Historic Structural Detail: A Clever Composite

by Don Friedman on February 2, 2016

We live in an age with two overwhelmingly dominant fire-rated floor systems: concrete in metal deck in steel-frame buildings, and formed, bar-reinforced concrete slabs in concrete-frame buildings. (Plywood deck is used with wood and light metal joists, but that’s not are rated.) The past was more complicated. There were dozens of floor systems in use between the 1890s and the 1950s that are now largely forgotten…except when they turn up in a building investigation.

That’s a picture of a composite concrete-joist/concrete block floor. The thin tan-ish stripes are the underside of the reinforced-concrete joists; the wide gray-ish stripes are the underside of the concrete blocks. Above this is a thin non-structural finish slab resting on loose fill, resting on the tops of the joists and blocks. Why would anyone build something like that?

This building was constructed in the 1910s and there are a few facts about construction at that time worth remembering. First, reinforced concrete was still a new technology at that time. It had been in use for buildings in a form in some way recognizable to us for about fifteen years. The equivalent in modern tech is that fifteen years into the PC era we were using Windows 95. What does new technology mean? Among other things, concrete material quality was variable and known to be variable. Second, complex formwork was expensive, the same as it is now. Third, gypsum board didn’t exist, so ceilings were plaster on lath., applied to a solid substrate.

The idea behind concrete joist construction is to reduce the volume of concrete required for a floor by concentrating the structural capacity at discrete, closely spaced locations. You can build a whole lot of wood forms to create all those joists, or you can use metal joist forms (that look a lot like upside-down canoes), or you put rows of concrete block on top of a flat wood form. The blocks tend to adhere to the concrete, so rather than removing them you can use blocks with corrugated sides that will adhere well and stay in place. That gives you a continuous plane of material to serve as a base for the ceiling plaster.

Placing concrete so that it goes into the joists – into the spaces between the rows of blocks – and also goes on top of the blocks to form a slab is difficult and liable to fail unless you have high-quality concrete and good vibration, so you eliminate the top slab. Who needs a top slab anyway, when you’ve got well-adhered blocks giving you a top surface? But the blocks are a bit irregular compared to a cast slab, so you’d better use some fill to even things out and a topping slab to hold the fill in place and give a smooth walking surface.

To repeat, why would anyone build something like that? Because it made sense in their circumstances.

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