The steel skeleton frame

node · earth · North America, then worldwide

A three-dimensional cage of metal columns and beams that carries the entire weight of a building, including the weight of its own external walls, floor by floor. Its consequence is categorical rather than incremental: in a load-bearing masonry building every extra storey thickens the walls at the bottom, so height is bought by losing ground-floor area, and a sixteen-storey masonry building such as Chicago's Monadnock Block ends up with walls roughly two metres thick at street level. In a frame, the wall carries nothing but itself and the wind, so it can be thin, or glass, or hung from the frame like a curtain. The frame also needs lateral bracing against wind, fireproofing of the metal (early frames were clad in terracotta or concrete because unprotected steel loses strength in a fire), and a mechanical way of reaching the upper floors. The system spreads from Chicago and New York in the 1880s and 1890s and is still the basis of tall building construction.