Yet Again, Everything Is A Trade-Off

If the first thing you look at in this picture are the columns with the conical capitals or the waffle slab, you may be in the AEC professions or otherwise interested in buildings. If the first thing you look at is the track hung from the ceiling and snaking its way around the floor, you may have some experience with meat processing. That track carries animal carcasses on meat hooks along a disassembly line. This building, the M.T. Marcello Company Building in Providence, Rhode Island, was built in 1947 to turn carcasses into sausage. Structure was of concern only in so far as it would be good that the building stood up, so that gives us a sense of the logic that went into choosing this particular structural system.

The choice of concrete is no surprise. Concrete multi-story industrial buildings were first built in the US around 1900 and this form – curtain walls that were mostly window, two-way slabs, round columns with splayed capitals to address the punch-through shear problem arising from heavy loads and relatively weak concrete – had been regularly built in cities across the country since the early 1910s. By 1947, the concrete code existed under its current name: ACI 318. It was still based on linear-elastic, allowable-stress design, but it had broken out of twenty years stuck in a multi-organization committee that couldn’t move it forward. Allowable stresses, for example, had increased in ACI 318 compared to the older, pre-ACI code of the 30s.

Waffles are a form of two-way slab, in the same way that joists are a form of one-way slab. Regular column spacing, as you typically have in an industrial building, simplifies things a lot. In an ordinary two-way slab, you have bottom and top rebar distributed evenly within each “strip” of the slab, with one set of strips following the columns lines in each direction and the other set of “middle” strips missing the columns in each direction. Basically, it’s a plaid pattern. In a waffle, you group the reinforcing together every 12 or 16 inches and reduce the piece of slab between to a minimum thickness. This works fine for the portions of slab with positive moment, since you still have the reduced slab to carry compression in those areas, countering the bottom reinforcing’s tension. It works less well in the negative-moment areas because there is no slab there for compression, only the ribs. The negative moments are highest at the columns, so eliminating the coffers (that define the waffle) in those areas in favor of solid slabs addresses that problem as well as helping with shear.

Looking at this solely from an economic perspective – as you might if building a factory for a meat packer who just wants a safe building for the least money – the cost of the reinforcing and concrete material is about the same for waffles and flat slabs. The waffle is slightly more efficient in design, so the amount of rebar might be slightly smaller; the amount of concrete might be slightly smaller as well, or the same, or even a little bigger depending on the geometry. But those are minor, incremental changes in cost. A big difference is that the waffle has to be built in negative in the forms for the wet concrete. Pre-fab forms for the coffers have been around since at least the 1910s, but they still have to be installed and removed, and that’s a good amount of extra labor for the waffle slab. If you play your cards right and the waffle has less total concrete than the flat slab, then the dead load is reduced, making the waffle lighter still, and making the columns and foundations lighter.

So the choice between a flat slab and a waffle, if made rationally, comes down to the savings on material costs for the waffle versus the savings on labor costs for the flat slab. The fact that both were used suggests that the overall costs were close, and local conditions – a temporary spike in rebar costs, or a shortage of carpenters – might sway the decision.

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