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6th Street Viaduct Replacement Project

Owner: City of Los Angeles  |  Los Angeles, California


About the Project

The largest, most complex bridge project in Los Angeles’ (L.A.) history, the 6th Street Viaduct Replacement sets a new threshold for seismic safety and community integration. The new viaduct opened to traffic on July 10, 2022, replacing a beloved double steel arch landmark with 10 pairs of sculptural arches spanning 930m across a highway, multiple railway lines, the L.A. River and industrial lowland.

COWI worked on the project for seven years, beginning in 2015, performing Erection Engineering and Construction Services. As the Erection Engineer with extensive experience in complex structures, COWI worked closely with the Contractor Skanska Stacey and Witbeck (SSW) to create a functional erection design, develop a constructable erection sequence, including falsework design, step-by-step camber and stress analyses, and a hanger installation and stressing procedure of the bridge to achieve target geometry and target hanger forces at 10 years after End of Construction (EoC). Key safety aspects and schedule improvements were discussed when finding cost efficient and sustainable solutions.

The City’s goals were to preserve mobility, encourage multimodal travel, achieved through an ADA – (Americans with Disabilities Act Standards for Accessible Design) compliant helical ramp structure located near the center of the Viaduct, instill civic pride and ‘sense of place’ as a world-class L.A. icon, improve community safety, increase transportation equity in the community of Boyle Heights, support long-term sustainable growth and development, unify Boyle Heights with the L.A. Arts District, provide much-needed public space and amenities through a future 12-acre public park below the viaduct, and last but not least rebuild an iconic structure. The design incorporated resiliency which will require minimal maintenance for the City of L.A. for the next 100 years.


COWI used the “SOFiSTiK” analysis software for structural computations and was able to simulate the construction sequence within the finite element model, considering complex behaviors such as structure falsework interaction and imposed deformations. Final submittal comprised 140 drawings, 3 reports and 191 memos. COWI was in continuous contact with the Contractor, attending weekly meetings with Designer, Owner and others as needed. While the project team communicated with distributed engineering tools such as Teams/Outlook, in addition to site visits, the pandemic imposed further restrictions making site visits impossible for over a year. During this time COWI followed critical site operations by video link providing the same support as if physically on site. COWI was on site regularly to meet with construction crew and check on construction progress. We responded immediately to any issues and if needed would be on site within 24 hours. COWI helped to improve the construction sequence, which was partially imposed by the design, without changing the latter. We developed efficient ways to stabilize the seismic isolators during construction and to account for all post-tensioning and long-term effects. We carefully evaluated curing times and demonstrated in a comprehensive construction scheme the different stages of construction. COWI also provided expert services during critical issues during the erection process and helped the Contractor to swiftly recover the schedule impact.


COWI’s Erection engineering expertise made the construction of this complex bridge a success. Staffing with erection engineering experts familiar with the complexity of jointless continuous structures, network arches and seismic requirements allowed us to leverage our full potential. Our team worked closely with Owner, Designer and Contractor to detect potential issues early on. The result is a viaduct that met the Owner’s and Architect’s expectations and earned overwhelming public acceptance.

Service(s) Provided
Erection Engineering
Design Services during Construction

Project Team (Consultants)
Skanska Stacy and Witbeck, A Joint Venture
Owner’s Engineer – HNTB
Construction Management: TYLin


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