There’s a beam inside that wall and nobody knows what it is. You’ll design onto it anyway, probably this week, and nothing about that is unusual. The footing is under the floor, somebody put it there before you were born, and either nobody drew it or somebody drew it and the builder did something else on the day.
The analysis is going, and honestly most of it went years ago. Frames have been sized by software since before you qualified, and your trade has been complaining about graduates who can drive a package and not sketch a bending moment for at least that long. What is arriving now takes the next layer off. A model built from the drawings. The load takedown, the sizing, the connections, the schedules, the clause by clause check against the standard, the calculation package written up so a stranger can follow it. Specification and report as well. Nobody sane enjoys the write-up, and I’d hand the lot over without an argument.
The usual reply is that the calculation was never the job, which is true, and then comes the sentence I want to argue with. The job is deciding what to worry about. I don’t think it is, and the reason is a compliment to your profession. Your worry list is published. A century of failures got turned into load combinations, factors, deflection limits, tying requirements, accidental loads, fire, wind, the lot. That is what a design standard is. It is a written list of what to worry about, paid for in collapses, and it is the most successful piece of knowledge transfer any trade has ever managed. Anything written down that plainly is available to whatever reads it next.
The judgment is somewhere else, and there are two of them, and they turn out to be one.
Most of what’s published assumes new work. New steel of a known grade, concrete somebody will test, timber with a stamp on it. There is guidance for assessing what is already standing, it is better than it was, and it runs out early. Then you get handed a building with cast iron in it, or wrought iron, or lime mortar, or a floor of shallow brick arches sitting on iron beams, or timber that nobody graded because grading did not exist. There is no clause for a beam a plumber notched to get a pipe through. There is also no clause for the fact that this floor has carried a working machine shop for generations without a murmur, which is evidence, and rather good evidence, and there is nowhere to put it in the calculation. You use it anyway. Every engineer does. Nobody has written down how.
The other one eats your week. How much to open up, and where. Every hole costs money, needs access, wrecks a finish, and has to be argued for with a client who cannot see why they are paying you to make holes in their own building. So you ask for a few in the right places instead of a lot in the wrong ones. Most of the job then rests on that choice, and you made it by guessing what was likely to be in there.
The guess is the trade. Not construction in general, which is published, taught and examined. What you are running on is a private catalogue of buildings you have had open. The era, the town, what the builders around here did when they were behind, which decade put a steel in properly and which one took the wall out and hoped. A floor that moves under you as you cross it has told you the span before you measure anything. A crack has categories and the categories are printed, but whether it is still moving is settled by paint, by dust, by whether the last repair has opened again, and by whether the door at that end binds in winter. That is where the two judgments turn out to be one. Where to open up, and what to make of what you find when you do, come out of the same catalogue, and there is nothing else to answer either of them with.
Then the ceiling comes down and there is a steel in there nobody drew, sitting on half a brick, painted the same color as everything else, and the floor above has been fine since somebody’s grandfather put it in. You have seen it now. There is no clause for that either.
What is happening with that knowledge is not the familiar story about wisdom being too deep for words. It is smaller and more annoying than that. Every piece of it is worth almost nothing on its own. One street, one decade, one builder’s habit. Write it down and you have produced a sentence nobody will ever go looking for. It pays only as an accumulation, held in one head, sorted by resemblance, and handed up without being asked for. Firms have tried to keep it. Defect libraries, lessons learned, the folder on the server that somebody set up with real enthusiasm. They fail, and not because engineers are lazy. The unit of this knowledge is too small to be worth recording and there is too much of it to record all of.
Scanners are getting better faster than the argument I’ve just made. Radar through concrete, a camera down a small hole, meters that find the reinforcement, monitoring left on a crack through a season. Feed a point cloud into a model and a lot of what I have been calling unknown stops being unknown. I think that is true and I think it takes a real bite out of my argument. What a scan gives you is shape. It does not give you the grade of the steel, or whether the connection was ever finished. It won’t tell you what the mortar has turned into inside the joint, or whether the bearing under that beam is brick or rubble with a face on it. Somebody still has to decide what to scan and get it paid for. I don’t know how far those tools go and I wouldn’t trust my own guess about it.
Which leaves the awkward fact about the catalogue. It is built out of whatever you happened to have open, so it is local, lopsided and full of holes you cannot see from the inside. Send somebody to a town they have never worked in and they are confident in exactly the same way, right up until they aren’t. Nobody in the office can tell the difference, and on a bad job neither can they.
The next time a ceiling comes down in front of you, stand there for a minute before anybody starts sweeping up. Everything you can see was invisible on Monday, and the drawing you designed from is still in the car with none of it on it. That drawing was wrong long before you got there. So is the next one.