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One Of The Rare Exceptions

As a coda to the issue of using the proper words to describe things – critical for engineers (as it is for pretty much all professionals of any kind) in order to keep from being misunderstood – I present: a structure that is a mechanism. The picture above shows the Grand Street Bridge in Bridgeport, Connecticut, in 1995. This was after the bridge had been removed from service and several years before it was demolished. The bridge was a double-bascule bridge, AKA a drawbridge where both halves of the span rotated upwards when it opened. In other words, this is what I meant when I said there were rare exceptions to the general rule that structures should not move perceptibly.

The bridge opened in 1919 over the the Pequonnock River, connecting two industrial neighborhoods in an industrial city. While it’s entirely possible that the bridge had reached the end of its useful life when it was closed in the 1990s, the fact that it had been rehabbed in the 60s and 80s suggests that maybe there was a different cause for abandonment. By the time the HAER survey took place in 1995, the industry was gone from both ends of the bridge, as were many of the industrial buildings. Maintaining a bascule bridge is more expensive than maintaining a fixed bridge, which is not necessarily cheap; doing so when there’s little traffic may seem like a pointless endeavor.

Here’s a view under the deck, showing one of the main girders on the left, deck girders in the middle, the 1960s steel-grating deck, some of the lateral braking (the seemingly-random diagonal angle at the top right), and a maintenance walkway from center right running to the center. The nearest deck girder is haunched where it connects to the main girder, which seems odd at first glance: this is not a moment connection, since there is no equivalent connection for the top flange of the deck girder. Looking at the other side of the main girder clarifies things:

The sidewalk on each side overhangs the main girders by over 7 feet, and it carried on these triangular brackets. Those brackets impose a torque on the main girders, and torque was something that designers of that era tried as best they could to eliminate. (Even today, it’s a pain when dealing with slender I-shaped beams like these.) The inward pressure from the lower leg of the bracket is aligned with the main bottom flange of the deck girder, which helps a lot; the haunch serves to provide lateral bracing to the bottom flange of the main girder.

But this is a moveable bridge! On to the fun stuff. Here’s a motor and part of the gearing for one of the trunnions:

And here’s the support for the trunnion (the pivot). The big featureless block on the upper left is the concrete counterweight.

The concrete weight is a tip-off that this was a product of the Strauss Bascule Bridge Company, which was one of the leading moveable bridge companies in the US in the early twentieth century. I like the incongruity of two of the moving parts of this bridge are huge blocks of concrete.