(Above, the personification of Prudence, with pet dragon.) A few years ago, I went down a rabbit hole of reading on the topic of epistemology. This is an incredibly complex topic, in part because it’s reflexive (how do you know that the theory of knowledge you have is actually a meaningful theory of knowledge?) and I just scratched the surface. That all started because I was trying to answer the question, for myself, of how I know what I think I know as an engineer. Or, more simply, why do I think some buildings are safe and some are not?
I know as a fact – which means that I am convinced but have to spend time convincing others – that there are plenty of buildings that do not meet current code and are safe. There are plenty of buildings that do not meet any code and are safe. The obvious companion question – are there buildings that meet code but are not safe? – is a more interesting one. In order to answer it, we need to move one notch up the scale of abstraction and ask what we mean by “safe.” Prior to April 2019, few people would have said the cathedral of Notre Dame was unsafe, but it turned out to not be safe in a fire. In other words, extreme conditions will destroy any building, so “safe” does not mean indestructible. A building can be safe today but if not properly maintained it will be unsafe in a few years. (This is the basis of New York’s FISP law.) In other words,”safe” is not an inherent condition but is contingent on time and effort. Opening this up to some real argument, a building can be safe even if I do not know exactly how it functions because most or all of the structure is hidden from view. In other words, “safe” does not (or maybe should not) depend on personal opinion.
If we use, for new and newish buildings, the minimum standard of code compliance, we run into a well-known problem. Every change in every code, whether the IBC (and its derivatives like the NYC Building Code), a material code like the AISC code for steel, or a general engineering code like the loading described in ASCE-7, has the effect of making things that used to be considered “unsafe” now “safe,” making things that used to be considered “safe” now “unsafe,” or both. If I designed a new building (or even just a single beam) under the old code and the rules changed in the new code, was the structure I previously designed now unsafe? The code answer is no, it is not, and that’s why there are grandfathering provisions in code, but it pretty much is the killer argument for why a lack of code compliance is not proof of an unsafe condition.
To get back to my original question, there are a few basic concepts that are, at least in my experience, infallible. Gravity pulls downward. The wind will sometimes blow, and the earth may sometimes move. Loading causes stress, and stress (pressures within materials) and strain (deformation of materials) are always linked. Every materials fails at some level of stress; we call “brittle” those materials that fail at low levels of strain. All of these concepts, and similar platitudes, lead to looking at how structures have performed as an indicator for how they will perform. An old unreinforced-masonry building exposed to real loads (a church on a corner lot, for example, exposed to wind loads on a regular basis) that does not have cracks in brittle materials (the masonry, plaster ceilings, plaster-covered partitions) reads to me as “safe” because it has a history of sustaining load without ill effect. I can’t perform that kind of analysis on a new building because it does not have that history: maybe it will suffer damage the next time there is an abnormally high wind.
The danger in empirical analysis is that it may be interpreted as “you’re telling me that because it hasn’t fallen down it won’t fall down?” I don’t think an old building is safe simply because it exists. That’s a pretty clear example of survivorship bias. I think an old building is safe because the visible conditions (and they have to be seen throughout the building, not just seen from one angle) do not indicate overstress (which will lead to material failures) or excessive movement (which will lead to instability and/or material failures). That logic is both independent of codes and subject to personal interpretation, which is probably why it makes people uncomfortable. And if I argue (as I did above) that the condition of “safe” does not depend on who is looking at a building, then I’m not actually determining what is safe. Every building is, at any given moment, safe for some loads and some external conditions and unsafe for others; so what I’m doing is deciding if I believe the building I’m looking at is safe for some set of loads (ordinary occupancy, or temporary construction conditions, to give two possible examples) for some period of time (today, or for the next five years for FISP, or the next twenty to thirty years for ordinary structure). That’s a reasonably consistent answer but perhaps one that lacks a satisfying air of certainty.
