By Surur Sheikh
After 22 years in the City of Los Angeles’ books, the Taylor Yard Bikeway and Pedestrian Bridge is underway. Earlier this year, the groundbreaking ceremony was attended by Los Angeles mayor Eric Garcetti, LA Metro CEO Phillip Washington, council members and local officials, to celebrate the progress of this project and its value to the community. Working closely with the City of Los Angeles and Studio Pali Fekete Architects, Arup is the bridge design consultant and engineer of record.
The Taylor Yard Bikeway and Pedestrian Bridge will connect the existing Elysian Valley neighborhood south of the river to the newly developed Cypress Park community to the north. It will provide a safe travel path for both pedestrians and cyclists, connecting to an existing bike path south of the river and a future bike path along North San Fernando Road.
The bridge will also connect to a future river revitalization project, the Taylor Yard G2 River Park Project. The project will be built on the 42-acre site adjacent to the river known as the G2 parcel. Currently, the G2 parcel serves as a construction staging area to assemble the bridge’s truss segments.
Once the bridge construction is complete, the G2 parcel will be transformed into a park and recreational greenspace that is sustainable, safe and inviting. Taylor Yard Bikeway and Pedestrian Bridge is one of three pedestrian crossings under construction along the Los Angeles River as part of the City’s river revitalization plan, which aims to serve local communities, connect neighborhoods, and maintain flood protection and safety.
The proposed two-span steel-truss box structure bridge spans 400 feet over the Los Angeles River and is designed to stand as a landmark with its distinctive profile and unique orange color. The structural steel frame system consists of square hollow structural sections (HSS). The vertical and horizontal HSS members are connected to form a box truss structure. Diagonal tension rods triangulate the truss to provide lateral stiffness and transmit shearing force vertically and transversally.
The design team’s selection of steel as the bridge’s structural material provides many benefits, including expanded design options, faster construction, broad architectural possibilities, and sustainable offerings. These advantages helped the design team mitigate project constraints and meet the client’s goals, architectural vision, and architectural aesthetic requirements. The high strength-to-weight ratio of the steel also helped minimize the substructure and foundation costs.
Steel truss offers a lower structural depth below deck for the span compared to a concrete bridge of similar span. This was achieved by increasing the height of the vertical truss member, which provided a longer lever arm for the top and bottom chords, reducing their axial forces and increasing the bending capacity. This helped overcome the vertical clearance constraints at the north abutment, while providing a low-profile, aesthetically pleasing section. In addition, the prefabrication of steel truss members reduced the construction time and minimized the ecological footprint and disturbance to the river’s natural habitat.
As one of the most recycled construction materials, steel also offers sustainability benefits. At the end of a steel bridge’s useful life, the steel structural members are sent back to mills or manufacturers to create new steel products.
The superstructure consists of HSS modules that are 30 feet tall, 30 feet wide and 22 feet 7 inches long. Designed to create a floating appearance, the concrete deck consists of an 18-foot-wide, 8-inch-thick reinforced-concrete deck slab that is cast on a stay-in-place corrugated steel metal decking form. The slab is supported on secondary longitudinal wide-flange beams that run the full length of the bridge. The longitudinal beams are supported on the primary transverse wide-flange beams spanning between the vertical HSS members and spaced at 22 feet 7 inches along the full length of the bridge. The stainless-steel tension rods triangulating the bridge vary in size, from 2-inch diameter to 3.5-inch diameter. They provide stability, improving the lateral stiffness and strength of the bridge’s structural system during construction and over its service life.
The bridge substructure and foundation system consist of a reinforced pier wall at mid-span. The wall is supported on four 7-foot-diameter cast-in-drilled-hole piles connected by a reinforced-concrete pile cap. At the north and south abutments, the bridge is supported on four 3.5-foot cast-in-drilled-hole piles connected by a reinforced-concrete pile cap. The steel superstructure is supported on bearings at the pier wall and at the abutments to transfer the forces from the bridge superstructure to the substructure.
The City’s vision for this landmark bridge included a high level of seismic resilience, so they requested that the structure adhere to the California Department of Transportation’s (Caltrans) seismic design criteria and performance goals. Under these criteria, the bridge is categorized as Ordinary and classified as Standard, which triggers a Safety-Evaluation Ground Motion Assessment. This assessment is based on probabilistic ground motion, and the design spectrum is based on a 5 percent in 50 years probability of exceedance, or an approximately 975-year return period.
The seismic design strategy adopted is ductile substructure and elastic superstructure. The ductile (inelastic) behavior of the bridge is limited to the bottom of the pier wall. The structural steel HSS truss members are designed to withstand the deformation imposed by the design earthquake, with sufficient strength and reasonable reserve capacity to ensure it will not collapse during an earthquake. The HSS members are supported on bearings at the north abutment, south abutment and at the center pier. The longitudinal wide-flange steel beams cantilevering north of the bridge are also supported on bearings that connect to the retaining wall. All bearings are designed to resist the lateral seismic force and displacements in an extreme event.
The hydraulic study for the Taylor Yard Bikeway and Pedestrian Bridge concluded that the proposed bridge will increase the design-flood water-surface elevation by up to one inch at the upstream bridge face. However, this increase will not reduce the top-of-bank freeboard – the distance between the high-water elevation and the bottom of the structure – beyond the minimum level recommended by codes and local authorities. This design ensures the structure will remain safe and above water in a 100-year flood event.
The design team also considered strategies to build the bridge sustainably and minimize disruption to the waterway and the ecological habitats. At an early stage in the design development, the Arup team carried out an extensive constructability study, which articulated a feasible construction sequence option, as well as Caltrans inspection and maintenance protocol for steel bridges with fracture critical members. The constructability white paper informed the City of the complexity and size of the project, which enabled them to grant additional funding.
As a result of this initial sustainability-focused constructability research, the project will be built only during the dry season to reduce impact on the river habitat. That means construction can only be done six months out of the year, from April to October. To increase the speed of construction and minimize environmental impact, segments of the bridge will be built on the off-site fabrication yard adjacent to the river. The steel box truss will be built in five segments, with each segment made up of four to five bays that will be pre-assembled off-site and then lowered to the river and spliced to the next segment.
The bridge is schedule to be completed in 2021 and is targeting Envision Platinum certification, which is awarded to sustainable and resilient infrastructure. When the bridge is completed, it will serve as a connector between two communities, as well as a key component to revitalizing the Los Angeles River.
Surur Sheikh is a senior bridge engineer based in Arup’s Los Angeles office. She has more than 16 years of local and international structural engineering design and project management experience, working on a range of projects in the built environment. She has designed and delivered infrastructure projects in the UK, UAE and USA. Her expertise includes structural analyses, detailed design of reinforced concrete, structural steel, precast and prestressed concrete, and composite construction.