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Transit Agency Repairs Historic Eads Bridge

 

Civil Engineering News
March 2013
by Jeff. L. Brown
 
Transit Agency Repairs Historic Eads Bridge
 
James Eads’s double-deck Mississippi River crossing at St. Louis made history as the first major steel arch bridge in the world. Now crews have begun work on the first complete rehabilitation of the lower deck since the bridge opened, in 1874.
 
At first, the joint owners of the bridge—the City of St. Louis and the Bi-State Development Agency, commonly known as Metro—planned only to apply a new coating to the bridge to address corrosion issues, says Fred Bakarich, P.E., the project director for Metro. However, annual inspections in 2007 and 2008 revealed extensive fatigue cracking in the lower-deck floor system, suggesting that the floor was nearing the end of its service life.
 
That finding prompted a contract with TranSystems, Inc., which is based in Kansas City, Missouri, to inspect the structure more thoroughly. That inspection found 26 previously undocumented cracks, confirming that the floor system was deteriorating rapidly, says Allen Smith, P.E., M.ASCE, the firm’s project manager. TranSystems went on to develop recommendations for repair and has been retained to oversee inspections during construction, which is now under way.
 
Originally designed for highway traffic and freight rail, Eads Bridge today carries a four-lane highway on its upper deck and two light-rail tracks on its lower deck. The crossing stretches a total of 6,442 ft, including a 520 ft center span and two 502 ft secondary spans. Each of the three large spans is supported by four pairs of tubular arch ribs. The upper and lower ribs that make up each pair are braced by bars and tubular struts.
 
The existing lower-deck floor is supported by transverse floor beams spaced approximately 12 ft apart. Two longitudinal stringers span the distance from one floor beam to the next. The stringers are attached to the web of the floor beam at each end.
 
The floor beams are supported in different ways depending on their location. Near the ends of each of the three main spans, the floor beams are attached to columns that rise from pin connections in the arch ribs below the deck.
 
At the center of each span, however, the upper arch rib extends above the level of the deck. Here the floor beams are attached to hangers that are suspended from pins in the arch rib above them.
 
The differences between various parts of the floor system do not stop there. Near the middle of each span, where the upper rib rises above the floor, the arch rib bracing passes through holes in the stringers and floor beams. The space between the floor beams varies slightly from one panel to the next. Furthermore, the stringers and the floor beams vary in depth.
 
The variations in the existing floor system made it impractical to repair or replace individual members, says Bakarich. Instead, TranSystems recommended replacing the entire floor system. This approach created an opportunity to devise a design that would simplify construction, reduce labor costs, and give the structural system greater redundancy.
 
The new floor system employs the same basic arrangement of beams and stringers, Smith explains. Instead of connecting to the web of each floor beam, however, the stringers rest on top of the floor beams. This simple change makes it possible to use longer stringers that stretch continuously across three 12 ft panels instead of one, significantly reducing the number of connections.
 
Because the present-day light-rail loads are lighter than the freight rail loads of the late 19th century, the depth of the beams and stringers can be reduced as well. Thus, the stringers can be stacked on the beams without reducing the vertical clearance for the trains or encroaching on the navigational clearance beneath the bridge. In a few places near the center of the spans, however, the stacked floor system cannot be used because of conflicts with the lateral bracing.
 
The arch ribs also require repair. These unique structures consist of six steel members arranged like barrel staves. A 1/4 in. thick steel envelope encases the staves to form a tube 16 in. in diameter.
 
To assess the condition of the ribs, workers drilled 1 in. diameter holes in them and inserted a borescope. In the process, they discovered that the steel Eads used for the ribs was incredibly hard.
 
“It took the ironworkers more than a day to drill through the steel,” Smith recalls. “It was like drilling through a sledgehammer head.”
 
The borescope inspection revealed that the staves on the interior of the ribswere in surprisingly good condition. The exterior envelope, however, was severely corroded. Workers will repair the envelope by welding plates over the affected areas.
 
The rehabilitation project includes other repairs as well. Various lateral bracing members and fasteners will be replaced, as will the hangers that link the floor system to the upper ribs. Another aspect of the project involves removing the existing coating and repainting the entire superstructure below the lower deck. KTA-Tator, Inc., of Pittsburgh, designed the new coating system.
 
Analyzing the loads that would be imposed on the bridge during construction was vital to a successful start for the project. Because the structure is so complex, the task was unusually time consuming, requiring several months to complete, says Bakarich.
 
“This is a really complicated bridge with a lot of different load paths,” he explains. “It was a huge challenge working through the various load combinations that would be in place during construction and making sure that we were not going to overstress any element of the bridge.”
 
To accomplish the task, TranSystems employed STAAD.Pro software, developed by Bentley Systems, Inc., of Exton, Pennsylvania, to create a three dimensional model of the bridge’s center span. The model was used to calculate the stresses that construction loads would impose on each of the arch ribs.
 
These stresses, in conjunction with historical estimates of the ribs’ material properties, yielded a load rating that was found to be acceptable. The model will also make it possible to carry out further analysis in the event that any material defects are discovered during construction,
Smith notes.
 
The $36.3-million construction contract was awarded to the St. Louis Bridge Construction Company, which began demolishing the floor system in December.
 
Thomas Industrial Coatings, also based in St. Louis, will paint the bridge. The project, which is funded in part by federal grants obtained through the American Recovery and Reinvestment Act of 2009, is scheduled for completion in late 2015.

 

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