July 30, 2010 § Leave a comment
JSAH readers will recognize the thing on the left as one of the most important structural innovations of the 1890s. The Gray Column was the invention of New York engineer J. H. Gray, who patented the shape and then licensed it to engineers and builders (saving him the trouble of actually fabricating anything–pretty clever). If you know your structural theory, you’ll remember that the ideal shape for a simple column is a hollow circle (or, almost as good, a square). The column has to take an axial load, but because buildings sway in the wind it will also have to take some bending, and there’s no real way of knowing which direction that bending is going to come from. So by putting all of the column material as far from the column center (or its neutral axis, for you SCI-TECH grads out there), you guarantee that the column will be reasonably efficient in bending no matter which direction the wind’s coming from.
There are two problems with a hollow round shape, though. One is fabricational–it was difficult to either cast or bend steel to a consistent, tight radius and end up with a product that you could trust. Hollow cast iron columns were notorious for being asymmetrical, as the mold that formed the void tended to float, rather than to remain in place. Hollow steel pipe is still expensive today, as it involves rolling a sheet of steel around a hardened steel die.
The other problem was more critical in the 1890s. Riveting was key to forming monolithic connections between girders and beams, which was in turn a key element of any good wind bracing scheme. But there was no way to rivet to a hollow steel section, since hammering rivets required access to both the front and the back of the steel member. The Phoenix Company produced quarter-round sections that could be combined with flat plates to provide room for riveted connections, but this was complex and needlessly heavy.
Gray’s innovation was to assemble a column section that approached a hollow round tube in its radius of gyration (a measure of how much of a shape’s cross section is a useful distance away from the center) by using standard steel shapes. If you look at the cross section you can see that it’s really eight regular steel angles riveted to one another, with some intermittent diagonal pieces–sort of a chassis–that keeps them in place relative to one another. Not only is there no material in the center, there are plenty of nice flat surfaces at the very edge where girders and joists can easily be attached. Gray described the relationship of the shape to a successful wind bracing scheme in his 1896 catalogue:
“The primary result of the method of construction is an entirely independent steel structure, thoroughly braced and vertical, which does not need partitions or walls, either inside or out, to keep it in position, and capable of supporting all of the other materials used in the construction of the building.”
The catalogue (in the Canadian Centre for Architecture’s collection) contained all sorts of tables for safe loads given standard angle sections from Carnegie and other steel works, along with dimensional information for proprietary steel shapes and their resultant configuration as Gray columns. This was just about contemporary with Carnegie’s first steel handbook, and it shows a steel industry on the verge of standardizing its shapes and testing. The catalogue also shows a number of important skyscrapers of the era–the Guaranty in Buffalo and the Fisher and Reliance in Chicago–in construction. Key to the columns’ light weight and strength are the hollows that read as dashed white lines in the center. There’s no steel there, and because of the intermittent nature of the “chassis” pieces you can in fact look right through the column centers.