A Bridge Seen Out Of Context

That beauty is the Fort Point Channel bridge for the New Haven Railroad, part of the final approach to Boston’s South Station when built in 1898, and still serving in that capacity today. Fort Point Channel separates the center of Boston from South Boston, and the trains on this line are now all commuter trains for Boston’s southern suburbs. This bridge is one of the surveys that got the “engineering” tag from HAER.

It’s a rolling lift bridge, which is a bascule bridge (i.e., the deck rotates upward) where the hinge is a set of tracks. I associate these bridges with Chicago: they were used elsewhere, but various forms of bascule bridge have been used since the beginning on the Chicago River. This bridge was, in fact, designed and built by the  Scherzer Rolling Lift Bridge Company of Chicago. The big round pier abandoned in the channel made me think the previous bridge (there’s been a railroad line crossing here since 1845) was a rotating bridge, but HAER says it was a jackknife draw, where cables from a tower pulled the deck up and over. Maybe that pier is where the tower was, or maybe it’s from a different bridge entirely. The aerial view…

shows some important facts about the bridge. There are six trusses, each (obviously) with its own rolling lift mechanism, the space between each pair of trusses had (when built) two tracks, and Fort Point Channel is both shallow and narrow. One more fact that I am sure greatly complicated the design: the bridge is built on a skew because the angle between the rail line and the channel is about 60 degrees rather than a right angle.

Skews are a serious design problem in masonry arch bridges, where they affect both analysis, which is difficult enough under any circumstances, and construction, where they create complicated 3D geometry for stones. In an ordinary steel-truss bridge, the skew affects the loading from the cross-deck girders to the trusses and it affects the wind bracing, but is otherwise not so bad. In a lift bridge, skew is going to require extra lateral bracing to avoid racking. Lifting the bridge (which could happen independently for the three segments) meant that two trusses are moving up at the same time and in parallel planes, but with different pivot points. The fact that this bridge didn’t become famous for jamming open suggests a solid design.

As seen in the last picture, only four of the six tracks are still in use; the bridge no longer opens or is openable, as the mechanism has been removed.

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