Engineering Visible

Structural engineering for buildings may be generally hidden, but there are a lot of exceptions. I’ll exclude exposed wood joists in small buildings from being exceptions since those buildings were not, until relatively recently, designed by engineers. But, for example, plenty of concrete factories have exposed structure (albeit opaque structure that is difficult for non-engineers to read), and old-fashioned train stations often had exposed iron- or steel-truss roofs.

Multiple-story steel-frame buildings tend to have very little exposed structure because the steel needs to be fire protected. But sometimes, some steel pokes through. The loft building in the center of the photo above, seen from a roof a block away, is an example. We’re looking at a side wall (on the right) and the rear. The side wall has windows and a narrow light well cut into it. The light well was probably a hedge against future development: if a similarly tall building were to be constructed immediately east of this building, it would block all of the side-wall windows, but not the windows facing the light well. In any case, this is a steel-frame building and I know this (a) because that’s what just about every old high-rise in New York is, and (b) the topic at hand, a portion of the frame is outside the wall.

Those are braces – what used to be called wind girders – that are part of the frame of the building. There’s probably another one five stories below the lower one, hidden behind the somewhat ragged white terra-cotta nearer my vantage point. I’ve shown these kind of braces before, and they’re the inevitable result of the many light wells and independent wings of the pre-air-conditioning, pre-fluorescent-light era, when windows were needed to make interiors spaces habitable.

Those partially-arched beams connect on each end to the columns at the corners of the light well and, through them, to the internal frame just behind the side wall. They were designed to have no role in carrying gravity load, but they permit that frame to act as a single entity (across both its front and rear portions) in resisting wind load. This is a narrow building (look at the rear facade) and probably only has three column lines running parallel to the side walls. As a result, this frame – the eastmost column line – was needed. If the building were twice as wide, those braces would probably not be there.

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