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A Chicken and Broken-Egg Problem

The list of natural enemies includes not just Montagues and Capulets, and roadrunners and coyotes, but plumbers and wood framing. This is a bearing wall in a building in Greenwich Village and a plumber with a saw did a number on those studs. In case it’s not obvious from the wood lath, this is a nineteenth-century building and it’s been modified a number of times.

The first question that comes to mind is: why has the wall performed well structurally with chunks taken out of the studs at mid-height? The real load in the studs has probably been less than the design loads because of the probabilistic effect used in live-load reduction.[efn_note]The studs don’t qualify for code live-load reduction because their tributary area is too small, but the effect is still there. It’s just less predictable for a small area. [/efn_note] But more importantly, material that we don’t think of as structure can carry load and will if load is applied to it. In this case, it’s fairly easy to show that the plaster sheathing on this wall can work to restrain the studs against buckling without exceeding the (rather low) allowable stress for plaster. Obviously, the people who were involved in constructing this building and those like it didn’t think in those terms. But they knew from experience that a plastered wood-stud wall would work as a structural bearing wall.

A problem the comes up with this kind of thing is that by the time you know the details of the construction, you’ve damaged or destroyed possible oddball structural mechanisms like the plaster. One of the first stages in any construction project is interior demolition, and the plaster here was removed so that the plumber for the current project can do his thing.[efn_note]Hopefully without any more cutting of wood.[/efn_note] This is, in my experience, a real issue with wood-framed buildings. I’ve seen old houses that racked sideways during alteration because all of the interior plaster was removed and, without it, the walls lost enough of their stiffness to stop functioning properly.

In other words, wholesale removal of plaster and lath can interfere with the functioning of structure that we can’t see until we remove the plaster and lath. There’s a simple solution froom the engineering standpoint: before construction, remove only enough plaster to get the information needed about the base structure; during construction, shore the wood framing if significant amounts of plaster will be removed. The trick to this is getting everyone else to agree.