The Sixth Street Viaduct will be slightly curved, making the multiple network arches visible from nearly every point on the bridge. Rendering courtesy of HNTB Corporation
With a concept inspired by the existing bridge, HNTB wins a competition to design the new Sixth Street Viaduct in Los Angeles.
November 27, 2012—A series of 10 network arch spans, the new Sixth Street Viaduct in Los Angeles will be a modern interpretation of the one it will replace over the Los Angeles River, says Theodore P. “Ted” Zoli, P.E., M.ASCE, the national bridge chief engineer for HNTB Corporation, in New York City.
The City of Los Angeles hosted an international design competition to replace the existing 80-year-old viaduct, one of a series of nine bridges designed by the city engineer Merrill Butler over two decades spanning the 1920s and 1930s. Although eligible for listing in the National Register of Historic Places, the 3,500 ft long concrete viaduct has suffered from alkali silica reactivity and has deteriorated to an extent that it is beyond repair, according to the Los Angeles Bureau of Engineering’s website.
After narrowing the competition entries to three, Los Angeles Mayor Antonio Villaraigosa announced in October that HNTB, an engineering and architecture firm headquartered in Kansas City, Missouri, won the $400-million contract with its “world-class, iconic” concept. HNTB is collaborating with Los Angeles-based Michael Maltzan Architecture on the project. The other competition finalists were the New York City-based engineering firm Parsons Brinckerhoff and the New York City office of AECOM.
Zoli says he was drawn to the competition by the challenge of designing a viaduct for a major city but also replacing a bridge that has become so identified with Los Angeles, particularly in film. “A long viaduct in a major city is really an unusual typology, not at all like a bridge over a major river or the more mundane highway overpass; this is really an unusual form, to say nothing of the channelized Los Angeles River,” Zoli says. “And whenever you start with something that’s unusual, I think you’re trying to get to the essence of what it should be.” He added that a design competition affords the opportunity for engineers and architects to collaborate to develop a design that is best able to “explore an unusual form, but to invest in it a sense of place.
“For us, the concept had to begin with the premise of concrete arches, which are so characteristic of Merrill Butler’s other bridges, but it was also essential that these arches form a chain that crosses the entire floodplain, not just the Los Angeles River and adjacent railroad tracks.”
To that end, the bridge will be 3,500 ft long, extending in a sweeping curve that will take it up to 100 ft north of the existing alignment. The curvature creates a subtle progression of perspective, Zoli says. “This long, continuous series of arches helps to emphasize the scale of the viaduct; but it’s the progressive shift in perspective that creates the drama. You don’t get that with a straight alignment.”
The Sixth Street Viaduct will feature a series of 10 network arches
spanning the Los Angeles River. Rendering courtesy of HNTB
In addition to four traffic lanes, two in each direction, the 98 ft wide bridge will carry two pedestrian and bicycle pathways, which will be accessible vertically via stairs and ramps. As more people bicycle and walk in Los Angeles and other major cities, designers must be much more aware of the different character of these transportation modes, Zoli says. “It’s that question of, ‘What is the relationship between people who drive cars and people who ride bicycles and people who walk?’” he says. “Especially in an urban environment, I think that dynamic is changing dramatically in my practicing lifetime. We’re at a point where our focus is no longer the automobile first; we are going to see more pedestrian and bicycle facilities incorporated into our infrastructure, and that these modes are becoming ever more vital to urban mobility.”
The plan also calls for parks beneath the bridge, although funding for the parks has not yet been identified, Zoli says. “We will do our best to identify alternative funding sources to allow us to develop the park space underneath the bridge,” he says. The parks will extend from beneath the bridge and into the surrounding communities. “If the viaduct can produce a series of underdeck spaces—Michael Maltzan refers to them as rooms—which become inhabited with different programs, whether park space, areas of recreation, or gathering spaces, it makes the bridge so much more a part of the city and so much better at delivering on the promise of a viaduct in a city,” Zoli says. He adds that in addition to the budget, the greatest challenge of the project will be constructing the bridge in the densely populated area without disrupting existing utilities and infrastructure.
HNTB and its partners plan to begin substantial design around the beginning of 2013. Construction is slated to commence in 2015.