The $18-million bridge project, being constructed by a Skanska USA-led design-build contract, is more than 60% complete, with two 28-ft-tall concrete abutments flanking the freeway. Motorists are driving under a 330-ft-long falsework tunnel structure, which give workers access to erect the permanent 584-ft-long bridge.
The precast “baskets,” measuring 25 ft tall and 17 ft in diameter, will be assembled in segments off site by Masonry Concepts, Santa Fe Springs, Calif., and will be trucked in during August. Designed by renowned public artist Andrew Leicester, the columns, along with a serpentine-like underbelly of the superstructure, are a tribute to the San Gabriel Valley and the Native American Tongva.
“The Tongva were the earliest dwellers in this area and are famous for their skill in basket making,” says Leicester, who was selected by the Foothill Extension Construction Authority, the entity in charge of building the Gold Line light rail line from Downtown Los Angeles to the eastern county line, through an international competition.
“I was thinking how the structure of a basket is similar to the construction method of concrete,” adds Leicester. “Metal rebar is the skeleton and it is clad with skin of concrete, and with a basket you have structural members inside the basket that give it its silhouette; it has a decorative skin woven around the members.”
Crews erecting the steel and wooden falsework beams spanning the freeway are dealing with tight spaces.
“This biggest challenge is working in such a small area with traffic very close,” says Lawrence Damore, project executive with Skanksa USA. He says crews have a five-hour window each night between 12 am and 5 am to stage materials, put up traffic signage and lighting, and complete spanning work.
While restrictive space is a definite obstacle on the project, Habib F. Balian, CEO of the Authority, says it was subsurface conditions that posed the biggest challenge.
“We identified three earthquake faults here, and we had old column bases to work around,” says Balian. He says original plans called for four bridge foundation columns, but because of fault lines they went with three.
“We solved the problem by going deeper and putting in three [110-ft-deep by 11-ft-diameter] columns instead of four,” says Balian.
As added security against a seismic event, the design-build team's main designer AECOM has equipped the bridge with Time Domain Reflectometry (TDR) technology, which can help assess damage following an earthquake. The system involves wiring columns with electrical feedback monitors.
“When engineers design bridges, they can predict approximately where damage may occur during a seismic event,” says Gary Baker, the authority's director of construction.
He says in the eventuality of a seismic event, this “smart column” technology will allow engineers to make an initial assessment regarding the integrity of the columns’ structures electronically rather than relying on the traditional method of digging trenches adjacent to areas where engineers suspect damage. The authority claims it has never been used on bridge foundations before.
The project is slated to finish this summer. Baker says falsework will be complete in late March. Then crews can begin two months of formwork and installing steel and rebar. In late April, there will be a massive 2,100-cu-yd concrete superstructure pour, which Damore says will last 18 hours, from midnight to the 4pm the next day.
Dubbed the “Gateway to the San Gabriel Valley,” the first phase of the Metro Gold Line Foothill Extension from Pasadena to Azusa is scheduled for completion in 2015. Baker says the contractor faces a potential $6,000 per day in late penalties.
The second phase of the program will extend the line from Pasadena to Azusa. A $486-million contract has been awarded to a joint venture of Kiewit Corp. and Parsons Corp.