Inside the making of the world’s longest cantilever

Few aspects of architecture are as dramatic as the cantilever, a rigid structural element that extends horizontally, unsupported at one end, from a building. It’s the closest an edifice can come to flying. Arguably the world’s most famous cantilever belongs to Frank Lloyd Wright’s masterpiece, Fallingwater, whose room-size concrete balconies float mesmerizingly over a waterfall in Bear Run, Pennsylvania. “I want you to live with the waterfall, not just to look at it, but for it...