LOS ANGELES, CA — HNTB’s Southern California structures practice continues to expand under the leadership of Yung-Nien Wang, PE, HNTB Southern California Structures Group Director. The practice has grown to a team of 25 bridge and tunnel experts who are tapped to design some of the region’s most challenging structures.

“Across the Southern California landscape, from the Sixth Street Viaduct Replacement project to the recently opened Gerald Desmond Bridge and the I-405 Sepulveda Pass Mulholland Bridge Replacement, our structures team continues to deliver iconic and complex bridge projects for our clients,” Wang said.

For Los Angeles’ Sixth Street Viaduct Replacement project, HNTB serves as architect and engineer-of-record. With the bridge on track to open in 2022, construction crews are finishing the viaduct and its signature arches, designed so they appear to be a “ribbon of light” weaving together the communities below. The Sixth Street Viaduct’s substructure features concrete “Y-Bents” that flow into the arches and use grade 80 reinforcement instead of grade 60, which is a first for a California bridge.

“Seismic isolation bearings were combined with the unique viaduct framing to achieve outstanding seismic performance,” said Mike Jones, PE, SE, HNTB Bridge Manager for the project and HNTB Fellow.

In Long Beach, California, HNTB provided preliminary design and prepared design-build bid documents and then provided design-build support to the Port of Long Beach and Caltrans during construction for the newly opened 2,000-foot-long Gerald Desmond Bridge. As the first vehicular cable-stayed bridge in the state, it includes three lanes in each direction with wide shoulders for improved traffic flow. The new bridge has two single mast towers at the middle of the roadway, and at 165 feet, the bridge is one of the widest cable-stayed bridges in the world.

Envisioned as the first signature span bridge in the region and an iconic bridge for Long Beach, the California Transportation Foundation recently named the Gerald Desmond Bridge Replacement as Project of the Year.

“It is California’s first cable-stayed roadway bridge,” Jones said, who was local office Bridge Design Manager. “The new class of container ships could not pass under the previous bridge. The new bridge provides the clearance necessary to accommodate growing international trade at the Port of Long Beach.”

For the I-405 Sepulveda Pass Widening Design-Build Project in Los Angeles, HNTB was lead designer for the replacement and widening of 23 bridges, including replacing the Mulholland Bridge with a 300-foot-long main span structure over the busiest highway in the U.S.

“The biggest structural challenge was the magnitude of the project — each bridge, one through 23, had its own set of challenges in design or during construction,” said Aamir Durrani, PE, HNTB Bridge Lead and Construction Manager. “Our technical experts implemented a range of innovative solutions on the structures to expedite construction, such as use of precast panels to special software to help the contractor with complex formwork geometry. These innovations substantially contributed to reducing the overall construction costs.”

On LA Metro’s Crenshaw Light Rail project, HNTB was lead designer for the design-build contract that integrates the Los Angeles International Airport with the regional rail network. For the I-405 UP bridge, the initial proposal included a center bent for the portion spanning over I-405. To avoid a maintenance and traffic nightmare while reducing Caltrans' review process, HNTB developed a design using a 315-foot-long main span that spanned the entire freeway and right-of-way without a center bent. The bridge profile ducked under high voltage lines while clearing local side roads.

“Setting the bridge geometry was like threading a needle with the horizontal and vertical constraints. HNTB’s design avoided another ‘Carmageddon’ freeway closure and minimized disruption to the citizens of Los Angeles. Today, the two bridges — Mulholland and I-405 UP with nearly identical lines — serve as bookends to the City of Los Angeles,” Durrani said.

“HNTB’s high-profile, complex bridge construction and replacement projects span across the entire United States and can be seen in most major cities,” Jones said. “Bridges not only connect people and communities, but they can serve as recognizable landmarks that elevate a city’s profile and improve mobility and quality of life.”

Since its founding, HNTB has been nationally recognized for its bridge expertise. The firm has roots in Southern California dating back to 1914 with the design of Pasadena’s historic Arroyo Seco Bridge. Spanning 1,486 feet and notable for its distinctive Beaux Arts arches, light standards, and railings, the bridge is on the National Register of Historic Places and has been designated a National Historic Civil Engineering Landmark by the American Society of Civil Engineers.