Damian Hadley described a particular moment in a meeting to me.
The architect asked how thick the slab needed to be, and everyone turned to look at him, expecting Damian to produce a number out of thin air. “Like a magician.”
He suggested I build it as a problem for falsewrk. Span, load, deflection limit, single span or continuous - guess the thickness.
I’m a bridge engineer. I’ve spent 10+ years on things that carry traffic, not floors that carry people and partitions. I know how the structural mechanics work, but I don’t know what “normal” looks like for problems in this space.
So I sketched it out and sent it to Damian to check.
“It is actually challenging. We definitely don’t use that equation.”
The equation was AS 3600’s deemed-to-comply span-to-depth rule. There’s a single equation that can give you slab depth, and while I figured it was conservative, I also figured it’d be good enough for the kind of answer you give in a meeting.
“Throw out the deemed-to-comply. No one uses that.”
He threw down the gauntlet. We could do better.
Damian went away, called in a few other engineers, and came back with a spreadsheet. Not easy, he said.
A question that gets asked casually in a meeting with the architect is something that takes experienced engineers a weekend to agree on.
I’d asked him a side question along the way:
Do you shorten the end spans to balance the moments out? The ideal arrangement to balance the moments is to have the end spans ~80% of the internal spans.
In thousands of projects Damian’s had the opportunity to do it twice. Site geometry and the architectural setout decide where the columns go.
You don’t get to pick the ideal engineering solution. You design the floor you’re given, and almost always, the end span governs.
The full calculation - 3 No. 7 m spans, a one-way slab, constant thickness - is a problem you can drill yourself on falsewrk. Use the deemed-to-comply as a starting point, but that’s it.