Another Popular Oddity

That’s the Turn-of-River Bridge in Stamford, Connecticut. (Photo courtesy of Magicpiano.) Putting aside anything I have to say about the structure, that’s a fantastic name for a bridge. It’s (obviously) quite small and has been converted from a road bridge to a footbridge on a trail, but it’s looking okay for a neglected 1893 bridge.

It’s a lenticular truss, one of some 400 churned out by the Berlin Iron Bridge Company of Berlin, Connecticut. BIBCO had bought the patent for lenticular trusses in 1878 and used it to its fullest. This bridge, like most of the 400 others, is a pony truss. The pony configuration, with trusses standing isolated on either side of the deck, is inevitable with lenticular trusses unless they are quite large: the curved or faceted top chords mean that wind-bracing between those chords will interfere with traffic unless the trusses are big enough so that vehicles can pass under the low ends. Since the trusses are symmetrical and supported so that the lowest point on the curved or faceted bottom chord is just about at deck level, and you’d want (depending on the era) somewhere between 12 and 15 feet clearance between the deck and the bracing, the trusses have to be around 30 feet high for end-to-end bracing, and you’re only likely to see that if they’re at least 150 feet long. The span at Turn-of-River is 53 feet.

It’s a well-designed bridge and dealt with the bracing problems of pony trusses in a reasonable way. The verticals are built-up I-shapes with latticed webs, bearing against the top of the cross-deck girder. The girders are hung by U-bolts from the pins of the lower-chord connections. The verticals are restrained against rotating out of plane by the moment couple developed between the U-bolt and the flange on the leeward side of the rotation. I’d prefer that there be riveted connections between the vertical bottoms and the girders, but it’s not necessary; there’s no obvious reason that the verticals should extend down to the the girder other than to create that restraint for bracing.

Otherwise: loop eyebars, big pins for the connections, wood deck spanning between the cross-deck girders, wind-bracing below the deck by diagonal rods…it’s almost like there were a set of conventions developed by the bridge-building community.

Bridgehunter has a whole bunch of good pictures.