TLAW

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Mar 152018

There is an acronym that is commonly used in the various engineering disciplines (it certainly was in aerospace): TLAR.

That Looks About Right

What it means is simply that some things are so well understood and characterized that at least at first glance, to first approximation, at the back of the envelope stage, a design can look like it will work. Someone can, say, sketch out a jetliner… a tubular fuselage, modestly swept low-mounted wings, swept tail surfaces, podded engines suspended below the wings – and it will look like a “proper” design. TLAR is useful for things people have really nailed down the design of over the years. Entirely new stuff? An Alcubierre Warp Drive ring assembly, for example… who the frak knows right from wrong on that. But jetliners? Ships? Automobiles? Launch vehicles? Sure. An engineer can look at a design and say “that looks about right.”

And bridges. A good engineer can take a look at a design and say “that looks about right.” And sometimes, even an engineer from another discipline with rusty skills can take a look at a bridge design, and his engineering-spidey senses will start tingling, and “TLAR” is *not* engaged.

I look at the design of the failed FIU pedestrian bridge and man, TLAR is *not* what pops into my head. Instead I get a distinctly That Looks About Wrong feeling.

To be fair, the design of the *completed* cable-stayed bridge (by Munilla Construction Management, whose website still hilariously claims: “Safety first! At MCM Safety is paramount and we are committed to zero accidents on all projects.“) looks pretty ok to me:

It looks fine. It has two spans, each supported at the ends atop piers, and then in five places along each span by what appear to be quite stout tension cables connected to a central tower. It looks nice. Completed, it looks nice. Incomplete, it scares the pants off me:

Note how here, during the rapid assembly process, the bridge is supported from below at four points: the two ends on the piers, and within the span by temporary supports. This is a perfectly good way to install a suspension or cable stayed bridge: support it from below until you can get the cables in place. Really, there aren’t too many other practical ways to do it for a bridge like this. But where me “I want to be elsewhere and unassociated with this project” response kicks in is when they remove those central supports… without having the suspension cables in place. The design of this span just does not Look About Right for something supported only at the ends. You have a great big and seemingly massive deck at the bottom, a few centrally located diagonal supports, and then a relatively narrow structural span running along the top.

Note that the deck certainly looks pretty thin… it appears to be one, maybe two feet thick. Doubtless of steel and concrete construction, but still quite thin. As a cable-stayed span, the deck would be hanging every however many feet from those diagonal supports; the upper structure could (*could*) be virtually cosmetic. However, as a simply supported bridge, that lower deck is under a *lot* of tension, the upper structure under a *lot* of compression, and the diagonal supports transmitting those loads in a way much different from when it’s a cable-stayed span (cable stayed, they’d be in tension; incomplete, they’re in compression).

In its incomplete state, it just doesn’t *look* like a decent structure.

That’s of course easy to say now that its laying in the street. And let me be clear: an engineer should never, EVER say that something is good unless they’ve run the numbers, and should avoid saying something is bad unless they’ve run the numbers. Engineering is the wrong discipline for anyone who operates by “feelings;” it is the place for hard numbers, hard facts, objective reality. Merit rather than politics. Still: the reality is that in a world of hard facts, some things are WRONG, and you don’t need to do a whole lot of math because the facts have already been long demonstrated. You can’t run an internal combustion engine on water, nor can you tinker with your carburetor to make your otherwise unmodified Ford F-150 go 200 miles per hour and get 500 miles per gallon. You can’t make the spar of your jetliner out of butter. You can’t use a pound of dynamite to blow the Moon to flinders, nor can you make a perpetual motion machine out of a cordless drill and some weights. These are of course ridiculous examples, but there is a spectrum between “that’s obviously so stupid I don’t need to do the math” and “that looks about right.” And the FIU-Sweetwater bridge certainly falls between the two. “Feelings,” I found during my engineering days, were, when applied properly, an appropriate and useful check against unwarranted enthusiasm and optimism. I get the feeling here that someone should have been a bit more pessimistic during the design process.