Toward The Perimeter

I want to illustrate something I said in passing yesterday. In describing how to make columns stronger by decreasing their slenderness, I said “pushing the material from the center of the section to the outer edges increases the radius of gyration without increasing the weight.” The picture above is from the Lenox Avenue branch of the Interborough subway, which opened in 1904 about a month after the original main line. The main roof girders span perpendicular to the tracks, as is true in all of New York’s old subways; the girders here are divided into four spans by a line of columns between the two tracks and a lines of columns near the track-side edges of each of the platforms. The column in the center of the photo is one of the platforms columns, and it is cast iron. Cast iron columns were used only in the oldest sections of the IRT subway.

It’s hard to make out, but the center columns have a form that, to our modern sensibilities about steel, is odd. They’re built-up H sections, but they’re not built up of a web plate and four angles like most H sections of that era, they’re built up of a plate and four “bulb angles.”

The logic behind that oddball shape isn’t very clear when you look at one in isolation or, as above, when you compare it to ordinary angles. It’s clearer when you compare built-up column sections:

Figure 96 is from slightly later – the Dual Contracts extension of the subways began in 1913 – but the principle of pushing material to the outer edges is visible here. Section A shows a built-up box made of two channels and lacing. The lacing is discontinuous, so it does not contribute to the main load-carrying area of the section. The channels, which are that load-carrying area, are at the edges of the square, as far from the center as is possible.

Sections C and D create ersatz H sections by adding secondary flanges onto a narrow I section, an I-beam in C and a built-up I in D. Those secondary flanges are, again, at the perimeter of the section, as far from the center as possible. The overall section is less efficient because the central I is not far from the center.

Section B is similar to the central columns in my photo above. The column is a built-up H that uses bulb angles for the flanges. This is not a particularly efficient shape, with only the flanges far from the center, but the bulbs put that much more material there, at the edges, rather than closer to the center.

There’s been a long debate in history of technology circles as to whether “evolution” is a good description for how technologies change over time. The problem with that usage is that changes in technology are not blind, but guided by people’s conscious decisions. The advantage to that usage is that it provides a way to talk about issues like the use of bulb angles in columns. Once engineers understood that having more material at the perimeter created more efficient columns, they searched for ways to get the material there. The bulb angle was an evolutionary dead end but well suited for its specific purpose.

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