Drag to turn. Scroll or pinch to zoom in where you point. Right-drag, or drag with two fingers, to slide. Double-click a piece to go to it, and the sky to see it all.
Why does the vault need flying buttresses? The same physics on one arch, in a slice: build it on its centering, strike it, and watch it stand or fall.
How an arch standsHow high could you build? Design your own cathedral: the vault, the walls, the flying buttresses. See whether it stands, then build it here and watch it go up, or come down.
Design your ownWhat you're building
The forces
The red line is the line of thrust through the vault's ribs, down the wall, along each flyer and through each buttress pier, worked out the same way as in How an arch stands. They appear when the vault's centering is struck, the moment the vault starts carrying itself.
Color by load follows each piece's weight down to the ground and divides by the area it rests on. It ignores the sideways push, so it shows how load piles up, not exact stresses.
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A treadwheel crane lifting each stone, real physics when something falls, the cut face drawn as a solid section, and the dome of Florence built the same way.