When 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. [For all photos, See PDF or View in Full Issue.]
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 the mighty Mississippi River. Therefore, a novel approach was taken on the broken Eads’ truss. A 1,000,000 lbs. capacity hydraulic jack would be inserted in the open section of the truss. The ram of the jack would then be extended until enough load was reintroduced into the broken truss. But how could the magnitude of that load be determined? That’s where engineering ingenuity came into play.
With a newer bridge, the requisite load could be roughly determined from analysis of structural drawings. But with the 100-year-old Eads, no calculations survived, if any had existed (James Eads had lots of practical experience but no engineering schooling). So, the engineers of the venerable American Bridge Company devised an innovative empirical approach and called on my college professor, Dr. Harry Duffy, to provide the instrumentation and its implementation, and I was fortunate enough to be on his crew. (The photo shows unrelated wood truss testing for which Dr. Duffy was periodically engaged as an independent PE. This took place in the rear of the aged stamping plant of the company that would become MiTek).
The devices used, called strain gauges, were less than an inch long, and when bonded to a surface could measure minute amounts of deflection. Our team’s job was to clean the steel surfaces in the areas that would receive the gauges and then glue the gauges securely in place (as in the example photo that uses much more modern gauges). We affixed many dozens of these up and down all four bridge trusses and brought the lead wires from each gauge to a central location where they were connected to measuring equipment. The equipment was to translate the electrical signals from each strain gauge into measurements of the magnitude of the stretching or contracting (i.e., the deflection) of truss members during jacking. Because these deflection readings would be proportional to the magnitude of loads, its readings would show when load was being picked up by the repaired truss, and when it was being shed by the adjacent trusses. Interpreting this multitude of readings required the studied collaboration by several engineers onsite during the jacking process. But, remarkably, after only a couple of hours of methodical jacking, they determined that enough load had been transferred into the damaged truss and the repair was deemed to be complete.
In 1998, the Eads Bridge (and its repaired truss) were tested again when a runaway barge hit one of its supports, similar to the March 26, 2024 incident at the Key Bridge. In that case, however, the barge broke into two pieces and no one was hurt. Today, the venerable Eads is still carrying traffic, even as it nears its July 4, 2024 Sesquicentennial marking 150 years in service, thanks, in part, to one of the most ingenious truss repairs in history!