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New Los Angeles Viaduct Makes Headway

Thursday, February 4, 2021

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According to reports, the new Sixth Street Viaduct in Los Angeles—also referred to as the “The Ribbon of Light”—is on track to be completed by mid-2022, despite having undergone unique design and construction challenges that delayed the construction schedule by 18 months.

The $588 million viaduct is currently the city’s largest infrastructure project underway.

The Original Viaduct

The original Sixth Street Viaduct was once touted as one of the 14 historic structures in Los Angeles, having been built in 1932. The structure connected neighborhoods between the Arts District on the west side and Boyle Heights on the east side.

Measuring 3,500 feet in length with a 46-foot-wide, four-lane roadway with 11-foot eastbound and westbound inside traffic lanes, the structure also featured 12-foot outside lines with no shoulders. However, the viaduct managed to feature sidewalks of varying widths.

Constructed using an onsite concrete mixer, nearly 20 years after its completion was diagnosed as having “concrete cancer,” or an alkali silica reaction, meaning that the aggregate used in the concrete caused a chemical reaction that deteriorated the concrete. The structure was later ruled to be functionally obsolete in term of national bridge design standards and was also seismically vulnerable.

While restorative methods were attempted and failed, studies eventually showed that the structure would require a full replacement and was demolished in February 2016.

Ribbon of Light

Prior to the structure’s demolition, infrastructure design firm HNTB and Los Angeles architect Michael Maltzan won a design competition regarding the replacement of the old viaduct in 2012. The team was amongst six of nine to be shortlisted in the design competition and one of three to be given a stipend for further design by the Bureau of Engineering, under the leadership of City Engineer Gary Lee Moore and in partnership with the City's Bureau of Contract Administration,.

“It was necessary to take it down from a technical and safety standpoint, but there was a lot of pushback by communities who had an investment, emotionally and culturally, with the bridge,” said Maltzan. “[We needed to] create a new bridge that had as strong a character as the original bridge—one that captured the ambitions and spirit of the city.”

In an attempt to do just that, Maltzan and HNTB designed the new structure to feature 10 pairs of continuous arches stretching roughly 300 feet long, down the 3,060-foot-long and 100-foot-wide viaduct. The arches will also be supported using 388 hanger cables and post-tensioning in edge girders and floor beams.

Designed to serve as both a destination as well as a multimodal link and a driving force in transforming the urban landscape, according to Maltzan, the columns and arches will create an image of a “ribbon of light.”

The scope of work for the project will not be simple, however, as it calls for the demolition of 13 buildings in addition to the relocation of five overhead powerline systems and various improvements to 10 street intersections. The city also plans to build a 12-acre park underneath the viaduct.

But that’s not all, the construction of the actual viaduct will also require 27 million pounds of falsework to support the new deck and 80 feet wide, 30 feet tall Y-shaped columns which are designed to support the arches. Crews will also have to employ customized lock-up devices designed by erection engineer COWI to support each column during construction, which will eventually serve as shear keys to accommodate anticipated changes in the alignment of the continuous cast-in-place structure due to creep and shrinkage over 10 years and could be further utilized in the case of a seismic event.

According to Julie Allen, principal engineer with the city, the Earthquake Protection Systems-designed friction-pendulum bearings and second-generation triple-pendulum bearings will allow for the structure to move up to 30 inches in any direction and is designed to withstand a 1,000-year seismic event.

In addition to the use of bearings, the viaduct will be supported on 10-foot in diameter cast-in-drilled-hole piles up to 165 feet deep. Already, crews have poured the transverse floor beams with huge edge girders and post-tensioned them with the assist of falsework, which is slated to remain throughout cable installations.

“The superstructure is post-tensioned both transversely and longitudinally to reduce cracking,” said Michael H. Jones, senior civil engineer with HNTB. “The concrete mix has polymer fibers and admixtures to reduce shrinkage.”

Jones also added that structural steel for the project is galvanized and that the arch hanger brackets will have a zinc-aluminum alloy coating which is expected to have nearly triple the design life of conventional zinc coatings.

Construction manager/general contractor (CM/GC) team of Skanska USA Civil and Stacy and Witbeck (SSW) also worked with the city and design team to address any possible rebar congestion said Jerry Iniguez, Skanska project executive. “As you come out of these wide Y arms, the bridge is post-tensioned, so we had to create enough room for the stressing ducts to fit.”

Moore recently reported that the viaduct is 65% complete. “By the end of January, we’ll have 2,000 linear feet of superstructure up to the deck level. Construction for the first of our six out of 20 arches will be complete by the end of February.”

On that note, SSW is preparing to place an average of 300 cubic yards per arch at a rate of four vertical feet per hour, maximum. While the rate is reportedly slow, the measure is being taken due to the volume of the project. Once concrete pours are completed, crews are scheduled to transition to cabling and hanger bracket installation.

At peak construction, the 32 crews and nearly 200 craftworkers have already completed concrete pours of up to 200 cubic yards each for the Y-shaped columns. On track for the mid-2022 competition, the contract includes $40,000 a day in potential liquidated damages or an early completion incentive of $200,000 a day up to $20 million. SSW also intends to achieve the federal goal of including Disadvantaged Business Enterprise involvement at 23.95%.

Moore reports that this create $60 million in opportunities.

“It’s not simply a bridge crossing the river; arguably that’s one of the smallest parts of the exercise,” says Maltzan. “It was more important to not celebrate this as a line dividing the city, but as a way to knit it together.”

   

Tagged categories: Design build; Infrastructure; Infrastructure; NA; North America; Ongoing projects; Program/Project Management; Project Management

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