However, there are a few cases in which, by some expedient and careful design of the construction process, some structures have been erected without any centring. A widely known example is the domical vault of the Florence Cathedral, built by Filippo Brunelleschi in the 15 century. He achieved such a challenge by building a massive structure, mechanically behaving like a spherical dome, but with large ribs and exploiting the masonry herringbone bond to lean and fix every new layer on the previous one. Each layer of the structure seems to be composed by many small arches. The vault is also double-skin, with an intermediate hollow space hosting the staircases, through which air can flow to avoid humidity concentration. To resist parallel tensile stresses which may separate the fuses of the vault, Brunelleschi arranged sandstone chain along some parallel plane.