There Now, That’s Not So Good, Is it?

From 2018, a nasty surprise after the ceiling was removed: More specifically, the gypsum board ceiling that had been put in place below the original plaster was removed, exposing the plaster, and the failure of the plaster, and the failure of the bottom of the cinder-concrete slab, and the failure of much of the wire-mesh reinforcing. The short version: the roof terrace above leaked, and the rusting mesh spalled the slab and plaster. Obviously installing a new gypsum-board ceiling had solved the problem. The more interesting question is: why didn’t more bad things happen? Or, why wasn’t there a visible collapse? There’s no one answer to that question, but rather a bunch of partial answers which more or less get added together. First, despite appearances, the wire mesh wasn’t completely rusted: there was still some viable steel there. Second, the old designs for catenary slabs undercounted the strength of the steel, limiting wire tension to 20,000 psi when the material used could be…