Sideways

One of our stranger projects… The pictures below show a sidewalk vault in Manhattan. The vault extends from the sidewalk (its roof) down two floors or about 25 feet, and was built around 1895. Since reinforced-concrete was still in its infancy then, it was not used for foundation walls, and the outside wall of the vault (near, but not quite in line with the curb) was made of masonry vaults spanning horizontally between steel beams.

I took this picture standing on the lower vault floor looking more or less horizontally toward the outer wall. You can see the curve of the brick vaulting; the black stripes flush with the brick are the inboard flanges of the support beams. Note that these beams are oriented vertically: they span from the slab on grade at the lower level to the upper vault floor above, and from the upper vault floor to the sidewalk. At the top, above the yellow pipe, you see the underside of the upper vault floor.

For non-engineers – and, unfortunately, for some engineers – the whole “vertically-oriented beam” thing is apparently confusing. A column is a structural member whose primary load is compression along its main axis. It may have bending in one or both other directions, it may have shear, it may have torsion, but the main load it carries in compression. On the other hand, a beam’s primary loads are bending and shear. The vertical steel members embedded in that wall are carrying bending from the horizontal pressure of earth on the wall, and little to no vertical compression. So they are vertically-oriented beams.

The brick vaults are in good condition after some 130 years, maybe because their outboard (earth-facing) sides are waterproofed with a thick layer of tar and maybe because whatever water they’ve been exposed to has simply worked its way through to the inside space where it evaporated. The beams were rusted where we could see them, and that raised a question about the condition of the parts we couldn’t see. You can sort-of see the solution above, but here’s a better picture:

We ran new vertically-oriented beams floor to ceiling and used them to shorten the span of the old beams to about 20 percent of their full length, which reduces the bending stress in the old beams to around 4 percent of its original amount. The new horizontals carry compression from the old beams to the new, so they are compression struts or, basically, columns. If it bothers you, look at the second picture and rotate your head down to the left so that your right eye is on top, and all will be well.