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May 2024 #16298 Page #150
The Last Word How to Repair a Bridge Truss, or Any Sagging Truss
W
hen a ship hit the Francis Scott Key Bridge in Baltimore, MD, its fate was never in doubt. But that was not the case when a barge hit the Eads Bridge near St. Louis, MO, another steel arched truss bridge. The Key Bridge lost one of its main supports, while the Eads Bridge lost one of its truss members, but was subsequently repaired. Surprisingly, the way the Eads was repaired was not fundamentally different than the way a sagging wood truss should be repaired, at least in principle, but certainly not in execution. And that repair is an object lesson, and I had a hand in it in 1972.
Joe Kannapell, P.E.
Fortunately, James Eads incorporated four parallel steel trusses in his 1874 design, providing incredible redundancy. When the bottom chord on the outermost truss was severed in 1972, the load that had been carried by that truss was transferred to the adjacent trusses, theoretically removing one-fourth of the Eads’ overall carrying capacity, and potentially overstressing it. As a result, the authorities halted train traffic across the bridge immediately, but didn’t interrupt vehicular traffic, since cars and trucks would impart only a small fraction of the load of freight trains. As with any sagging wood truss, simply inserting a new member and scabbing it in place does little good, because that new member doesn’t pick up any of the load that had already been transferred to the other trusses. Of course, the typical method of restoring loads in a roof truss by jacking it up would not have been feasible 100 feet above
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