The marvel here is structural sleight of hand: the monument carries its own weight through its outer skin, with no hidden frame inside. Each leg is a sandwich of two walls, stainless on the outside and carbon steel within, the lower reaches filled solid with reinforced concrete and the whole rising from 142 stacked triangular sections that taper as they climb. Because no scaffold on earth could wrap a leaning tower this tall, crews used a pair of creeper cranes that rode rails clamped to the steel and hoisted themselves upward as the legs lengthened. The two halves climbed from separate footings, angling outward, and had to converge within a fraction of an inch hundreds of feet overhead. It stands as engineering balanced on pure arithmetic.
Run your eye up the nearest leg and take in something that's genuinely hard to believe from down here: that bright skin is not a wrapper stretched over some hidden steel skeleton. There is no skeleton. The skin is the structure. Each leg is a double wall — an outer shell of stainless steel and an inner shell of carbon steel — and through the lower reaches the space between those two walls is packed solid with reinforced concrete. The whole Arch was built up from a hundred and forty-two triangular sections, stacked one atop the next up each leg like sleeves up an arm. The monument holds itself up by its own surface. And the reason it can is the curve. Saarinen's curve isn't shaped that way only to look graceful, though it surely does — it's shaped so that every ton of this thing presses straight down through the steel and into the bedrock, the whole load running through the legs in pure compression, nothing anywhere trying to bend it. Draw that line exactly right and the Arch all but wants to stand. Draw it a little wrong and it pulls itself apart. So before you picture the cranes and the crews, picture this: a tower with its bones on the outside, balanced on a line of arithmetic. Linger here a moment — how they actually raised it is the better half of the tale.
Photo: Rwietlake · CC BY-SA 4.0
