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Why and How?

Besides being a nice example of Gotham City architecture in the real world, that photo – Staple Street looking south from Harrison Street – accidentally shows a problem with old bearing-wall buildings. A lot of them don’t have readily-definable lateral load systems. I’ve taken part in some serious discussion recently about buildings of this type and they pose some problems.

The building on the left, 5 Harrison Street, is about twenty years old. It’s a decent attempt at contextual architecture, trying to fit into its Tribeca surroundings, but it has modern structure and therefore is not part of the topic. The building on the right, 7 Harrison Street, was built in the 1890s as loft (i.e., industrial) space. Steel skeleton framing had been established before this building was constructed, but was not yet being used for buildings of this size or use. It has masonry bearing walls. Or rather, it has masonry bearing walls on its east and west sides, a somewhat-discontinuous bearing wall in the center, and non-bearing walls at its north side (facing Harrison Street) and its south side (facing a lot line). The front and rear walls should serve as shear walls to provide lateral bracing against east-west wind[efn_note]and theoretically seismic, but that’s anachronistic[/efn_note] load. And that’s the problem: the first-floor base of the north wall consists of cast-iron storefronts with very little lateral strength.

In other words, the ordinary engineering description of a bearing-wall building fails. The front wall above the second floor can carry lateral load as a shear wall, but it can’t transfer that load to the foundation in the expected manner. On the other hand, the building has been standing for over 125 years so something is working.[efn_note]Few things annoy me more than engineers throwing up their hands at this point. If you’re not willing to analyze a peculiar structure that requires thinking outside your comfort zone, you’re not much of an engineer.[/efn_note]

It’s unlikely that the building has ever seen the level of wind pressure mandated by current code. Wind, like any fluid pressure, can only exert force perpendicular to a solid, which means that a building of similar height to most of its neighbors and located on narrow gridded streets is protected against full wind load. Wind blowing down a street can’t exert much load on the buildings facing the street. That said, this building has received some wind load and even if it hadn’t lateral stability is required to keep a structure from simply slumping over like a house of cards.

There are all sorts of non-traditional mechanisms that can carry lateral load. The cast-iron storefront columns, if connected to the stone piers at the corners and at the center wall, might be acting as composite columns that are part of a stiffer first-floor frame than the cast-iron alone is. Interior partitions – particularly those around the stairs and elevator shafts – might be serving as shear walls. Depending on how the floor beams are anchored to the bearing walls, there may be some lateral-load resistance built in there. None of these (and other oddball possibilities) are addressed particularly well by building codes and they can be difficult to prove. They are somewhat easier to disprove, so a process of elimination helps.

Old buildings aren’t bad versions of modern structure. They’re their own thing, and you have to try to figure out how and why they are standing if you plan to work with them.