I’ve got questions about the 1899 Amory-Bigbee Bridge over the Tombigbee River in Mississippi that will likely never be answered. This is, to date, the demolished bridge presented here that I most wish still existed. It was demolished around 1960, after a new bridge was constructed nearby, and the brief description in a HAER report on a group of bridges – “Bridges of the Upper Tombigbee River Valley, Columbus, Lowndes County, MS” – has some tantalizing clues.

The big round pier in the middle of the river is a hint as to what makes this bridge special: it was a center-pivot swing bridge. The bridge itself is a Pratt though-truss – technically a Parker truss, since the top chord is segmental, but if you don’t count the portal frames at the end there are only three segments in an eleven-bay bridge, so maybe somewhere between Parker and Pratt – doubly cantilevered from the center pier. That’s most dramatic when the bridge is opened:

The right angle, of course, allows you to see the center span in isolation:

As a road bridge, the trusses and deck are lighter than they would be for a railroad. My first question is about the center bay, directly over the pivot pier. The top chords, which are built-up box beams in the cantilevered spans, become pinned eye-bars here, which is to say that they were meant as pure tension members. Tension follows logically from the double-cantilever layout, but this is a moving structure. Assuming that the rotation was slow, it still has a start and a stop, which is to say acceleration and deceleration. So did the designer calculate the top-chord compression from the accelerations and confirm that it was less than the dead-load tension, or did they get lucky? Structural engineers in general, including me, tend to think statically and have to remind ourselves to consider dynamic effects like the acceleration of a swing bridge, and that was more true in the past. Wouldn’t it have been easier to just continue the built-up boxes across the center bay?

My second question concerns the foundation for the center pier, which is not described in any detail in the HAER report. The pier is a “steel cylinder filled with concrete” but sitting on what? For the pivot to function properly, the center pier had to be relatively more exactly vertical and more rigid than an ordinary bridge pier. The pier still exists, abandoned some 40 feet form the replacement bridge, so whatever it sits on is doing well.

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