New And Improved Graphics!

Seven years ago [!] I spent some time looking at the Pearl Street overpass portion of the Manhattan approach of the Brooklyn Bridge. The short version is that it’s an 1880s steel truss reinforced in the mid-1900s with a steel arch below. The long discussion is: here and here and here. Anyway, here’s yesterday’s photo, taken while walking from Foley Square to the office:

The pedestrian walkway is directly above; the truss-and-arch combos on each side are below the roadways. The masonry at the far end of the overpass is the Manhattan cable anchorage.

First, just as a work of inadvertent graphic design, the contrast of the curved wide-flange arches and the eyebar and built-up truss members above is fantastic. Throw in the concrete pedestrian deck and the granite of the anchorage and we’ve got some very different textures.

The connection between the truss and arch is interesting. There’s no easy way to connect to an eyebar chain (the truss bottom chord) with six parallel bars, and the presence of the pin connection makes it more difficult. So the designers for the arch simply put a big heavy plate on top of a compression strut at each pin location and let the truss bottom chords bear on them. For some reason a nursery rhyme about “sat on a tuffet” springs to mind.

I’ve written a lot here over the years about trusses. I like them aesthetically and they are structurally quite efficient. I think this photo shows the difficulty in maintaining old-fashioned trusses compared to new structure: think about the effort to paint each surface of those eyebar chains, and each internal surface of the built-up struts.

Finally, and this is probably the most personal-taste-centric comment I’m making here, I think there’s something to be said for a structure that is both complicated and readable. At first glance this is a blacksmith’s shop, but a little time spent looking at it makes the “truss propped by an arch” general scheme apparent, and the difference between lateral (wind) bracing and the main gravity structure apparent as well.