photo by Ollie Craig: https://www.pexels.com/photo/bird-s-eye-view-of-st-paul-s-cathedral-on-a-hazy-weather-6398539/
Sources
https://www.stpauls.co.uk/5-designs-for-dome-c-1685-1710
https://en.wikipedia.org/wiki/St_Paul%27s_Cathedral
https://en.wikipedia.org/wiki/Christopher_Wren
https://www.explore-stpauls.net/oct03/textMM/DomeConstructionN.htm
https://www.london-tickets.co.uk/st-pauls-cathedral/dome/
https://plus.maths.org/content/maths-minute-st-pauls-dome
https://en.wikipedia.org/wiki/Catenary
https://diamondgeezer.blogspot.com/2020/11/tall-buildings-in-city-of-london.html