Clever Design in an Unnamable Form

by Don Friedman on December 29, 2015

I’ve seen a lot of long-span wood roof trusses, both good and bad. Most of them are hidden, but some of the best trusses are the ones that were meant to be exposed as part of the architectural interior design. Case in point (click to enlarge):

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The trusses are not a standard form, but are perhaps best thought of as modified queen-post trusses. In a queen post truss, two verticals run from the upper chords (See “A” on the marked photo below) to the lower chord, usually near the mid-points of the upper chords (the quarter-points of the lower chords), and a horizontal strut runs across between the points where those verticals (the queen posts) intersect the upper chords. (There’s a good, if slightly overcomplicated picture here.) The way that this type of truss works is that the side triangles (made up of the queen posts, the far ends of the upper chords, and the far ends of the lower chord) are rigid, the top triangle (made up of the horizontal strut and the inside ends of the upper chords) is rigid, and the middle of the lower chord is a tension tie that keeps those three triangles from rotating relative to one another.

In this church, the upper triangle is deformed, with the horizontal strut taking the form of the middle of an arch. (This is a fake hammerbeam truss – a real one would have horizontals coming off the top of the side walls that were cantilevers, and no lower chord between the ends of those cantilevers.) The center portion of the lower chord has been replaced by an iron tension rod (C). This truss has no real name, but fortunately it doesn’t stop working just because we can’t classify it. In a particularly fine detail, the metal connectors (F) from the wood lower-chord ends (B) to the iron lower-chord center also support the lantern hangers. The trusses support purlins (D) which support common rafters (E) which support the roof plank sheathing.

The best detail of this well-designed roof, in my opinion, is that there are two diagonal trusses at the crossing, and the two iron rods are connected with a decorative flourish where they intersect. Can’t find it? Look at G below.

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