Atwater Multimodal Bridge

Buro Happold

30/04/2012 Written by: Steve Chucovich No comments

In 2007 the City of Los Angeles released “The Los Angeles River Master Plan,” the culmination of a five- year effort involving thousands of citizens and untold city, state and federal agencies. The plan outlines a blueprint for reclaiming and restoring 32 miles of the Los Angeles River ecosystem over the next 25 years.

The Atwater Multimodal Crossing is the first large project to stem from this plan. The structure will complete the connection from the historic Atwater neighborhood to the LA River bike trail and ultimately to Griffith Park, a 4,310-acre (1,744ha), half-wild oasis in the center of Los Angeles. It is also the first new bridge to span the Los Angeles River in twenty years, and the first large-scale piece of infrastructure to be fully funded by private donation.

These firsts have created some unique challenges that influenced the structure’s design. Programmatically, the crossing must accommodate pedestrians, bicyclists and equestrians, requiring two very distinct pathways that separate horses from pedestrians and cyclists.

The second and more challenging requirement came from a private donor, who specified that the structure be a cable-stayed bridge. Given the project’s constraints—a 325-foot (100 meter) span, minimal allowable incursion in the center of the river, and high-voltage power lines along the far river bank—a “balanced” or symmetrical solution was impossible. Alternative designs posed their own structural challenges.

The structural design integrates architectural and site solutions by employing a center, single-cable-row solution that ties together the 120-foot-high (37-meter-high), off-set mast with the horizontal deck beam. The cables are arrayed between the mast and central horizontal spine, to create a structure that spans the river yet avoids power lines. In order to accentuate the main cables’ fan-like pattern, the backstay cables are grouped unobtrusively behind the mast.

Engineered to resist torsion in the deck, the spine supports cantilevered deck beams on either side. The structure creates a natural separation between pedestrian and equestrian pathways.

The constricted site calls for an orchestrated construction sequence, since any work in the river can occur only during the summer months, and requires stringent safeguards to protect the surrounding environment and the river’s water quality. To minimize sitework, steel bridge components will be pre-fabricated and pre-assembled for fit before being brought to and assembled on site.

The multi-modal crossing continues a tradition of landmark bridge design that shaped the city’s landscape for 80 years. Much like the well-known structures of the past, this bridge has an opportunity to inform the city’s future by catalyzing further river redevelopment. Through its scale and image the bridge will provide a recognizable landmark within the complex landscape of multilane highways and migrating birds, while the quality of its detailing, materials, and lighting will resonate with its pedestrian, cyclist, and equestrian users.

The Atwater Multimodal Crossing design team includes Buro Happold, Mia Lehrer + Associates, Fuscoe Engineering, Gardiner & Theobald, Leighton Engineering, and Tetra Tech.