X-Ray

by Don Friedman on August 20, 2015

For someone working with old buildings, this picture (click on it for a larger version) tells a sad story: a 1920s skyscraper is being demolished. Of course, for someone working with new buildings, the story is a happy one: a 2010s skyscraper will shortly be built on this site. But that’s not what I want to talk about.

The removal of the side wall of the building has exposed the steel frame and provides a beautifully clear explanation of how solid brick can deserve the name “curtain wall.” The edge of the (three-window wide) street facade on the left looks paper thin even though it is 12 inches thick; similarly the lack of depth to the demolished side wall is visible in how close the steel columns and beams are to the outer brick face.

For an engineer, the visible steel tells several stories. The steel is in good condition, which is a function of good maintenance on the brick, luck, the red-lead paint coating (the reason that the steel is reddish brown) working as intended, or some combination of the three. Bracket connections – barely visible as diamond-shaped steel plates at the junctures of the beams and columns – indicate that the lateral-load resistance in the building is provided by a moment frame. Of course, the phrase “lateral-load resistance” is an anachronism, since the engineer who designed this building almost certainly referred to the bracing as “wind frames,” a phrase still in use in New York into the 1980s. Comparing the column locations to the windows in the undemolished wall area shows that each column is marked by a wider-than-normal brick pier between windows. The lighter brick at the window heads indicates areas where steel lintels had rusted and had to be replaced, and the new brick matches the original (AKA clean) color of the old brick.

Finally, a small, older building was demolished before this picture was taken. The red-brick remnants of one wall can be seen on the right, where they contrast with the yellow brick. Note that the window pattern is distorted where it runs up against the small building and that the red brick appears to be flush with the yellow brick. This was most likely an old party wall that was incorporated into the new steel-frame building as part of its exterior enclosure.

This kind of analysis of what you see by just standing still and looking at an old building is somewhere in-between a parlor trick and real work. If, for some reason, we were hired to review the condition of the soon-to-be-demolished skyscraper, the information I picked up on the sidewalk would save me time on the inside. On the other hand, most of the time doing this is simply fun…and maybe practice.

 

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