Today’s post was added to this series in response to a comment regarding yesterday’s post. I said yesterday “The question we have to ask is not whether it meets the specific requirements of today’s code (it almost certainly does not) or the code in force when it was built (it might or it might not). It’s whether we can show that the way it works is equivalent to what it was meant to be. This is both more difficult than simply looking for code compliance and much more interesting.” The response was:
In case the embedded tweet doesn’t show, Bill Harvey said “Oh, indeed, much more interesting, but not for the faint hearted. The question of competence requires serious introspection.”
I believe I’ve already addressed the first half when I said that you had to have the strength of your convictions when using a method other than those specifically present in the codes. The second half is more important and deserves a longer answer. The first part of the answer is that discussions of licensure are meaningless. Every structural engineer in legal charge of work in the US is licensed. (Unlicensed engineers legally work under supervision of licensed engineers.) In most states, they are Professional Engineers who have taken a test on general civil engineering, which is a far broader field than structural engineering; in a few states they may be Structural Engineers. But SEs are tested on general structural engineering, not any of the subspecialties within the field. We rely on people being trained on the job and being ethical enough to work only in their field of expertise to prevent, say, someone whose work revolves around wood-framed low-rise apartment hoses from taking a project involving a long-span suspension bridge. (We also rely on market forces to direct projects to those capable of handling them, but that’s a fallible system.) Unlike doctors, we do not have specialty certifications, so there is no official body that has said, for example, that I am qualified to work on old buildings.
This may sound naive, but I rely on the engineering basics to make the distinction between people who understand old buildings and those who don’t. When I rely on an alternate method, I analyze it using tools based in stability, statics, material behavior, and other building blocks of structural engineering. You can analyze a stone cornice, for example, using free-body diagrams, which are about as low-level as you can get in structural analysis. This is an obvious way to proceed and actually present in codes. IBC section 1604: “Load effects on structural members and their connections shall be determined by methods of structural analysis that take into account equilibrium, general stability, geometric compatibility and both short- and long-term material properties….Any system or method of construction to be used shall be based on a rational analysis in accordance with well- established principles of mechanics. Such analysis shall result in a system that provides a complete load path capable of transferring loads from their point of origin to the load-resisting elements.” For what it’s worth, “geometric compatibility” was what I was talking about on Tuesday. I’ll take any argument made on those grounds seriously, although I may disagree with it.
What don’t I take seriously? Arguments based on codes. Basic codes are meant for new construction and (in the US, at least) existing-building codes don’t have enough detail about how to perform structural analysis of existing buildings to be helpful. The IEBC discusses when you have to use the new-building code because of the extent of work, but not what to do otherwise. Following the IBC or IBC-based local codes on existing buildings leads to over-conservative design, outright errors, and gaps where the code is silent on obsolete and archaic structural materials and systems.
In other words, you can simplify the question of who is competent to analyze old structure using alternate methods to looking at who discusses analysis based on first principles of engineering and who uses modern analysis and current code requirements. It’s bit crude but in my experience it’s reasonably accurate.