Historic Structural Detail: Non-Skid

by Don Friedman on June 13, 2016

A nice picture by Mona of the sidewalk over a vault, with the vault light on the left, and the granite-slab sidewalk on the right:

Note that the glass lenses in the vault light vary greatly in condition, with some looking very good, some cracked, some chipped, and some missing and apparently replaced with hardened bubble gum. The purplish color of some lenses is probably due to the presence of manganese dioxide in the material.

I normally look at sidewalk vaults in terms of their structural adequacy, but this photograph nicely illustrates how well thought out they were. They were designed, in the same sense we would use that word today. First, and most obviously, is the existence of sidewalk vaults. Their presence is not an accident or a by-product of constructing a building. They are a logical response to the technological conditions of their era: buildings were nearly all flammable and were all lit by flame. Cellars were therefore the worst-lit spaces in a building, with no possibility of light penetrating from windows, and were used to store, among other things, coal. Any way of getting light into a cellar was an improvement, and the possibility of storage outside the volume of the building proper was also helpful.

Next, the iron-and-glass vault lights are located adjacent to the building, where the light they provide is most needed in the vault below. The granite is next to the street, where its greater toughness was good to resist abuse from wagon wheels.

Finally, note that the vault-light iron has a raised pattern of nubs, the iron band that separates the vault light from the granite has a cast-in hatch pattern, and the granite has been scored. The issue of providing a sidewalk surface that can be safely walked on when it is wet is not a modern concern, and arguably was even more important when everyone was wearing shoes with leather soles.

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