A Moment In Time, 150 Years Later

That’s the West Main Street bridge in Clinton, New Jersey, spanning the south branch of the Raritan River. Clinton* is in western Jersey, due west of New York City and about two-thirds of the way form NYC to Allentown, Pennsylvania. The Raritan River takes a somewhat circuitous path before joining New York Bay at Perth Amboy. The bridge was built in 1870 and it’s still in service, with load restrictions, carrying local traffic as it always has.

If we compare this small bridge to the cutting edge of bridge technology of its day, it’s hopelessly backwards. It has two spans of roughly** 80 feet and structure that is a composite of cast and wrought iron; in 1870, the Eads Bridge was in construction over the Mississippi, with 520-foot steel arches, and the Brooklyn Bridge had been started in New York and would be built with a suspension span of 1600 feet supported by steel cables. But if we step back and look at this bridge for what it is, there’s something to be learned about mid-1800s structure.

Cast iron was still a perfectly respectable structural material in 1870 and would only decline in use after (a) the Chicago fire of 1871 showed once and for all that it provided no protection against fire beyond not actually burning, just like other non-flammable materials, and (b) its use in poorly-designed partial-frame buildings led to a number of collapses in the 1880s and 90s. It was known to be the strongest available material for compression and to be weak in tension. Wrought iron was equally strong in compression and tension, and failed in a ductile manner rather than brittlely, but was about a third less strong in compression. Designs in the 1860s therefore often combined the two metals to take advantage of the strengths of each.

Here’s the main Historic American Engineering Record drawing of the bridge:

The main trusses, as shown in elevation at the bottom, have the Pratt configuration (tension diagonals) with additional smaller back-diagonals added for local loads. (In several panels, the main diagonals are double rods, while the back-diagonals are single rods.) The presence of wind-bracing below the deck (middle plan) is evidence of real engineering design, as the original roadway deck was wood and would have contributed nothing to sideway resistance. Putting the main trusses between the pedestrian paths and the roadway (i.e., treating the pedestrian paths of cantilevered outriggers from the main structure) shortens the span of the main side-to-side deck girders and provide the pedestrians with some little bit of protection from horse carts. The main deck girders (top section) are also composite: tension rod girders, where wrought iron tie rods strengthen cast iron beams, but this is apparently a 1910s retrofit, where the rods were added to increase the capacity.

Close up of a main truss, showing the cast-iron compression vertical, wrought-iron rod diagonals, and cast-iron compression top chord:

Finally, the HAER drawings showing the complexity of the connections at the lower-chord nodes, where the truss members, below-deck rod wind-bracing, main deck girder, and sidewalk deck girder come together:

It’s a good design given the constraints/assumptions that went into it. Increased experience with wrought iron (and later steel) and the failures that highlighted the weaknesses of cast iron meant that this type of design was only used for a short time. Cast iron use had already been abandoned for railroad bridges because the impact loading from the reciprocating steam engines in locomotives had been proven dangerous to the metal.

* Named after New York governor DeWitt Clinton. Lions and lambs, Hatfields and McCoys, etc.
** The banks of the river (and thus the end abutments) are not parallel and the mid-river pier is not parallel to either of them, so giving an exact span length is difficult.

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