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 is, of course, a suspension bridge, but Pearl Street is inland of the Manhattan cable anchorage and therefore this part of the approach is basically a huge ramp. That ramp is supported on bearing masonry walls for most of its length, but there are steel-framed bridges to carry the ramp roadway over the streets below.
The ramp’s bridges were all upgraded in the course of the twentieth century to carry heavier loads. So in this picture we’re looking at original steel from the 1880s and newer steel probably installed during the 1950s renovations. There are real differences between the two eras that are worth pointing out. The biggest is the presence of large wide-flange sections in the newer steel, specifically the big arches at the bottom. The technology that made wide-flange sections possible was still twenty years in the future when the bridge was first built. Beams in the original construction were the older I-beam shape or built up of plates, angles, and rivets. This difference is one of technology only: the engineers in both eras understood what an efficient steel section should look like, but in the 1950s such sections were ready-made while in the 1880s they were a do-it-yourself project.
The more interesting difference is visible above the arches. The structural form of the original Pearl Street bridge was a deck truss, where the road runs over the top chord of the trusses. The truss form was a double-diagonal Warren,*** where the top and bottom chords are parallel and each panel marked by a pair of verticals has two diagonals running from the top to the bottom. In elevation, a truss of this type is a series of large Xs side by side. The bridge consisted of several such trusses side by side, and one is visible running diagonally down to the right from the upper left corner. In such a truss, designed to carry heavy gravity loads and little else, the bottom chord of the truss is in tension only. The form of the bottom chord is a group of parallel eyebars forming a kind of chain. This chain cannot carry any load but tension because (a) the bars themselves will buckle sideways if loaded in compression and (b) the joints between the bars can rotate** and so allow the whole chain to buckle vertically.
I remember being taught how to design a steel eyebar in 1985. I don’t remember the details of how to design one because I’ve never done so. The use of eyebars in steel structures may not be entirely over, but it’s close. That technology is an expression of several design ideas – including static determinacy of structures and non-reversal of load – that have lost favor. There’s nothing inherently wrong with those ideas, but steel technology has moved in a different direction. Even the uses of trusses for a short bridge like this has fallen out of style because changes in the relative cost of steel material, steel fabrication, and erection labor have made girder bridges cheaper.
Within the broad topic of steel construction, there are many competing sub-technologies. The use of any one of them changes over time. For example, bolting came, went, and came back again. It’s not often you can see two different eras of construction expressing two different sub-technologies side by side like this, and both are equally well designed.
* That phrase was occasionally used by professors when I was an undergrad, most often after they had covered several blackboards with equations. In retrospect, I assume it was a joke gone stale; at the time it seemed like torturers laughing at their victims. My sophomore-year roommate amended the phrase to “intuitively obvious to even the most casual observer on roller skates” which is my preferred version.
** In theory, anyway. In practice, friction, rust, and paint have the joints pretty well locked up.
*** (Footnote added 7/17/17): I was wrong, it’s not a warren truss. See here.
Edited: Not any more. On another topic: the easiest way to get notification of new blog posts is to follow us on Twitter.
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