In talking about the truss roof at the 1890 second Madison Square Garden, I mentioned that it was one of the relatively few long-span roofs in the city at that time. That raises the question of what did the other ones look like? By far, the most common long-span roofs were over church sanctuaries, most with spans around 40 feet, but those tend to be fairly straightforward gable truss (king-post or queen-post) or scissors trusses; every nineteenth-century church roof I’ve seen had trusses made primarily of wood. In short, those roofs are interesting in their own way, but they are structurally no more advanced than similar roofs built hundreds of years ago. In the last third of the nineteenth century, structural technology was advancing very rapidly worldwide, and New York was in a position to benefit from and take part in that race.
The distinguished building above is the Seventh Regiment Armory, at Park Avenue and 65th Street. The beautiful rooms in the front of the building tend to attract the most attention, but like all such buildings, the back of the building is a large open drill hall where maneuvers could be practiced. This space was completed in 1880 along with the rest of the building and shows the state of long-span roofs a decade earlier than Madison Square Garden. That’s a short generation, but a meaningful one in terms of the advancement in construction that took place. Here’s the drill hall:
And here’s a close up at the “eave” elevation that shows some of the details of construction:
A few things worthy of note… First, the trusses are wrought iron, because 1880 is just a bit too soon. The designer was Charles Macdonald of the Delaware Bridge Company. By the time Madison Square Garden was built, that scant decade later, steel had pretty much taken over. The roof itself consists of wood plank on wood rafter, spanning between iron purlins. The usual purlins are built-up I-section trusses with double angle flanges and fish-belly lattice webs, but there are solid I-beams thrown in here and there at edge conditions. The roof is not curved, despite being supported on curved trusses, because of the rafter configuration. The lower roof on each side is faceted, as the joists run from purlin to purlin; the middle roof on each side is a single slope supported on secondary iron above the trusses; and the center gable roof is supported on secondary iron partially in the form of iron arches.
The trusses themselves are two-hinged arches, with the bottom hinges hidden in modern times below the bleachers set up for sporting events. The top chord turns down at the ends so that it runs parallel to the side walls but is not laterally supported by them.
This is a hybrid scheme: not yet a really modern roof, but using the modern technology of those two-hinged arched trusses (or, if you prefer, trussed arches) to get the clear span of about 170 feet. The little arches below the center roof are pointless and use of wood rafters and plank made the whole thing unnecessarily vulnerable to fire. But it’s a step in the right direction, and Madison Square Garden was the next step. Tomorrow: back another decade for the previous step.