Failure Portrait: The Last Extremity

by Don Friedman on January 12, 2016

Sidewalk vaults used to be considered parts of the main space of the building rather than just holes in the ground, and their construction reflected this importance. They had cast- and wrought-iron beams, cast-iron columns, granite-slab and brick vault roofs, and most famously cast-iron vault lights. [link] There are a lot of buildings where the sidewalk vault is the only portion that is completely non-combustable, unlike the wood floor structure upstairs.

It takes a lot of weathering to destroy these solidly-built structures, but it happens. Like here:


The upper left is a cast-iron vault light. The big girder running from the middle left to the upper right is wrought iron, over-running a cast-iron column. The lower, solid sidewalk on the right rests on brick vaults; the vaults are supported on wrought-iron beams that rest on the girder. The underside of the brick vaults can be seen on the right; their ends are visible above the girder.

Everything that can be destroyed by water and overload is being destroyed. The wrought iron is rusting away – in case it’s not obvious, the main girder is largely missing to the right of the column. Cast-iron rusts slower and less destructively than wrought iron, but the column is still pretty heavily rusted and the vault light is just about gone. Mortar is missing from the brick walls and vaults, and some of the bricks themselves are falling apart.

The fact that this structure hasn’t yet collapsed is a function of how heavily it was built and how lightly it’s been loaded. If it weren’t over-designed and over-built, or if it had ever been loaded to the maximum for a sidewalk by having a fire truck park on it, it would have collapsed. Good luck is nice to have, but it doesn’t count as a rational structural design.



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