Endrestudio’s twist on the classic spiral stair appears in twin San Francisco condominiums. A triangular spine coils into a 360-degree helix and travels unsupported between two floors. Just below the second-floor landing, two steel arms project from the building to clamp the stair’s spine. Each stair tread hovers above the spine, carried by slim connecting wings. U-shaped tubes intersect these wings and become balusters for the handrails. The effect is of a taut spring, one fabricated from laser-cut plates of 3/16-inch steel.