A Mixed Palette

I feel like every time I mention the Williamsburg I disparage it – because I don’t like its appearance and several of the design details that distinguish it – but it’s important to remember that it was an important milestone in suspension-bridge construction. It took the main-span record from the Brooklyn Bridge (if only by a few feet) and held it for more than twenty years; it was the first bridge with such a long span to have steel towers. I stumbled across an Engineering Record write-up about its anchorages while looking for something else, and it’s quite interesting. Here’s the Brooklyn anchorage in the picture above:

Here are elevations and a top plan (minus the roadway) from ER in 1897:

In broad terms, this is very much like the Brooklyn Bridge design, with some updates based on the 25 years separating the designs. The cables come into the anchorage and end in eyebar chains that bend down to anchors at the bottom of the huge mass of masonry. Very simple vector diagrams will tell you that the bends in the chain cause compression radially downward at each chain joint. The tension in the cable is resolved by having gravity pulling down a dead weight on top of the anchors. Meanwhile, the weight of the anchorages needs to be supported so that it doesn’t sink into the riverbed, so it sits on piles.

First observation: the cables are steel, the eyebars are steel, the “reaction platforms” (the anchors, and what a great name for them) are steel, but the anchorage is ashlar masonry and the piles are wood. A small amount of unreinforced concrete was used as a form of masonry. In short, the materials used reflect the on-going transition at that time from traditional to modern construction. The lesser-controlled materials – wood, stone, and unreinforced concrete – were used in situations where variations in material strength would not be critical.

Second observation: the steel in the eyebars is described in some detail and is quite good. It had a yield stress of 35,000 psi, a minimum ultimate stress of 60,000 psi, and reasonably tightly controlled damaging impurities (sulfur, phosphorus, and silicon). It’s not all that far from the ASTM A7 and A9 steel that would be defined just a few years later or from the A36 steel that dominated American practice from the 1960s until after 2000.

Third observation: that lump of masonry is a highly-designed object. Here are various sections through it:

Years of effort by many talented people went into a design that, despite its aesthetic flaws, has carried pedestrians, trains, and vehicles from horse-wagons to large trucks for 119 years. You can look at the designers’ use of wood and stone as an anachronism that engineering eventually moved past, or you can argue that they used materials appropriate to the tasks at hand rather than relying on just steel and concrete.