February 2016

Historic Structural Detail: Diagonal Sheathing

by Don Friedman on February 29, 2016

Every once in while, when you strip the outer sheathing (clapboard or shingles) or the inner finish (plaster) off a wall, you find that the exterior sheathing is a bunch of diagonal planks.

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Why?

A topic that I keep returning to is that people in the past had to be clever because they had fewer tools than we did. That’s “tools” in the broad sense, meaning all of the technologies we use in construction today, from computerized analysis to manufactured lumber. In this case, the modern technology that is missing is plywood.

The traditional, pre-industrial method of providing lateral bracing to wood-frame building was to use diagonal braces between exterior-wall posts and the floor-level girts within the walls. This method works fine, but requires large-dimension timbers and a lot of fancy carpentry to create the mortise-and-tenon connections. The introduction in the mid-1800s of the houses framed with 2x and 3x “sticks” required a different system. One possible solution, used mostly before 1900, was to “let in” 2x braces running diagonally in the same plane as the 2×4 or 2×6 wall studs. This still required a lot of relative difficult work, notching each stud on an angle to create the space for the brace to fit. (The braces couldn’t be simply nailed to the stud faces or they would interfere with the sheathing.) These braces continued the previous idea of discrete braces providing stability.

A different idea was to use what we would now call a diaphragm: a planar structural element loaded in its strong direction, in-plane. When we use plywood as exterior-wall sheathing, that’s what we’re doing, creating a lateral-load diaphragm. To do it without plywood took some thought. Horizontal plank is extremely inefficient as a diaphragm, as its lateral-load resistance is limited to the moment that can be developed by a pair of nails through a plank-to-stud connection. It’s true that there are a lot of those connections and the resistance is additive, but the resistance at each connection is so small that the totaling it from all of the connections is still weak. Diagonal sheathing, on the other hand, connects the studs to the floor structure (either rim joists, or the top and sole plates, depending on the frame type) and creates a whole lot of the triangles that engineers love for bracing. This form of bracing works so well that it is still in the codes as an acceptable method of bracing a building, and since the diagonal planks are a continuous plane of sheathing, they can just be nailed to the outer face of the studs.

That’s why.

More Familiarity

February 26, 2016

When we were working on this project, I didn’t know it would be Russ & Daughters. I just knew it would be a new cafe in the museum basement. The Jewish Museum is housed in an interesting building. Most of it is a 1908 mansion, with a 1960s expansion that was modified in the early […]

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They Look Familiar

February 25, 2016

The New York City Landmarks Preservation Commission has been acting recently to clear up a backlog of buildings that had been “calendared” – that is, marked as buildings that would be researched for possible landmarking designation – that had built up before 2010. Reading the list was a trip down memory lane, as we have […]

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The Master of Cast Iron

February 24, 2016

When you mention cast iron, James Bogardus’s name comes up most often, but he’s not the guy I’m talking about. Bogardus greatly encouraged the use of structural cast iron in the mid-1800s and built some interesting structures, but his work output is eclipsed by that of Daniel Badger. Badger went from making door and window […]

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Best for NYC: Best Practices, Talking And Not

February 23, 2016

In the process of discussing our practices for the Best for NYC Challenge I’ve talked about talking  about our work as a form of education for the public, and I’ve talked about talking about our process to keep our employees up to date. That’s a lot of talking, which raises the question: when to shut up? […]

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Ambivalence Revisited

February 23, 2016

Some time ago, I discussed the EIFS facade being installed near to our office windows. It’s nearing completion: And it looks halfway decent. Dark gray is a reasonable color for a facade in New York and the workmanship looks good, given the material. Of course, we’ll see how it weathers over time.

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The Tallest In The World, Times Two

February 22, 2016

I previously mentioned our work at the Park Row Building, but the one picture I attached didn’t really do justice to the topic. This, on the other hand, shows off the building at its best: You may ask, what is that remarkably beautiful thing? That’s where a lot of our work, led by Ryan Cleary, is located […]

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Historic Structural Detail: Some Artifacts Are Bigger Than Others

February 19, 2016

In most alteration projects, the first stage of construction is demolition of the interior finishes. When that work is done, we often find conditions that we did not know existed and would not have known with the demolition. Sometimes those conditions affect our work, sometimes they are simply artifacts showing us some information about how the […]

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Changes Over Time and Changes In Loading

February 18, 2016

We’re currently working on a team that is creating a Historic Structures Report for Castle Clinton. Our concern, obviously, is the structure of the old fort. But for us, part of analyzing the current structural conditions is looking at the load history of the building. It’s current state is roughly similar to how it was during […]

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Not An Optical Illusion

February 17, 2016

A lot of what is now SoHo was once a swamp. Don’t take my word for it, just zoom in on the Viele map. What does that bit of history mean today? Among other things, it means that the soil in that area is relatively soft and tends to settle more than average under concentrated […]

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