The Next Level

A while ago, I discussed the use of simple trusses for long-span roofs in the late nineteenth century. Engineers being who we are, things didn’t stay simple for long, and I’d like to discuss an example of a more complex design at one of the most remarkable buildings of the beginning of the twentieth century, the 1901 West Baden Springs Hotel in West Baden Springs, Indiana.

The picture above, taken a couple of years after completion, gives a good sense of the interior. A common architectural layout for a hotel is the double-loaded corridor: a central corridor with rooms on both sides. The corridor can angle or branch, but having rooms on both sides stays constant. At West Baden Springs, the corridor forms a circle, so half the rooms face the outside and half face the interior atrium, which is a 195-foot diameter circle. From an engineering viewpoint, the roof of the atrium is the most interesting part of the building.

The roof is always described as a dome, and architecturally it is, but it is not a structural dome. A structural dome is a form of vault, using three-dimensional compression forces to span. Masonry domes develop circumferential tension in the lower part – hoop tension – which is why pure masonry domes either get thicker at the base (as at the Pantheon in Rome), have some form of reinforcing (as at Cattedrale di Santa Maria del Fiore in Florence), or have vertical (radial) cracks (as at a lot of domes). That’s obviously not how this roof works. A series of steel trusses span radially from the inner wall of the donut-shaped main building to the center of the atrium, supporting purlins which in turn support the roof sheathing. The fact that so much of the central portion of the roof is glass shows that this is not a structural dome, as the skylights are in areas that should have circumferential compression.

The structural engineer, Oliver Wescott, was a bridge engineer, and his training shows. A structural dome would create significant outward thrust at its base, and if the trusses were simply a series of arches, that thrust would be concentrated at each truss end support. Instead, the structure consists of 24 half-arch ribs around a central compression ring (deep enough to span the full truss depth from top to bottom, so perhaps better called a compression drum). There was a tension ring near but not at the rib bases, which were designed to be able to move (with thermal movement or wind pressure) so as to not exert too much lateral force on the building below.

So each pair of ribs opposite one another act as a two-hinged arch, free to rotate at the base, and transmitting their forces to each other through the central drum. Two-hinge arches can develop a fair amount of bending, which is fine since the steel trusses were specifically designed to take those forces. And the diagonals in plan are wind bracing that keep the whole roof from spiraling inward and collapsing.

This is a well-thought-out design responding to a challenge that even today is not so simple to meet.