The world of architecture, engineering, and construction is full of technical terms that sound like ordinary words but are not. Like every other field, we have our jargon; the problem is that so much of our jargon is made incomprehensible by being so close to ordinary word usage. Everyone knows what “heavy timbers” are: they’re big pieces of wood, as opposed to two-bys or three-bys. Except that’s not what “heavy timber construction” means.
I’ve talked a bit about systems in the past, and that type of logic is what’s necessary to understand heavy timber. That logic, and some history. The need for seriously fire-resistant buildings in the US didn’t start with office buildings (because prior to the end of the 1800s, the vast majority of office buildings were more or less house-sized) and didn’t start with residential buildings (because even today, we accept wood stick-built houses as ordinary). It started with industrial buildings and mostly with factories. Factories were big (for the era), had large open spaces and therefore fewer internal fire-breaks, were full of machinery (and therefore full of oil and the potential for sparks), and often full of flammable materials (cloth, thread, paper, wood, and so on). Structural fireproofing, as in modern materials that are good insulators and therefore can resist fire, was developed starting in the 1870s, after the Chicago and Boston fires and after iron framing had achieved some popularity. Before that, the only fireproof material we had was masonry, and building an all-masonry building (i.e., using masonry-vault floors supported only on masonry walls) was so expensive that US examples are incredibly rare.
Some mill owners and builders – starting in the 1840s, improved through iteration into the 1880s, and still building well into the twentieth century – came up with the answer: the heavy-timber system. It relies on the way wood burns: it chars on the outside faces where it’s burning. If you have a small-dimension stick (a two-by, for example) the corners and the large surface-area-to-volume ratio ensure that the unburned interior gets exposed to the flame quickly. If you have a large-dimension piece of wood, then the char acts as insulation, slowing down the spread of fire. Char is, of course, rather weak and will therefore break off and the fire will spread inward, but much more slowly than with small pieces of wood. This behavior is familiar to anyone who has ever set, or even watched, a fireplace or a cooking fire. We use small pieces of wood to get it started and larger logs that will burn slowly to keep it going.
Another critical part of the system is to stop, as best as is possible, heat from rising from one floor to the next. So not only are the floor planks very thick – 3 or 4 inches, usually – but they are either tongue-and-grooved or grooved-with-splines. And there are specific details to make sure that the floor-to-wall joints are tight enough to prevent hot air from streaming upward and setting the edges of the floor planks on fire.
In the picture above, the two beams are the roof framing to create the opening for the skylight and you can see the splines where there are gaps between the planks. The sheet-metal connectors for the framing are to keep the member sizes intact, rather than cut them down for traditional wood connections. The piece of wood that appears to be in the wall does not actually reduce the wall thickness because the three courses of brick below it are corbelled out. So we get the tight wood-overlapping-wood connection at the wall but the wall stays the same thickness and strength. Here’s a view from nearby that shows the corbelling more clearly:
This system, going on 200 years old, is recognized as a fire-rated form of construction by our current codes, even though its main construction element is known to be flammable. That’s a pretty clever design.
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