Yesterday I talked about the difficulty of fitting a code from one era to a building from another. A bit of creative interpretation may be necessary since some of the governing assumptions of the document may be wrong. The reverse effect can also be true, where a literal reading of current code has a different effect than intended, but one that is still beneficial. The example that comes to mind is the meaning of lateral-load provisions in current (as in today’s) code compared to the existing old-building stock.
The old masonry-bearing-wall buildings in New York were designed without analysis of their walls for load. From 1882 until 1916, the codes had complicated narratives describing the minimum thickness of masonry walls – generally assumed to be brick – for different occupancies, different heights, different distances between intersecting walls, and so on. Those rules provided a more or less rational empirical design – walls were increased in thickness at the lower floors of tall buildings, for heavier use, for longer spans between intersecting walls – but no analysis was required. When rational analysis of frames began to be written into the code, short and stocky buildings were excluded from wind analysis, and this is represented in the extant buildings: I’ve seen any number of low-rise steel-frame buildings with no defined lateral-load system. They depend on their masonry “curtain walls” for lateral resistance. In one case, the building in question was a city-built firehouse, constructed just before that exemption was removed in 1968. The other lateral load, earthquakes, didn’t show up in NYC codes until 1996, given our relatively low maximum ground acceleration compared to some other parts of the country.
I used to feel that the old wind exemption made sense and that seismic risk was overstated. I never argued either of the points, as the people who are looking at the analysis portions of the NYC code are more knowledgeable than I am, but I’m perfectly capable of following and promoting a code I don’t entirely agree with. My argument for the wind issue is that building collapses in NYC directly resulting from wind pressure are so rare and obscure an event that they’re hard to find. (The only example I know of had some pretty severe water damage as a contributing factor.) My argument for the seismic issue was the pair of questions: how many people have died in seismic-related building damage in New York since 1624? How many died in building fires last year? The answer to the first is zero, the answer to the second is more than zero. In short, even if any given rowhouse or tenement has no recognizable lateral-load system, they work together when located in rows, and for most of the twentieth century newer buildings were constructed tight against them, as seismic gaps were not required. In addition, the sheer density of development in Manhattan tends to protect old buildings against receiving the full force of wind.
Here’s why I was wrong even if my arguments were right: following the newer code provisions for new small buildings and for renovations of existing small buildings increases their resilience. Wind and seismic loading do not increase the same way based on height, width or use, so they tend to enforce better resilience from two different (philosophical rather than geometric) directions. And there are a pretty good number of failures related to lack of resilience: walls bulging or collapsing out of plane, for example. By demanding for renovations a modern form of analysis that was not used in the original design, the current-code wind and seismic provisions improve overall performance. There is also, obviously, the strong possibility that my old arguments are wrong.
The picture above shows a collapsed roof-top billboard and lights from just south of Times Square, after a wind storm in 1912 that supposedly had 110 mile-per-hour winds. That kind of wind is certainly possible here, facing the Atlantic Ocean and on some hurricane tracks.
