Science Puzzle
Why Do Cathedrals Have Flying Buttresses?
Gothic cathedrals such as Notre-Dame in Paris have soaring stone ceilings, very tall, thin walls and huge stained-glass windows. Outside, rows of stone arches called flying buttresses leap from high on the walls, over the roofs of the side aisles, to massive stone piers standing clear of the building.
Why do Gothic cathedrals need flying buttresses?
The Answer
A heavy stone vault doesn't only press down on the walls; it pushes them outwards. The flying buttresses catch that outward push high up and carry it out to the piers and down into the ground, so the walls between don't have to resist it.
An arch holds itself up by squeezing its stones together, and the squeeze runs round the curve and out through its feet at an angle. Press on an arch of books and its feet slide apart. A stone vault is a ceiling made of arches, weighing hundreds of tonnes, and its feet push outwards on the tops of the walls. A tall, thin wall pushed outwards near the top would bend and topple.
Earlier Romanesque churches solved this with brute force: walls metres thick and small windows. The Gothic builders found a better way. Each flying buttress is a half-arch that meets the wall exactly where the vault pushes and passes the push across to a heavy pier outside. The tall pinnacles on top of the piers aren't just decoration; their weight helps turn the sideways push downwards into the ground.
With the walls freed from that job, they could be thin and filled with glass, which is why Gothic cathedrals are so full of light. The flying buttresses of Notre-Dame, from around 1180, were among the first used on such a scale.
The principle: Arch thrust. A stone arch or vault pushes outwards at its base as well as down, and flying buttresses catch that push and carry it to the ground, so the walls between can be thin and full of glass.