If it’s difficult or impossible to easily categorize the structural types in buildings, as I suggested yesterday, then how do you do it? I guess the first question to answer is why would you do it?
Being able to categorize the structure of existing buildings greatly speeds investigation, alteration design, and repair design. Since most of our practice involves exactly that type of work, speeding it up is good for us and ultimately good for our clients. It’s certainly possible to work a project from first principles, and figure out every aspect of the existing construction that way, but it’s slow and difficult. I’d guess that just about everyone in our field has some kind of categorization that they use.
When we started working on our guide to the structure of New York buildings, the first task we had was deciding how to organize it. There are over a million buildings in New York and we were aiming at 90 to 100 pages; the only way to fit an average of 10,000 buildings per page is through categorization. The answer was to categorize structure by using visible architectural criteria: a building’s original occupancy, its age, and its size. This is quite different from categorizing structure by using structural criteria, because there can be multiple structural forms that meet the same architectural criteria and a single structural form can meet multiple architectural criteria.
The illustration above, from an 1890s report on tenements, gives a sense of the issue. Plan number 1 shows a pre-Civil-War house, and plan 2 shows that same house converted to a rooming house. Plan 3 shows a purpose-built rooming house with a rear building on the lot. Plans 4 and 5 shows pre-Old-Law tenements, with their interior rooms and maximized lot coverage, plan 7 was the competition winner for the design to improve tenements, and plan 6 shows a typical Old Law tenement. It’s probably worth pointing out that all of these except number 1 were terrible places to live in the nineteenth century, before their plumbing and heating were upgraded.
All seven types have the same basic structure – wood joists carried by brick bearing walls – but the differences are important. The Old Law went into effect in 1879, and New York’s first comprehensive building code went into effect in 1882. The code set minimum standards for wall thicknesses that were generally thicker than those previously used on multiple dwellings. So if you know that a building is an Old Law tenement – and the fastest way to make that identification is the presence of those narrow side light-wells – you can make an educated guess as to how thick its bearing walls are. If you know that a building was constructed before the Civil War (i.e., before the 1860), you can make an educated guess that the wood used is old-growth timber and therefore denser and stronger than more modern wood. If you see interior rooms, you can make an educated guess that the building was constructed before 1879 and therefore before the minimum wall thicknesses of the 1882 code. Structural information has been derived from architectural information.