Unique Bridge Over Tempe Town Lake
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.
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.
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
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."
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
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
here for Next Mass Transit Feature>>