Steel

Composite Structure

by Don Friedman on August 21, 2017


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.

Sidewalk Vaults and True Obsolescence

August 14, 2017

For people unfamiliar with sidewalk vaults, the illustration above, from 1865, might seem plausible, but it’s actually Daniel Badger’s fantasy of how he could sell more iron. The left-hand side is reasonably accurate for mid-1800s vaults: the facade columns extend down past the plane of the sidewalk, marking the separation of the cellar proper from […]

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Weathering Metals

August 9, 2017

Beautiful, right? Thanks to some old alteration work, we get to see the results of a nice little experiment in exposing metals to water. You’re looking at three beams here: a modern steel beam that had been directly supporting a sidewalk (middle left, outlined in purple below), the original cast-iron girder that supports that beam […]

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Self-Contradictory Structural Decoration

August 1, 2017

I thought I was done looking at the Pearl Street bridge that is part of the Brooklyn Bridge approach, but apparently not. This is another view of the new steel of the arch structure that was installed to resupport the original truss bridge. The big beam at the bottom and the two diagonal braces above […]

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Decorating Structure

July 18, 2017

The picture above is an oblique view of the Pearl Street bridge in the Brooklyn Bridge approach, showing the original (1880s) truss above and the newer (probably 1950s) arch below. As previously mentioned, one of the notable differences between the two eras is the type of steel member used: eyebar chains for the bottom chord […]

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Different Structural Forms Combined

July 17, 2017

Now that the weekend’s over, back to the Pearl Street bridge that is part of the approach to the Brooklyn Bridge. First, a minor correction to the first part of my street analysis of this structure: the original truss is not a double-diagonal warren truss. It’s a subdivided pratt truss, as some of the compression […]

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Different Eras With The Same Technology

July 14, 2017

There is so much going on in this picture that it’s going to take me two or three posts to cover it all. In case it’s not intuitively obvious to even the most casual observer* that’s a piece of the Manhattan approach of the Brooklyn Bridge, specifically the bridge over Pearl Street. The Brooklyn Bridge […]

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Riveted Lineage

June 22, 2017

National Geographic articles always have great pictures, and this one on the construction of Yankee Stadium is no exception. The article text tells the basic story; the best of the old drawings is, in my opinion, the 1934 extension steel diagram. (Yankees purists – which I am not – will probably prefer the architectural section […]

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Steel Trusses and Wood Purlins

May 19, 2017

No deep thoughts today, just appreciation for a well-designed roof. It’s an industrial building so finishes were kept to a minimum, and there was no requirement for fire-rating. The trusses are steel because they span about 50¬†feet; the purlins span 18. The end bays have diagonal rod bracing in the plane of the truss upper […]

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