Waffle, Iron

A mildly-unusual floor in an 1920s apartment house:

By the time this building was constructed, designers had, after several decades of indecision, settled on cinder-concrete draped-mesh slabs as the standard floor used for steel-frame buildings in New York. That floor system was perfectly fitted to the design criteria of the times and as the bulk volume of the slabs was made of coal cinders – which is to say someone else’s garbage – they were cheap. One of the limitations inherent in their analysis and design was that the length of a span had to be around 7 feet or less. This limitation was so much a part of their design that one name for the system was “short-span concrete floors.”

Occasionally, you will come across the floor in the picture above when designers wanted a long span. The most common reason is that they had a big room and didn’t want a deep girder running across it and lowering the ceiling. I’ve seen this floor in several high-end apartment houses from the 20s and 30s in living rooms that are more or less square in plan.

What you’re seeing: the floor is a concrete waffle slab, with ribs running in two directions at right angles to one another. Unlike modern waffles, the void space between the ribs isn’t empty. Rather, it’s filled with hollow precast concrete blocks (similar to modern concrete masonry units, but shaped differently). The interruption to the rib pattern on the left is a concrete-encased steel beam. The picture was taken during renovation, and you can see the line of the now-demolished dropped ceiling on the partition in back. The few random pieces of black iron wired to the floor underside are remnants of the original ceiling construction.

The way this floor was built is that a flat wood form was constructed, the concrete blocks put on the form, the rebar for the ribs placed between the blocks, and then concrete placed over the top of all of it. The blocks are permanently-embedded formwork for the ribs, and became non-structural as soon as the concrete cured. So the holes in the blocks, while disconcerting, do not in any way weaken the floor. One question I was asked to address was whether the holes made the floor unsafe. My answer was (1) only if a piece of a block was disconnected from the ribs enough to fall and (2) that the holes reduced the fire-rating of the floor, but that could be addressed by patching or using a fire-rated hung ceiling.

The most important advice I can give about dealing with oddball structure like this is to assume the people who built it were at least as smart as you. They had good reasons for doing what they did, just like you do. It’s your first job, today, to understand what they did. Then you can assess if their work is okay or not.