Historic Structural Detail: Triangular

by Don Friedman on July 19, 2016

We see draped-mesh floor slabs all the time. They were the best solution to the problem of creating a stable, strong, fire-resistant floor for steel-frame buildings from the 1910s into the 1950s. Usually the mesh is similar to modern steel-wire mesh except that the old mesh was asymmetrical, with heavier wires at a closer spacing in the carrying direction and thinner wires at a larger spacing running the other way.

Not always.

23rd street - triangular mesh 1

That’s a probe made by removing the concrete at the bottom of a slab that has exposed triangle mesh. (Click on the picture to enlarge.) The carrying wires are spanning left to right and the transverse wires run at 60 and 120 degrees, creating a series of equilateral triangles. The role of the transverse wires in either rectangular or triangular mesh is to tie the carrying wires together, and there’s no wrong way to do that.

Wires rust faster than rebar when exposed to the same conditions because the smaller wires have a larger ratio of surface area to volume and therefore a larger percentage of the steel is exposed to oxidation at any given moment. Here’s what it looks like when the wire rusts:

23rd street - triangular mesh 2

That’s…ungood.

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