Dumb Luck

by Don Friedman on February 27, 2017

That is not good. It’s an old concrete slab – the age is demonstrated by the board-form marks, meaning it was built before plywood was available – reinforced with wire mesh. The mesh is visible on the left and across the middle because the cover concrete has spalled off; the cover spalled off because the wire is rusted and the rust jacked the thin shell of concrete cover.

The problem here is that the wire is responsible for all of the strength in the slab. With the wire rusted to brittleness – I broke a piece off with little effort – the slab has only accidental strength. It can work as an unreinforced concrete slab in bending for maybe one-quarter the load that it can work for as a reinforced slab. And the reinforced slab will fail gradually if overloaded, while the unreinforced slab will collapse without warning when it reaches its unknown load limit.

So…why hasn’t this fallen down yet?* The short answer is that the load imposed on top of the slab hasn’t reached the unknown limit on the effectively-unreinforced slab. Live loads vary over time and follow a rough bell curve. (Not the normal curve, but an asymmetrical curve that reaches zero.) There’s an average load and any load higher than that is less likely, meaning it occurs less frequently. The load imposed on this slab simply hasn’t made it to the high end of the curve recently…which is not a reassuring thought.

* A few years ago, I used that as the title for a lecture. In a way, it was unfair, since I could use that title on three-quarters of what I write.

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