I’ve posted pictures of a lot of truss bridges here, ranging from spidery-thin to super-chunky. I’m not a bridge engineer and while I can appreciate those structures, they’re not what I deal with in projects. The picture above, on the other hand, is exactly the kind of thing I deal with all the time.
This is a boiler house at an industrial site in upstate New York, that pretty obviously has been abandoned for some time. It was built in the 1910s, long after steel skeleton framing was well established, but it’s a bearing-wall building. The brick wall you see is structural. The roof consists of a concrete slab supported on steel I-beam purlins (running up and down the photo) which are in turn supported on steel warren trusses that span the width of the building. The roof has a center ridge and slopes off to each side for drainage, which can be seen as the bend in the top chord just to the right of center. It’s not visible in this shot, but the trusses are supported on brick piers built integrally with the side walls.
We don’t use unreinforced masonry for structure much any more, but you could probably prove the walls and piers here work for modern code. It is, after all, only one story high, and the roof carries snow load and nothing more. But my attention keeps going back to those trusses, which almost certainly do meet modern code requirements. The main reason we got away from using structure like that was a change in the economics of steelwork. Steel material used to be more expensive relative to the labor needed for fabrication and erection, and the maximum available size of rolled beams used to be much smaller than it is now. We could design off-the-shelf beams today instead of those trusses, and they’d be heavier but cheaper. The design of a simple beam also requires a lot less engineering labor than the design of a truss. The design of roofs like this, and similar structures, is what encouraged some engineers in the late nineteenth century to abandon bridges and civil works and move into the design of buildings.
Trusses had much more flexibility in geometry than beams: making that ridge and slope using beams would mean a splice at midspan, which is not is bad but annoying; making a sawtooth roof for clerestory windows, which I’ve seen done with trusses in similar industrial buildings, would require so much work with beams that you might as well use a truss. The total weight of steel is less with the trusses, which matters if you have to erect the steel in a place where using a crane is not possible, like inside an existing building during an alteration. And, to my eyes, trusses are more graceful. On the other hand, a lot of fabrication effort goes into all of those connections, total weight usually is not that important, and to a lot of people trusses look overly busy compared to the simplicity of a single beam. De gustibus something something.
