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It Wasn’t A Skeleton To Them, Part 1

One of the biggest changes in the way that people in the design and construction field see buildings is the idea of frames. You can almost pinpoint the moment of triumph for frames as an idea: when Le Corbusier did his best to convince architects that houses should elevated on columns. Modern structural analysis software treats bearing walls as a specialized form of 2D frame elements, reflecting the way that a lot of engineers think.

The picture above shows the New York Public Library under construction in 1904. The vertical strip windows on the wall facing us are the west facade, so the stone yard is Bryant Park and we’re looking a bit north of east. The NYPL is not a frame building: the walls inside and out are bearing walls supporting steel beams and trusses. It’s probably the last large public building in Nw York design and constructed this way. (Interestingly, there are frames in the picture: the temporary cranes used to haul stone.)

Part of the concept (perhaps obsession) in the Beaux Arts style of having the plan match the section match the elevations has to do with structural logic. If your building will have bearing walls and you don’t have an easy way to transfer a wall laterally, you need to account for the wall locations from top to bottom. The poché space on your plans – the unusable locations of masonry walls – are as important to construction as the design of your floors. When the floors are vaulted masonry (rare in the US) the proportion of wall thickness to room size is important to make sure you have enough weight in the walls to counteract the vault thrust. With wood-joist floors, that issue goes away, but the wall layout still drives floor design, forcing a link between the vertical and horizontal. In other words, old bearing-wall buildings may have been simple geometrically, but they were always thought of as 3D objects. There’s no such thing as an isolated wall in that world, because an isolated wall will collapse.

Switching to frame construction meant that walls changed from unmovable planes to just another piece of the building. It took architects and engineers some time to understand this, but once they did, partial-bay setbacks and other shapes not possible with bearing walls became common. It was an architectural slippery slope, from skeleton framing (i.e., supporting masonry walls on a steel frame) to introducing partial-bay setbacks and other examples of walls supported over usable space, to having large storefront windows at the ground floor below ornate masonry facades, to all-glass facades. (See The Structure of Skyscrapers for more on this sequence.) That’s all fine, but reading that sequence backwards is a mistake. People in 1890 weren’t longing for a way to create building frames because they did not see buildings as a skeleton surrounded by flesh and skin. They wanted various improvements – ways to fireproof their buildings, ways to reduce the oppressive thickness of walls on tall buildings, ways to build faster – but it wasn’t until the new structural form was developed that they started to see existing buildings in that way.

Tomorrow: how engineering modeling is embedded in the idea of a building frame, for better or worse.