How did Brunelleschi build Florence's giant dome without a wooden frame?

Brunelleschi built the dome without a wooden frame by making it hold itself up as it rose. He used two lightweight brick shells, laid each ring of bricks so it worked like a horizontal arch, bound the dome with hidden stone chains to stop it spreading, and used a herringbone brick pattern to pass the weight of wet, freshly laid bricks to stiff ribs at the corners. Each finished ring could carry the next, so the dome never needed support from below.

It had to be done this way. Florence began its cathedral in 1296 and finished the nave by 1380, leaving a vast octagonal opening 44 metres across and 52 metres up. The usual method, a timber frame called centring holding the masonry until it set, would have needed more wood than Tuscany had. Buttresses, the Gothic answer to spreading, were out of the question: Florentines considered them ugly, and the style belonged to their northern enemies.

In 1418 the city held a competition, and the goldsmith Filippo Brunelleschi beat his rival Lorenzo Ghiberti. Building ran from 1420 to 1436. With no stress calculations, he relied on intuition and models, and invented machines to hoist some 37,000 tons of material, including more than 4 million bricks. The result is still the largest masonry dome ever built, though it has never stopped cracking.