Investigation

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.

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