The flying buttress

node · earth · Western Europe

A half-arch that springs from a free-standing masonry pier outside the building, leaps the aisle roof, and lands on the outer face of the high wall exactly where the vault pushes. It does one job: it takes the outward thrust of the high vault, and the wind load on the high wall and roof, and walks both down to the ground outside the building. Because the flyer intercepts the thrust above the aisle, the high wall no longer needs the mass that would otherwise be required to resist it, and the space between the piers can be opened as window. Robert Mark's photoelastic model studies at Princeton in the 1970s and 1980s argued that wind loading on the tall roof and clerestory was a far larger design driver than the vault thrust alone - which, if right, means the flyer is as much a wind brace as a vault prop. The upper flyers at Chartres and elsewhere sit high on the wall in a position that makes little sense as a vault prop and good sense as a wind brace.