Doing The Wrong Thing Right

by Don Friedman on December 14, 2017

I’ve talked before about my ambivalence concerning the way in which the Dickey house on Greenwich Street is being preserved as part of the development of a new tower next door. Façadism does not have a good reputation in the preservation world. Since no one has asked my opinion, the work continues regardless of what I say.

Taking the façadism as a given, the bracing seen in the photo looks pretty good. One of the problems in general with preservation engineering is getting people to understand that design criteria have to be different from those in modern codes. The tower going up on the site on the left is about 99 percent of this project and the Dickey house 1 percent, so the mindset will inevitably be skewed towards new construction. The walls of the Dickey house are made of early-nineteenth-century brick, which is soft and weak compared to modern brick, and have mortar that has almost certainly deteriorated from its original condition. The issue is not stress in the brick, as any engineered shoring would be intended to keep the stress from wind pressure low. The issue is movement and the secondary stresses it creates: it would be relatively simple to destroy the historic walls without them being overstressed in simple analysis by allowing them to move too much.

Allowable movement is defined in the building codes and elsewhere in at least two ways. First, beams supporting masonry walls* are limited to a deflection of 1/600 of the beam span. For comparison, deflection for human comfort is 1/240 of the span and deflection to prevent cracking in plaster ceilings is 1/360 of the span. In high-end residences, when we’re trying to limit perceptible movement, we usually aim for something like 1/480 of the span. One six-hundredth is quite restrictive. Second, in discussing foundation movement from construction adjacent to old buildings, absolute horizontal and vertical movements are restricted, usually to something like 1/2 or 3/8 inches.

As I’ve said before, buildings and building materials are dumb and can’t count. What matters is not the numbers but rather differential movement of one piece of structure when compared with another, as that is the situation that creates secondary stress. By limiting relative movement (as with beam deflection) or absolute movement (as with foundation work), the idea is to limit walls’ differential movement.

The amount of steel shoring seen above may seem excessive, but its purpose is to prevent movement of all of the masonry. Fewer and bigger steel supports could work for stress, but would allow more local movement of the brick. Having many separate struts to brace the walls is more work in constructing the temporary shoring, but it means less movement of the brick and therefore less damage during construction.

Well done, whoever designed that.


* It is assumed that the beams are more flexible than the walls. In all but extraordinarily weird and rare circumstances, this is a good assumption.

Rivets in U.S. Structures

December 8, 2017

(Picture: Three workers installing rivets in the construction of the Empire State Building in 1931, from the Lewis Hine collection at the NYPL.) Rivets in the metal structures now seems archaic and from other times. However, rivets were extensively used for metal structure in the 19th century and in the first half of the 20th. […]

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“The Bridge”

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Some great construction photographs of the Verrazano Narrows Bridge: here. I’ve worked on a few pedestrian bridges in the last thirty years and one dam, but that’s it: otherwise my projects have all been buildings. But bridges occupy a chunk of my brain and always will because of their nature as expressed engineering. There are a […]

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Everything In A City Is Manmade – Even The Water

November 30, 2017

Continuing with yesterday’s theme… New York is located on two medium-sized islands (Manhattan and Staten Island), a portion of a large island (Brooklyn and Queens are the west end of Long Island), a piece of mainland (the Bronx) and a whole bunch of small islands in the surrounding waterways. Those waterways include small streams (e.g., […]

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Just Like Us

November 28, 2017

A theme I tend to return to again and again is that people in the past acted the same way we do. They used the tools they had, worried about cost, sometimes had great ideas, sometime made dumb mistakes. They looked for ways to do things cheaper and easier, like we do, and that sometimes […]

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An Early Stage

November 25, 2017

I hadn’t seen this picture before. That’s the Woolworth Building under construction, looking northwest from the east side of Broadway, near Ann Street. The ornate building on the right is the old General Post Office. The low buildings on the left were part of the Astor Hotel and were replaced in the 1920s by the […]

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Another Old Joke

November 24, 2017

Level, flush, or plumb: pick one. In this case the designer and the bench operator disagreed as to which was important.

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Temporary Until It’s Not

November 21, 2017

That’s the underside of a highway overpass. It’s old enough that the built-up plate girders that are the main structural elements are riveted together rather than being bolted or welded. The concrete deck looks very new and is a replacement. Since water and (in the past) road salt attack this structure from the driving surface […]

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When Is An Arch Not An Arch?

November 15, 2017

That’s the F/G subway, paralleling 9th Street in Brooklyn, where the tracks cross over Fourth Avenue. The train is elevated here because it crosses over the Gowanus Canal a little to the west at the Smith/9th Street station. As the tracks head east, to the left, they stay at roughly the same elevation as the […]

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An Unseemly But Necessary Growth

November 3, 2017

There’s no mystery as to what that big thing sticking up above the roof of this building* is: it’s the top of an elevator shaft. The windows in line with the thing are all marked “SHAFT WAY,” which is a not-so-subtle hint; the fact that this is a mid-1800s commercial building that is still in […]

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