The Equitable Building was not just a steel-frame building. Its enormous size and almost mind-numbing regularity meant that it can be used as the platonic ideal of a steel-frame building designed in 1913. That date is important: the American Institute of Steel Construction was not founded until 1921 and did not publish its first specification until 1923; the amount of detail on steel construction in the 1908 New York Building Code was, by our standards, shockingly small. In other words, the specification book for the project had to contain basic information about steel construction because it wasn’t going to come from anywhere else. (I’m cheating a little here, but I’ll explain below.)
The steel specs start simply enough: a discussion of scope for the steel subcontractor, shop drawing submissions, and marking pieces. These are all items that you’ll see in a modern spec. Then things get more interesting: “All materials shall be steel, except separators between beams, which shall be cast iron, unless specifically shown otherwise.” Wide-flange beams were not in common use yet in 1913. Only Bethlehem Steel was manufacturing them (having licensed the patent from its European owner) and they cost more than the usual alternative of using a double I-beam instead. That’s what the separators were for: to hold the two pieces of a double beam at a given spacing and force them to share load equally. You could do the same thing by riveting together a bracket made of several steel angles, or plates and a piece of channel, but it would be more expensive than casting a spacer. Finally, there was still a decent capacity to cast structural iron available in 1913.
“All material for columns and riveted work shall be made by the Open Hearth process. All beams may be made either by the Open Hearth or Bessemer processes. The material must be uniform in quality and must not contain over 0.08 of one (1) per cent, of phosphor for Bessemer steel nor more than 0.06 of one (1) per cent, of phosphor for Open Hearth process steel.” In other words, the quality of steel for the columns was more critical than for the beams. Not something we think about today with steel, although we may specify higher strength steels for columns. But the interesting part comes next:
“The steel shall have an ultimate strength of 65,000 pounds per square inch and shall not vary from this more than 5,000 pounds per square inch either way. It shall have an elastic limit of not less than one-half of the ultimate strength; an elongation of 25 percent…” The yield stress, which controls most steel design, had to be at least 30,000 psi (65,000 – 5000, divided by 2) and the desired yield was 32,500 psi. First, using anecdotal data, that sounds a lot like what I’ve seen over the years in testing steel from the 1910s through the 1930s. Second, let me stop cheating: there was no general specification for steel construction in 1913, but there was a specification for steel material. The ASTM had first published steel specs in 1900, and the most recent version available for Equitable had been published in 1912. ASTM A7 steel was meant for bridges and A9 steel was meant for buildings. The 1912 version of A9 specified an ultimate stress of 55,000 to 65,000 psi and a yield stress of one-half the ultimate strength. In other words, the Equitable spec was asking for steel roughly ten percent stronger than the only national standard then in use.
So far, it sounds like the designs were looking for better-than-average quality, which is hard to argue with. But the New York City Building Code gave absolute values for bending stress in beams and compressive stress in columns. Using stronger steel did not, according to the code, change those values. So, were the designers cheating on the building code? Did they ask the building department for a variance on the basis of the better material? Was the purpose of the better material to create a larger margin of safety? I’ll probably never know, but it’s an interesting question to ask.
Some silliness coming for the weekend and then back to the Equitable specs next week.