The most important thing in this picture isn’t in the picture. Those joists are in a 150-year-old industrial building and what’s missing is creep. Sag. Curvature. Deflection.
I don’t know how heavily the floor above was loaded in the past, but it’s got some real load on it now and I have to believe that in such a long history and factory use that it hasn’t been loaded heavily in the past. When wood beams are heavily stressed for long periods of time, they “creep.” What that means is they start to develop a permanent sag that’s present whether they are loaded or not, as opposed to the ordinary deflection under load that goes away when the load is removed. The floors in factories are heavily loaded, so they are somewhere that you might expect to see creep.
Whether or not a given beam does this depends on a. lot of factors, but the two most important are the moment of inertia (a geometric property of the beam that varies with the beam’s depth and width) and the elastic modulus (a physical property of the material, which varies with the species and grade of the wood). Multiplied together, these two factors are the beam’s stiffness. In this case, the joists are relatively large for their span, are most likely a better species of wood than the fast-growing Doug-fir we use so much of today, and being so old are probably old-growth timber (and therefore a better grade than ordinary modern wood).
I’m speculating about the wood’s physical properties, but I’m doing so on the abyss of a critical piece of evidence: the missing creep/sag.
For those wondering about the title, I’m ripping off Sherlock Holmes.
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