This is a big topic, so my plan is to break it up as follows: today (The Good) I’ll talk about why people would build such oddball building structure as wood bowstring-truss roofs, tomorrow (The Bad) I’ll talk about why the extant roofs are a problem (with one example being their potential for outward thrust), and the day after (The Weird) I’ll talk about some of the really peculiar aspects of them. That’s the plan, which is subject to change if any one of the three segments gets to be too long, or there’s an emergency, or someone rolls a shiny ball past me.
The picture above is from another of our five-year-old investigations, and shows a beautifully average bowstring truss. This building has the peculiarity that the roof sheathing is carried on a lattice of very small, very closely-spaced purlins, which is definitely not average and gives the roof the feeling of a sweater knitted from coarse thread.
Thread number one: prior to the common use of wrought-iron, spanning a roof across a big interior space was a real problem in design. All sorts of heavy-timber trusses were developed for this purpose, but they all had the same flaw: they required heavy timber. Long, straight, large cross-section timbers are expensive and require fancy joinery at their connections. We see a lot of heavy-timber gable trusses (king-post, queen-post, and scissors, most often) in our practice in church roofs. It wasn’t worth the money and effort to build such a roof over a low-end industrial building like the one in the picture above.
Thread number two: the history of wood construction in the US in the nineteenth century is that of mass-produced small-section “sticks” (2x4s up to maybe 3x12s), connected with wire nails, driving our heavy timber on economic grounds and because stick construction is so much easier.
Tie the threads together, and you have the bowstring truss as we know it here in the US. Starting around maybe 1900, people started building long-span trusses out of 2xs. The chords were a bunch of 2xs ganged together and with the joints in each piece staggered, so that if you had six 2x6s as your chord, you could count of four or five of them transmitting load at any given point (which four or five varied along the length). The web was a lattice of 2x4s or 2x6s, not much different than that used in the wood lattice trusses of the early and mid-1800s. In a lot of the bowstring trusses, the lattice members are closer together at the ends, where the shear forces in the web are greatest. The curved upper chord, like the peak on a gable truss, gave the roof a built-in slope for drainage.
I’ve glided over the best feature of these trusses: the small size and light weight of each piece. The web is a bunch of 2xs that are at most ten feet long. The chords are a bunch of 2xs that are twenty feet long. And that’s it. Other than nails, bolts, and a steel strap at each end, that’s all the material used. You can span 120 feet using a bunch of 20-foot 2xs, which is a pretty neat trick. These trusses were cheap to build (cheap material and no special skill needed in fabrication or erection), and could give you a column-free floor as big as you’d need for a circa-1900 warehouse or factory. I was about to type “what’s not to like?” but the answer to that question is coming tomorrow or so. Meanwhile, here’s a close-up on the bottom chord:
There’s a non-structural fascia board obscuring the view, but if you look at the very end of the chord (bottom of the photo, center) you can see the exposed ends of the ganged 2xs where the fascia stops. You also see that weird lattice sheathing on the wall (below the sprinkler pipe) and at the end of the roof (above the modern electrical conduit). Here’s the top chord:
Note that the bottom chord 2xs are oriented vertically, while the top chord are horizontal. The chord members are nailed together to allow for continuity past the 2x “splices”, while the bolts connect the pieces of the chord con each side of the web through the web. You can also see how the lattice web becomes almost solid at the end of the truss.
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