Steps of Poured Concrete with Balanced Reinforcing
At the east end of our old house on South Avenue was a set of rotting wooden steps giving access to the outside door of what we called the Downstairs Cold Room—from the fact that, in our first year or so, that wing of the house had no heat and, in winter, the broken windows upstairs allowed snow to pile up on the floor.
Under the old steps was Meg’s “house” and she wasn’t happy to hear that progress was about to change it.
The existing steps at the front and kitchen doors (and the two terraces) were monolithic blocks simply poured against the foundation and, unfortunately, directly against the wooden siding of the house itself. Already, in a previous year, I had had to rebuild the front entrance owing to moisture induced rot in the sill and the consequent depredations of carpenter ants. I vowed that new steps would not so compromise the structure of the house.
In contrast to the solid mass of the existing steps I thought to make something “lighter” and more graceful, and so I set to work at the drawing board to see what might be forthcoming.
Beams of concrete alone, if relatively thin, cannot be employed in long spans. This is because concrete, while strong in compression, has only a small ability to resist corresponding tension. Gravity bends a simple beam under load downward; the beam resists the load by developing compression in its upper surface (chord) and tension in its lower chord. In the extreme the concrete cracks at the bottom. The solution to the weakness is to embed steel in those parts of concrete structures under tension. And this leads to a structural concept called balanced reinforcing. Concrete is used in compression up to its design load limit
on top, and embedded re-bars in tension to their limit at the bottom. That is, under extreme load, one could not predict for sure whether the concrete would crumble on top or the steel bars below pull apart. Achieving such a balance yields the most economical use of material in the structure as a whole.
Fortunately my engineering handbooks provided sufficient technical detail to permit creating a balanced design using 3/8” rods (re-bars). The straight rods had to be bent and for this I made a jig embedded in a sturdy block of concrete buried in the ground with two bending posts protruding at the surface.
At the top a 4-way* slab for the landing, so called because of its proposed support at the four corners producing bending in two directions. I assumed as design load a 250 pound mover in the center carrying half a piano. The steps going down are the same as the slab only narrow. In the center the re-bars are in the lower chord and, at the overhangs, in the upper chord because here the bending tension has moved to the top.
Slab and step spans are three inches thick at the center, tapering to two and a half at the ends to give a light effect.
Slab and steps are supported on two arches leaning against the house foundation.
I assembled the wooden concrete pouring form first for the left arch. After that pour having set we stripped it and reassembled it for the right arch. Matthew helped me mix concrete and aggregate in the wheelbarrow. The form for the landing slab included pockets for the end posts. Re-bars wired in place and supported on “chairs” of small stones become incorporated into the final mass. The form for the narrow steps was a strip cut out of the slab form. By the time we poured the bottom step that form had been stripped and reassembled twice and was almost a wreck.
We cast the date (1971) into the upper slab and all three children added their hand prints. When one jumps on the slab and steps they have a nice stiff “ring”. And Meg got her “house” back. Today the steps are forty-five years old; a bit weathered, but as stiff as ever.
*Technically not quite true.