Composite Structure

That’s the side wall and an oblique view of the front of a small building on Nassau Street. The front appears to be early twentieth century, but the odds are good that this is an older building that has been modified multiple times. If nothing else, the storefront – visible as sheet metal at the lower left – dates from the 1970s or so.

The very dark vertical stripe at the juncture of the side and front walls is a rectangular-section cast-iron column. Just barely visible above it, where the sheet metal is bent, is a pair of I beams. The column and those beams, along with whatever unseen columns are on the opposite side of the building, are the support for the front-wall masonry over the glass void of the storefront.

This is absolutely standard and common construction from around 1900. It represents the result of a rational thought process: masonry is good for enclosure, cast iron is strong in compression, steel is good for bending, so let’s use each material in the form to which it is best suited. The result is a composite structure, where different materials work together in a designed manner.

By our standards today, there’s a lot wrong with this. Cast iron use was abandoned because the material can fail catastrophically and without warning. There is no provision for lateral-load resistance in the column-and-beam frame. The metal is not fireprotected. None of those issues means that this design was a mistake when it was built and it should not be treated as such now: people building it then could not know how our standards would be different from theirs. Current use of the phrase “composite structure” is usually limited to steel frames with masonry and concrete shear walls, or steel beams that share load with concrete slabs. That seems, in light of this and many other combinations of materials that are no longer used, unnecessarily restrictive.