Atwater Bridge is designed to fit within a confined site bordered by an interstate just to the west and power lines to the east. Buro Happold Consulting Engineers, Inc.
A new bridge over the Los Angeles River will improve access between two city parks for pedestrians, bicyclists, and equestrians.
March 12, 2013—For the first time in nearly 20 years, a pedestrian bridge is being constructed over the Los Angeles River in Los Angeles. While it’s not too surprising, given the history of signature bridges across the river, that the crossing will be architecturally stunning, the bridge’s multimodal function is somewhat unexpected. The structure will carry not only pedestrians and bicyclists but also a group not often associated with one of the nation’s most populous cities: equestrians.
Atwater Bridge is being constructed in Atwater Village, a neighborhood in northeast Los Angeles. The crossing will be located between Atwater Park, which is home to horse stables and training facilities, and Griffith Park, which has many riding trails. Without a bridge over the river, equestrians have had to traverse the riverbed to move from one park to the other. “The equestrian community had been trying for years to get a crossing here of some nature,” says Steve Chucovich, LEED AP, an associate principal at Buro Happold, an engineering firm headquartered in Bath, Somerset, United Kingdom, which designed the bridge from its Los Angeles office. “So the idea was that the bridge would be multimodal. It would become both pedestrian and bicycle as well as equestrian so that you would have the ability for all of the uses to happen here at this location.”
Atwater Bridge is one of the first significant structures to be constructed as part of the Los Angeles River Revitalization Master Plan, a 20-year blueprint adopted by Los Angeles’s city council in 2007 to develop and manage the Los Angeles River. The Los Angeles River Revitalization Corporation, a nonprofit group responsible for implementing the plan, is overseeing the design and construction of the bridge. The approximately $5-million project is being funded mostly by a private donation from a resident who requested that the new bridge have a distinctive appearance. Buro Happold presented several bridge types, and the donor thought a cable-stayed structure would best answer the purpose, says Greg Otto, M.ASCE, a principal at Buro Happold.
Atwater Bridge will have two separate tracks: one for pedestrians
and bicycles and the other for equestrians. Buro Happold
Consulting Engineers, Inc.
Once the bridge type was determined, engineers began analyzing how to make all of the intended uses work in a single structure. It quickly became clear that the pedestrians and bicyclists would need to be separate from the equestrians. To that end, a two-track bridge was necessary. “As we began exploring the architecture of the bridge and then the engineering and sort of melding engineering and architecture as one approach, we started defining the geometry of how this would come together,” Chucovich recalls. In so doing, the engineers also considered the restrictive site configuration along the river. On the west side the shoulder of Interstate 5 is a mere 18 ft from the river’s edge, and on the east side overhead power lines run parallel to the river, Chucovich says. “There were a lot of urban elements, infrastructure elements, that we had to needle in and work around,” he says.
To fit the 306 ft long bridge in the confined site, the engineers designed it with a central pylon that is offset from the center of the bridge. “It’s offset to the west, keeping it a little farther from the power lines,” Chucovich explains. The pylon will be of steel and will have an unusual boomerang-like appearance. The top of the pylon will be 120 ft from the river bottom; the pylon will be founded on piles driven to a depth of approximately 30 ft and covered by a pile cap. “The piles and the pile cap that make the foundation are under the riverbed,” says Jean-Pierre Chakar, P.E., an associate principal at Buro Happold. “All that sticks out is the first stem of the mast, which is the concrete mast pier.”
The bridge’s western abutment will be founded in similar fashion. On the east side, however, the power lines make it difficult to drive piles, so that abutment will be founded on a large mat foundation, Chucovich says. All of the foundations will be constructed between April and October because it will be possible then to work in the water. The work will include channelizing the river to the extreme east side so that the pylon foundation can be constructed in the dry. “All work in the river has to be completed by October, before the rainy season starts,” Chakar says. “Once we start work on the foundations, the riverbed needs to be restored to its original state by October.”
Crews will construct the foundations for Atwater Bridge between
April and October because work in the water is allowed during
that period. Buro Happold Consulting Engineers, Inc.
Each of the bridge’s two tracks will be 12 ft wide. However, the overall width of the bridge will be roughly 36 ft to accommodate the connections of the 26 prestressed, galvanized structural stay cables, which will extend from sockets embedded in the pylon to anchors housed in tubes along the perimeter of the bridge’s steel-frame superstructure. Timber decking will be used on both tracks, but on the equestrian side the decking will be covered with a membrane and a 1 ¾ in. thick rubber paver system expressly designed for horses. The pavers “properly allow the horse to have enough friction, enough pressure resistance when walking on the surface and allow for ease of cleanup of the bridge afterward,” Chakar explains.
Bridge construction is scheduled to begin this spring, and completion is anticipated by the end of the year. The engineers say they are proud that, despite having to meet the requirements of several government agencies along the way, they have created a bridge that is both multifunctional and aesthetically captivating. “We have built something that is unique and basically will begin to set a precedent for future projects up and down the river,” Chucovich says. “There’s this new opportunity to sort of put this new layer of infrastructure over the river that people will use in a much more twenty-first-century way.”