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Around here (San Francisco), there are a lot of industrial buildings that I think were built in the ’20s, ’30s, and ’40s. Generally the roof is supported by barrel-vaulted lumber trusses. The bottom chord runs straight across the building, and the top is an arc of a circle. The top chords are not piecewise-linear; they are cut to the arc. I’ve seen spans from 30 feet to 150 feet. The roof decking is lumber set at roughly 45 degrees to the truss direction, so the lumber has to bend to form the roof.
I can’t see why the builders used this design. A truss with straight members is easier to build. Pitched roofs shed water more reliably. Bending the decking is an added installation process. So why did the old-timers use this design?
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I think what you're referring to is called a "bowstring "truss. They were really popular for a while, then they kind of faded in oblivion.
There's a picture at:
Bowstring Truss
There are 2 reasons that I can suggest as to why these trusses were used: First, what else did they have ? Metal plate connected wood trusses didn't turn up until sometime in the 1950's, and weren't popular until probably 1970 or so.
ALso - the rounded design means that the forces in the top chord were roughly the same all the way around the top of the truss. This type of truss was the best thing they had to work with during that time period.
That's about all I know.........
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Around here (San Francisco), there are a lot of industrial buildings that I think were built in the '20s, '30s, and '40s. Generally the roof is supported by barrel-vaulted lumber trusses. The bottom chord runs straight across the building, and the top is an arc of a circle. The top chords are not piecewise-linear; they are cut to the arc. I've seen spans from 30 feet to 150 feet. The roof decking is lumber set at roughly 45 degrees to the truss direction, so the lumber has to bend to form the roof.
I can't see why the builders used this design. A truss with straight members is easier to build. Pitched roofs shed water more reliably. Bending the decking is an added installation process. So why did the old-timers use this design?