The original 3,500-ft-long, 46-ft-wide viaduct was built in 1932 and since then has served as a backdrop for dozens of movies, TV shows, music videos and video games. But that structure, which consisted of two concrete end spans and a central steel span, suffered from alkali-silica reaction, a sort of concrete “cancer” that rendered it seismically unsafe.

The original Sixth Street Viaduct served as a setting for scores of movies, TV shows and videos.
Photo courtesy of City of LA Bureau of Engineering

Initially, Angelenos wanted a carbon copy of the iconic bridge, recalls Moore. “We asked them to ‘give us a chance to let us dream with you.’ They let us dream.”

That dream has been transformed into a 3,500-ft-long, 100-ft-wide structure comprised of 10 sets of arches that spans the Los Angeles River just east of downtown.

The goal was for the bridge to be “more than just a place for cars. It had to be for people walking, bikes,” Moore says. The wider bridge allows for two bicycle lanes and a separate pedestrian lane, including two helix-shaped stairwells at the midpoint that lead 60 ft down to a planned future park.

Helix-shaped stairwells will connect the bridge with a future park below.
Photo courtesy of City of LA Bureau of Engineering

The columns and arches will create an image of a “ribbon of light” stretching between the East LA neighborhood of Boyle Heights and downtown. It was designed to be a destination as well as a multimodal link and a driving force in transforming the urban landscape, architect Michael Maltzan told ENR in 2021. Maltzan worked with the HNTB-led design team that won the design competition to replace the original viaduct in 2012.

A construction manager-general contractor joint venture of Skanska USA Civil and Stacy and Witbeck (SSW) completed the final arch pour and removed the massive falsework this January. Crews are now installing 7,000 ft of barrier rail that separates the pedestrian lanes from the bike lanes, says Moore. “The railing cants [up to] 9 degrees to follow the arches,” he adds. Work on the barrier, typically about 36 in. high, must follow the tensioning of the 388 cables in each arch.

The 10 pairs of arches are not uniform. “Seven are 30 feet high. One is 40 feet over the 101 freeway, and two are 60 feet over the railroads,” says Moore. While each pair is tensioned, strain gauges atop them monitor the process.

“Due to the complex interaction between the concrete and steel bridge elements, temperature effects became an issue in the correct understanding of cable force readings,” says Karen Cormier, civil engineer with T.Y. Lin International, the construction management and engineering services provider.

“Ultimately, the strain gauge readings were collected at night when the concrete and steel elements reached near-equilibrium temperature to better correlate cable force tensioning with temperature effects,” Cormier explains. “For this complex network-tied arch structure, an elaborate tensioning sequence had to be developed by the construction engineer to balance the cable forces between arch ribs and adjacent cables, challenging the contractor’s team to perform multiple tensioning operations on each cable in a specified sequence.”

On average, the tension is at 150 kips, depending on each cable’s position and length, says Geraldo Iniguez, SSW project executive. “It’s like trying to fine-tune a guitar—you can’t just do one step and move forward. You have to come back and double-check the vital positions during that stressing sequence.”

SSW also had to create shop drawings after the concrete was placed for each arch to fabricate the fin plates that support the cables within the arches, says Robert Thorpe, SSW deputy project manager. “The fin plates needed to be fabricated to ¾-degree tolerance. Once you place the concrete for the arch, the support system deflects. To get within that tolerance, we had to use as-built conditions to develop them.”

While the fin plates met tight tolerances, special seismic features will allow the bridge to move within very broad ones—up to 30 in. in any direction.

Ten pairs of arches will have LED lights to create a dramatic effect.
Photo courtesy of City of LA Bureau of Engineering

Seismic Safety

The viaduct is supported on 10-ft-dia cast-in-drilled-hole piles up to 165 ft deep. Crews placed the transverse floor beams with huge edge girders, then post-tensioned them with 27 million lb of falsework still in place. Steel plates and bolts keep the Y-shaped columns stable during construction. Once the deck and transverse and edge girders were formed and placed, crews also erected falsework on top of the bridge for the arches.

Erection engineer COWI designed unique lock-up devices that hold the Y bents in place during construction. The devices will eventually serve as shear keys that will accommodate anticipated changes in the alignment of the continuous cast-in-place structure due to creep and shrinkage over 10 years.

Earthquake Protection Systems, Vallejo, Calif., developed and patented friction-pendulum bearings as well as second-generation triple-pendulum bearings that will rapidly stiffen after an earthquake, allowing for the activation of the secondary system.

“In the case of a 1,000-year event, the bridge will remain undamaged,” says Michael H. Jones, senior civil engineer with HNTB. “The viaduct will resist as an isolated structure. If ground motions are larger than anticipated, the bearings stiffen and the structure reverts to being a conventional structure.”

Anoop Mokha, vice president with Earthquake Protection Systems, says the technology was invented in 1986. The purpose was to design and construct structures that can survive big quakes and remain functional and operational. But existing bridge codes only address the purpose of keeping structures from failing, not from staying functional, he says.

“We couldn’t change the codes, but we can attempt to educate clients and engineers,” Mokha says, adding that EPS now works with clients around the world. The technology has been proven to work in recent large earthquakes in areas like Turkey, Ecuador and Mexico, he says. “HNTB came to us and said, ‘We have this bridge that is a critical lifeline.’”

The Sixth Street Viaduct is “probably the only bridge in the world that is not only designed to remain functional but at same time can handle an earthquake larger than the design level. If there is a magnitude 9 earthquake, this bridge will remain standing,” he says.

The elbow joints would flex back and forth while holding the bridge in place. Plastic hinging at the base of the piers includes an intentional weak element. “There are isolators on top of the pier. They are not rigidly connected to the deck; they can go up and down and swing like a pendulum,” says Mokha. “The isolator lateral strength is about 75% of the weight of the deck sitting on it. Normally, the isolators would be about 20%, after which the bridge will collapse and deck will fall down. Here, it is a unique design. A quake can get bigger, but the bearings will not collapse. At the base of pier, the hinges will be activated as a second level of defense.”

With the seismic isolation allowing for only two joints along the length of the structure, “it’s not only seismic advantage but a performance advantage, as joints are always a bit of a maintenance issue,” he says. All structural steel is galvanized and the hangers sport zinc alloy coating with the goal of a 75-year service life.

The bridge’s unique configuration—including its unbraced arch ribs—informed the seismic design, according to Hubert Law, principal with Earth Mechanics Inc., Fountain Valley, Calif., which provided field investigations, analysis and foundation design. The ribs, which canted outward, had a “violent” response to ground motions during simulations, Law says. “So we decided to go with seismic isolation. Normally isolators are at the top of substructure. Here, that did not work well due to the continuous arches and flow into the unique Y bents. So they inserted isolation bearings into the bridge mid-height. To our knowledge, this is the first time.”

Located in downtown Los Angeles, the bridge will link to East LA neighborhoods.
Photo courtesy of City of LA Bureau of Engineering

Pouring It On

By the time crews finished concrete placement for the arches, they had gone from 24-hour timeframes to 12, says Iniguez. “You had to find the sweet spot in the pour rate, where you want to pour fast enough so the concrete doesn’t create undesirable joints, but not so fast that the loads on the formwork are too stressful.” The biggest arches required 300 cu yd.

Iniguez notes that the outside portion of each arch is architectural, while the inside portion with imbeds and cable hangers are structural. “For the outside void spaces, we used Styrofoam on lower sections and Sonotubes for the top sections,” says Thorpe. Sonotubes are a proprietary product that is “a sort of hard cardboard treated so it can handle wet concrete,” says Thorpe.

Triple-pendulum bearings will help the bridge not only survive a major quake but stay operational.
Photo courtesy of EPS

The barrier rail that separates cyclists from pedestrians is modified to accommodate the bridge’s 3,400 LED fixtures. “There are only two light poles on the entire structure,” says Thorpe. Crews installed pull boxes and conduits in the lower curb section. “Hanging from the top beam section, there’s linear LED lights in every gap between posts. Those lights will shine inward to illuminate road, and outward to illuminate pedestrian access with no light poles,” he adds.

Moore hopes the brightly lit bridge will motivate people to use the future park and spur development of public spaces in an area typically deserted at night. When complete, the new Sixth Street Viaduct will allow motorists, cyclists and pedestrians to traverse between East LA and downtown, and potentially create the impetus for public spaces such as athletic facilities and plazas.

Regarding the imminent bridge completion and his retirement, he adds, “It’s a very special way to go out.”