Design

Different Logic For Different Elements

by Don Friedman on March 27, 2018



Those are two views of the same rather boring industrial building in the Bronx, built in the first half of the twentieth century. The exterior walls are solid brick, with a reasonably nice hard-burned face brick as the outer wythe; the floors are very heavy wood joists supporting a wood plank subfloor. It’s a long, relatively narrow building (two bays in one direction, six in the other), and there’s a line of steel girders and columns running down the middle. There are a lot of these two-bay-wide industrial buildings in New York, and I always think of them as “fish-skeleton buildings” because of that one line of girders in the middle.

The outside picture shows the typical lintel type used in this building: two small I-beams connected with spacers (which we can’t see but can infer from the visible bolt heads). The inside picture shows the wood joists and bare steel of the girders and columns.

This building is not fire protected in any way. There’s exposed wood structure that can burn and exposed steel structure that will collapse if overheated. Fair enough: we don’t require every building to be fire-rated* so the use of unfireprotected interior structure for this building makes sense. The more interesting issue is the exposed steel lintels. They will also collapse if overheated, and fires spreading from one building to another via windows and damaged exterior walls is a real phenomenon. This building was simply built to an old code that allowed unprotected lintels like these** and so is grandfathered under current code.

If there were a serious fire, this building would not last long. Because it’s set up for industrial use, it has entirely open floors, with nothing to stop horizontal fire spread. Because the interior structure is unprotected, fire will spread from floor to floor. If a similar building were to be built today (called Type III construction in the 2014 New York Building Code), the exterior walls would be significantly better at resisting fire because the lintels would be protected. In other words, the building would still burn relatively quickly in a fire*** but it would contain the fire within itself better and therefore would be less likely to cause its neighbors to burn.

There’s a fine line being drawn by the current code, where we accept the loss of an unfireproofed building but expect the neighbors to be protected from risk. That line didn’t used to exist, and is present now because of lessons learned the hard way.


* Whether we should require every building in a city to be fire-rated is, obviously, a different question.

** Current code allows unprotected lintels up to four feet in length, on the theory that the masonry can span across an opening that small via arching action if the lintel fails.

*** Sprinklers can help both the theoretical new building and actual old building resist fires.

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