Light Span
Unique Bridge Over Tempe Town Lake

By Tony Illia & Scott Blair

A unique triangular-shaped steel truss bridge over Tempe Town Lake, Ariz., that required six months of custom steel fabrication and welding, is now nearing completion. The 1,531-ft.-long, 11-span structure plays a critical role in Valley Metro's new $1.3-billion, 20-mile light-rail line from central Phoenix to Mesa. Denver-based contractor PCL Civil Constructors Inc. began bridge construction in Feb. 2005, under a $21.75-million unit-price contract.

 

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The structure consists of 42 trusses set 15-ft above the water. Each truss was hoisted into place using two floating barge cranes. Trusses were temporarily cantilevered up to 40-ft. from pier caps until the next section was erected, explains Shayne Waldron, PCL's construction manager.

"The longest span we lifted was 160-ft.," Waldron said. "It would come in two 80-ft. halves which we would bolt together on the barge. It weighed in the neighborhood of 160,000-lbs."

The trusses are supported by 10, 8-ft-dia. drilled shaft Y-capped piers and two bridge abutments. "The water averages 18-ft. deep and there is a layer of gravel and cobble below anywhere from 10 to 30 ft. deep," Waldron said. "They were socketed into the bedrock anywhere from 10 to 14 ft."

The low profile design by TY Lin International, San Francisco, came about as a result of public worry that the new structure would detract from a historic 1912 steel truss Union Pacific railroad bridge located 50-ft to the West. Valley Metro was 20 percent into design when it switched from a modified cable-stay bridge to its current scheme after the state Historic Preservation Office expressed concerns, said Joel G. Mona, Valley Metro's resident engineer.

The light rail bridge will sit at the same height and have the same pier spacing as the Union Pacific bridge for hydraulic reasons so that the Salt River's water flow is not affected, Waldron said.

Additional aesthetic design features include a cast-in-place retaining wall designed by Seattle environmental artist Buster Simpson which evokes cracked mud. A scientific equation is engraved into the concrete which mathematically describes how mud cracks in the desert. >>

A $750,000 color LED lighting system runs the length of the bridge. "The row of lights can be controlled by computer and will be reflected through a perforated sheet metal scrim," Waldron said.

Fabrication of the bridge's 880 pieces of diagonal bracing, 722 of which are unique, required a $250,000 one-of-a-kind computerized thermal cutting system, says Gary Gardner, quality control manager for Stinger Welding Inc., Coolidge, Ariz., the project's steel truss fabricator.

"AutoCad couldn't handle it," he says. "We had to create a special 3-D program due to the complexity of the design." The diagonal beveled braces consisted of 10-in. pipe, from 9-ft. to 10-ft-long, with wall interiors 1/2-in. to 1-1/8-in.-thick for positive and negative moment spans. Design specifications required pipe lengths to be accurate within 1/16-in., and clocking angles inside a 1/2-degree. There are a total of 3,000 to 4,000 full penetration welds for maximum strength to avoid premature structural fatigue and breakage during Arizona's hot summer months.

Tight site conditions made lifting difficult. "The erection was challenging because we've got parallel overhead power lines running directly overhead to the east and the bridge on the west side, so once you get the piers poured, it's pretty tough to get anything in there," Waldron said. "We spent a lot of time planning the right size barges, selecting the drill rig and conducting the steel erection, but it all went pretty smoothly."

The south bridge abutment was redesigned to a spread footing instead of drilled shafts for fear that the crane mounted drill rig would strike low hanging overhead power lines.

An underground 72-in.-dia. water pipeline, supplying 60 percent of downtown Phoenix with water, is located under the bridge alignment and entailed careful excavation and concrete reinforcement to withstand the additional bridge loading.

 
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