By T.R. Witcher
The bridge was extensively modified in 1914 to handle more traffic. According to an article on the website of the Texas State Historical Association, “The cable system was replaced, the roadway was reinforced with steel, and the towers were rebuilt and stuccoed.” Stiffening trusses were also added.
Vehicles used the bridge until 1971, when it was converted to pedestrian use. The prior year the bridge was listed on the National Register of Historic Places.
Removing the suspension
These days, the Waco Suspension Bridge anchors a network of parks in the center of Waco that hosts sporting events, concerts, and weddings. But in recent years, the cabling and anchors were approaching the ends of their service lives, so the city of Waco, starting in fall 2020, embarked on a $12.4 million renovation of the bridge to replace the cables and road deck and install a new anchorage system.
The city hired Balch Springs, Texas-based contractor Gibson & Associates Inc. to oversee renovation of the bridge. Gibson subcontracted bridge engineering firm Structural Technologies to perform the removal and replacement of the suspension system. Structural Technologies subcontracted engineering work on the suspension system to Modjeski and Masters, a Mechanicsburg, Pennsylvania-based bridge design firm.
Work began in fall 2020. Gibson designed and installed a temporary foundation of steel casings with steel bent caps in the river to take the load of the bridge while the suspension cables were removed. This temporary system also supported a work platform built beneath the bridge deck, which allowed workers efficient and safe access to the main span. (The platform and the support piers will be removed when the project is completed.)
“During construction it's no longer a suspension bridge; it's actually just a truss bridge on temporary supports,” says Tim Stuffle, P.E., a senior engineer at Modjeski and Masters. “It gives a lot of leeway to replace the components in their more or less original location.”
For the engineers, understanding the bridge’s geometry was crucial to manufacturing new components to the right lengths. Although they had the design plans from the bridge’s reconstruction in 1914, engineers decided to survey the existing structure to see whether settlement, movement, or other factors had changed its condition versus the original plans.
“We had to be able to really understand what we had to work with to make sure new components would fit correctly,” says Stuffle.
The geometry, it turned out, was substantially different from what they expected. One tower was several inches shorter than the other — the engineers do not know whether this was because of construction choices or settlement over time. The sag of the cables was also about 18 in. more than expected. In addition, the stiffening trusses were a little flatter and didn't have as much rise as the original plan called for. The height of the two main cables also varied by about 5 in.
Also of concern was the midspan hinge in the stiffening truss, which had some deterioration and had been repaired at previous points over the years. Contractors were worried about whether the hinge could take the articulation necessary to return it to the original rise profile called for in the 1914 plan.
“The city wanted to put the rise back on the bridge; they wanted to get the curvature of the deck back,” says Justin Campbell, a senior project manager in the Dallas-Fort Worth office of Structural Technologies.
The solution was a compromise between the original plan and the current condition. According to Stuffle, the 1914 plans called for an 18 in. rise of the stiffening truss (meaning the midspan is 18 in. higher than at the towers). The survey indicated the rise was about 1 in. for the north truss and 3 in. for the south truss. “The compromise was to target a 12 in. rise for both trusses in the rehabilitated structure,” says Campbell.
Releasing the tension
Stuffle notes that “cables stretch a fair amount when you subject them to load, and they're very long.” Modjeski and Masters and Structural Technologies had to figure out how to release the tension on the cables in a controlled way.
The firms considered using the midriver supports to lift the bridge deck, but they would have had to lift it a lot — almost 3 ft. They also considered de-tensioning the main cables, but the anchor houses where those cables terminated were very restricted, the condition of the existing anchor rods was questionable, and there wasn’t a lot of threaded length on the existing anchor rods to grab on to, Stuffle notes.
In the end, engineers chose to release the hangers — one by one or in pairs — connecting the suspension cable to the deck in a carefully designed sequence to prevent overstressing any part of the bridge. “As you start releasing the hangers, the stiffening truss is sitting on these temporary supports, and the cables start changing shape and rising,” Stuffle says. “If you do it without thinking about the sequence, you can end up with one of the existing hangers taking too much load.” This could have damaged the hangers or the existing cable clamps, which were being reused, he explains.
The work began in January 2021. Hydraulic jacks under the bridge deck were used to take the load off the hangers. Then the hangers were removed in a coordinated sequence: two or three in a row, then a pair would be skipped, then two or three more would be removed as the work progressed from the ends of the bridge to the center and back out again. They were left with several pairs of hangers spaced along the length of the bridge that they de-tensioned simultaneously. At the final three locations, the hangers were reinforced with supplemental tie-downs because the predicted force exceeded the capacity of the existing hangers. In all, removing the bridge’s sway cables, hangers, and suspension cables took about two months.
Beginning in December, Structural Technologies will start the reinstall of the suspension cable and new hangers. That should take about three months.
The hangers will be reinstalled with the same care given to proper sequencing as was their removal, according to Campbell. Two winches and a specially designed frame with rollers, which will cantilever off the tower footprint, will thread the cables through the suspension housing.
For reconstruction, the hangers are being manufactured several feet longer than is necessary for their final condition. This is so they are long enough to pull into position, and then they will be cut to the right length. “That gives you enough room to put a jacking chair and a hydraulic jack underneath the floor beam to pull the hangers to the correct length and adjust them to the right tension,” says Stuffle.