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.

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