Los Angeles: Sixth Street Viaduct Replacement Project

Sixth Street Viaduct, overhead view

From the July/August 2022 Issue of Wire Rope Exchange.  Above: Aeriel view of the Sixth Street Viaduct with downtown Los Angeles in the background – photo credit: shutterstock.com/Ringo Chiu

BY:  Patricia Glenn

To understand the significance of the Sixth Street Viaduct Replacement Project, it’s important to start at the beginning. Completed in 1932, the original Sixth Street Viaduct – also called the Sixth Street Bridge – was an engineering landmark for the City of Los Angeles (LA).

Sixth Street Viaduct, engineering team
The Engineering Team responsible for the Sixth Street Viaduct project.

The last in a set of 14 historical structures crossing the Los Angeles River, all 14 bridges were part of a project aimed at facilitating the smooth flow of different forms of transportation in LA – in particular separating car and railway infrastructure. Each bridge had a unique architectural theme, and at its completion, the Sixth Street Viaduct was the longest bridge in California, extending nearly 3,500 feet east-to-west from Whittier Boulevard to Sixth Street and crossing the LA River, multiple railroad tracks, highway US 101, and several local streets. At 46 feet wide, the bridge supported two sidewalks and a four-lane roadway with 11 feet eastbound and westbound inside traffic lanes and 12 feet outside lanes with no shoulders.

The Sixth Street Viaduct serves as a gateway between the Downtown Arts District and Boyle Heights, but over time, has evolved to embody Los Angeles history and culture. The viaduct connects two LA neighborhoods horizontally, but it is important to acknowledge the way the Sixth Street Viaduct functions from a vertical perspective. Underneath the viaduct, the LA River and its concrete slanted riverbanks have been the location of car chases and dystopian landscapes in numerous Hollywood productions.

Sixth Street Viaduct, old structure from below
Iconic view of the original structure, easily recognizable from a countless array of film productions. photo credit: shutterstock.com/Josh Cornish

A coveted filming location, on average, 80 movies, music videos and TV shows are shot underneath the Sixth Street Viaduct per year. In the background of Grease, Charlie’s Angels, The Terminator, and The Dark Knight Rises, the iconic steel double arches stretch above the viaduct into the sky, playing a silent starring role in the background. The art deco style and Hollywood recognition of the viaduct have been just as noteworthy in its history as the viaduct’s original purpose as transportation infrastructure.

Due to its large size, an onsite concrete mixing plant was used to construct the original Sixth Street Viaduct, but within 20 years of completion, the aggregate used in the concrete caused an Alkali Silica Reaction – a chemical reaction producing a gel that expands with the absorption of water, imposing pressure on and weakening surrounding concrete. The bridge was swiftly deteriorating.

Numerous costly restorative methods were implemented in an effort to save the viaduct, but all failed. Seismic vulnerability studies of the structure concluded the viaduct not only had geometric design and safety deficiencies, but a high probability of failure in the event of a major earthquake. Whereas the initial aim of separating transportation infrastructure was crucial in promoting more efficient traffic patterns in LA, the viaduct, which crossed multiple forms of transportation infrastructure, and served as a form of infrastructure itself, now posed great danger in the case of natural disaster.

The bridge had to be completely rebuilt, but when considering reconstruction, it was important to the City of Los Angeles that the bridge maintain its stature as equal parts transportation infrastructure, visual landmark, and neighborhood amenity.

Sixth Street Viaduct, stairway
This staircase, parallel to an archway, is one of the features vertically linking the bridge to the space that it spans. Photo Courtesy of City of Los Angeles Bureau of Engineering.

The design of the new Sixth Street Viaduct, called “The Ribbon of Light”, was created by HNTB and Los Angeles Architect Micheal Maltzan who was awarded the project after winning an international competition to redesign the Sixth Street Viaduct in 2012. Taking an architectural approach to an engineering problem, “The Ribbon of Light” takes the original two pairs of arches and expands them into a series of arches that connect the city like an extended ribbon. For inspiration, Maltzan looked at other iconic city structures like the Griffith Observatory in LA, the Statue of Liberty in New York, and the Eiffel Tower in Paris.

He found that each landmark had something in common: they were not only something to observe from the outside, but each served as an observatory on their own, offering a chance to look out upon their respective cities. In many ways, Maltzan redesigned the Sixth Street Viaduct to function not only as a structure to look at, but a structure to look from.

Revisiting the idea of horizontal and vertical space, Maltzan wanted the bridge to weave into its surroundings rather than exist over them. In 2023, the LA Bureau of Engineering plans to begin construction of the 12-acre Sixth Street PARC (Park, Arts, River, and Connectivity Improvements) Project under the viaduct. The PARC will serve as a recreational space, and will include access to the LA River, an arts plaza, public art, and numerous community amenities.

In 2016 the bridge was demolished completely, and construction on the new Sixth Street Viaduct began. Funded by the Federal Highway Transportation Administration, the California Department of Transportation, and the City of Los Angeles, the $588 million Sixth Street Viaduct Replacement Project was led by the Los Angeles Bureau of Engineering under City Engineer Gary Lee Moore in partnership with the LA Bureau of Contract Administration.

Sixth Street Viaduct, alternate overhead view
Overhead close-up view of the new viaduct structure and arches nearing completion, again facing west toward downtown. Photo Courtesy of City of Los Angeles Bureau of Engineering

The new Sixth Street Viaduct consists of 10 pairs of arches that range in heights from 30 feet to 60 feet tall. Each arch has a 9-degree outward cant, 10 foot width, and required 260 cubic yards of concrete (equivalent to 65+ truckloads). Arches had to be poured at a rate of 4 vertical feet per hour for a total of 12-14 hours per arch due to the formwork and intricate support system.

The arches join into a Y-shaped column, and in total, the 23 columns and 2 abutments of the bridge use triple friction pendulum bearings for seismic base isolation. These bearings allow movement up to 30 inches in any lateral direction, not only ensuring earthquake survival, but making the Sixth Street Viaduct one of the largest base-isolated structures in the world.

Generally, construction developed from east to west, and was led by contractor Skanska-Stacy and Witbeck Inc. Arches were poured without cables in place, then cables 2 ¾in. in diameter were added and tightened to create a network-tied arch structure. As cables settled, a computer system monitored cable tensions and performance, allowing engineers to fine-tune each cable. In total, 110,000 tons of concrete and 8,250 tons of steel were used to rebuild the Sixth Street Viaduct.

After 7 years, on July 9, 2022, Los Angeles celebrated the long-awaited grand opening of the Sixth Street Viaduct with a weekend-long community event. 15,000 people attended the day-long Saturday celebration with live music, food trucks, a market, vintage car display, a low-rider tribute, and a firework display and the official lighting of the bridge.

On Sunday, the bridge opened for pedestrians and bikers only, and the celebration continued with a symbolic run and bike ride led by Gary Lee Moore, the Boyle Heights Bridge Runners and the Skid Row Running Club who became the first individuals to run and ride across the bridge. On Sunday evening, vehicles were granted access and led by a low-rider parade.

Showcasing history, culture, and innovation in the grand opening, the redesign of the Sixth Street Viaduct has solidified it as an LA landmark. As transportation infrastructure it connects two neighborhoods, and visually it utilizes clever architecture and engineering to problem solve and reimagine the way the community interacts with the space – not only on the bridge, but in the space underneath.

Houston Structures, Inc. Supplies Major Components for Sixth Street Viaduct

Sixth Street Viaduct HSI Ulven Companies
One arch structure showing placement of wire rope assemblies supplied by Houston Structures, Inc. Photo courtesy of HSI/Ulven Companies

Three Hundred Thirty (330) socket-to-socket wire rope assemblies were supplied to the Sixth Street Viaduct project by Houston Structures, Inc. (HSI), part of the Ulven Companies.  The assemblies include cast sockets, bolts, nuts and forged rods manufactured in the United States by Ulven/HSI, and were fabricated and pre-tensioned with 2 ¾” wire rope sourced from WireCo WorldGroup.

The assemblies were supplied in a variety of pre-specified lengths for use in various positions on each of the arch structures on the bridge.  HSI also supplied the tensioning systems used to make finely tuned adjustments to each wire rope assembly, which by itself was an approximately sixteen-month process from start to finish.

Mike Denton, Director of Sales for HSI told Wire Rope Exchange that “this was a great project to be a part of, given its history and its place in the community it was really something special.”  Although he added, “we enjoy doing this kind of work on all sorts of bridge structures, whether they’re famous or not!”

Sixth Street Viaduct, HSI supplied fittings
Closer view below one arch of wire rope assemblies installed supplied by Houston Structures, inc. Photo courtesy of HSI/Ulven Companies

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