Historic and Modern Structural and Non-Structural Detail

by Don Friedman on September 11, 2017

I hope the title is broad enough…I wouldn’t want to leave anything out.

This is a probe in the facade of the penthouse of a 1920s apartment house. The exterior of the masonry facade has been stuccoed, which is why the background is so white. The pipe in the lower right corner is part of the scaffold resting on the main roof to allow access to the set-back penthouse facade.

Like a lot of buildings of this type, the penthouse was built much lighter than the main body of the building. The penthouse columns are angles and double angles; the main-building columns are built-up boxes of channels and plates. The penthouse spandrel beams are 7 and 8 inches deep, reflecting the light weight of the short parapets above; the main-building spandrel beams are 10 and 12 inches deep.* And, pertinent to this photo, the penthouse facade is stuccoed terra-cotta block; the main-building facade is solid brick.

On the left, the interior is visible of a terra-cotta block broken in the process of creating the probe, directly above the spandrel beam. More importantly the dark-brown** interior exposed at the probe shows the complexity of the hidden structure: the geometry of the steel beams, columns, and connections is complicated and bears little relation to that of the masonry curtain walls. The only geometric connection between steel and masonry is that the spandrel beams are parallel to, and partially embedded within, the planes of the walls.

This disconnect is not some peculiarity of this building or of its type. It is, rather, the whole point of skeleton framing, as it was developed in the late nineteenth century and as it is still used today. When we build a skeleton frame, we disconnect the construction of structure from the construction of non-structure. This speeds up construction, allows us to build exterior walls of any material or shape and with windows are big as 100 percent of the facade area, and removes the strength of masonry from the structural calculations. These benefits are so great that no one questions the concept of skeleton frames, but because the benefits so outweigh the drawbacks we rarely discuss the drawbacks.

The geometric*** disconnect between structure and exterior envelope is one of the drawbacks of skeleton-frame construction. It’s not that bad, but it does make it difficult to look at a building and know details of its structure. We regularly use destructive probes, like the one in the photo above, because it’s the only way for us to really understand the structure. Investigations of pre-modern buildings are in one sense easier: once you measure the masonry, you have a good idea of what the structure is.

* The columns at the penthouse are also closer-spaced than those below, further reducing the total load carried by each spandrel beam.

** The steel was coated with red-lead paint, which is reddish-brown. Some of the steel is rusted, and rust is reddish-brown. Everything we’re seeing is coated with dust from broken terra cotta block, which is reddish-brown.

*** Simply put, modern structure is hidden not just by exterior and interior architectural finishes that cover it, but by the fact that there is no way to know where it just by looking at the geometry of those finishes.

Anna Karenina in Concrete

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Tolstoy said that “Happy families are all alike; every unhappy family is unhappy in its own way.” This principle can be applied to buildings: every building in good condition is alike, every failing building fails in its own way. The white paint on the concrete does a great job, in my opinion, of highlighting where the […]

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August 30, 2017

The front entrance of Castle Clinton in 1896: The front entrance of Castle Clinton in the 1970s: We’ve been writing a lot of conditions reports lately. Just coincidence, really: as a small firm, the type of work we have at any given moment can vary. Some months are more design intensive, some more construction intensive, […]

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An Added Benefit

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We climb scaffold for a single purpose: to see some aspect of a building up close in order to better understand it. But a lot of the time (click on the panorama above to enlarge it) the view is worthwhile in itself.

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Is a Non-Dead Building Ghost a Building Zombie?

August 24, 2017

Another old picture, but this one is not quite a ghost. The curved mansard roof (that is visible as a curved pattern on the side wall) was part of the extent building before its unfortunate extension. The extension added one full story and made the mansard level the same size as the typical floors below, […]

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Building Ghosts From 15 Years Ago

August 23, 2017

I’ve been digitizing old rolls of film* and I came across a bunch of pictures I took, between 15 and 20 years ago, of building ghosts. Most of these ghosts are no longer visible: the current building boom in the city has resulted in the disappearance of a lot of vacant lots and parking lots, […]

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Beautiful Brickwork Marred

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Friend of OSE – and occasional collaborator – Glenn Boornazian sent me a few vacation photos from Massachusetts. That’s pretty nice masonry for an apparently abandoned building. In a non-aesthetic sense, the interesting stuff is going on near the eaves. That’s a damned big crack and displacement at the corner pier. The crack starts at […]

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A Square Peg In A Square Hole

July 20, 2017

If you wait long enough, every aspect of a given technology will change. Square reinforcing bars used to be fairly common in concrete, but they’re long gone in design practice. Plain reinforcing bars, without surface deformations, used to be fairly common in concrete, but they’re long gone in design practice. Of course, just because we […]

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Tragedy in Context

July 6, 2017

A fire engine on its way to the Triangle Fire, 1911: I’ve been reluctant to discuss the fire at the Grenfell Tower in London, largely because I didn’t see that I had anything original to say about it. I’m only slightly familiar with codes and practice in the UK and this horrific tragedy doesn’t lend […]

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Concrete Non-Failure

July 3, 2017

Concrete, as a composite material, has more potential modes of failure than steel. A steel beam can be overstressed (and yield or rupture), rust, or buckle sideways for lack of bracing. Barring some really esoteric failure modes, that’s about it and that’s plenty. A concrete beam (or, as pictured above, a slab) can be overstressed, […]

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