Historic Preservation

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.

A Long-Overdue Designation

December 12, 2017

The IRT Powerhouse has been designated as a New York City landmark. First, as the pictures may make clear, this is a huge building that is quite visible: it fills the block between 58th and 59th Streets, and between 11th and 12th Avenues. In other words, it’s 200 feet wide and about 600 feet long. […]

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What’s Been Lost

November 20, 2017

The green glass-walled building straight ahead is 2 Broadway. It’s as close to a generic circa-1960 office high-rise as you can get. There’s nothing about it that’s inherently bad…except….it was built on the site formerly occupied by the Produce Exchange. Here’s the Exchange, designed in the 1880s by George Post: In short, we lost a […]

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Something New

November 18, 2017

The idea of using LEGOs to provide temporary repairs to damaged masonry is either genius or moronic. I alternate between the two opinions.

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Missing One Thing

November 7, 2017

That’s a picture of demolition in progress at a 140-year-old house – a mansion, really – in Brooklyn. The bulk of the building will be demolished and/or altered into an apartment house that will look quite different. There is a need for more apartments in the city as construction of new units, particularly at the […]

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Three Myths About Brutalism

October 25, 2017

Neave Brown, the architect of the Alexandra Road housing estate in London (above, click to enlarge) as well as other brutalist housing projects, has recently been recognized for his work. In architectural terms, this is undiluted brutalism, with nearly all exterior surfaces as bare concrete or glass. The article at the second link above summarizes […]

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Road Trip: Adaptive Reuse Hotel

October 12, 2017

I’m currently at the APT conference in Ottawa, and that’s my hotel, the Metcalfe, above. It was built circa 1906 as a YMCA and has been converted to a nice boutique hotel. You can always repurpose a building if you want to.

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Better Than The Alternative, If You Squint

September 21, 2017

Richard and Anne Dickey, a wealthy couple of the era, had a house constructed for them in 1809-1810 on then-fashionable Greenwich Street. This was before rowhouses were being built in New York and long before the craze among the wealthy for ridiculously large mansions. Their house was about 40 feet by 60 feet and there […]

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A Site With Useful Information And An Unfortunate Name

September 14, 2017

Preservation engineering (or conservation engineering outside of North America) is relatively new and suffers from a number of problems common to newish subfields. The biggest problem, from my perspective anyway, is a lack of basic common information. If I want to explain to clients energy-code issues with glass curtain walls, there are any number of […]

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Code Intersectionality

September 12, 2017

There’s been some discussion in the last couple of weeks on the topic of Belgian Block paving – usually and somewhat incorrectly referred to as cobblestones – being impassable for people with mobility issues. A solution exists for this particular problem, which is to provide smoother pavement at crosswalks. This allows people in wheelchairs or […]

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