Located less than a quarter mile (400 m) south of a major freeway interchange, this busy railroad crossing is at the nexus of a significant freight and commercial truck corridor. Prior to construction, truck traffic exiting the freeway had to back up onto the freeway when a train blocked the grade crossing, causing major delays and a significant safety hazard.
Initially, the design featured a steel plate girder bridge with a steel plate forming the deck, says Jay Holombo of T Y. Lin International, the engineer of record of the redesigned underpass bridge. During the bidding phase an alternate superstructure design using precast prestressed concrete adjacent box girders was evaluated.
"This evaluation demonstrated that significant cost savings and reduced fabrication time could be realized if precast box beams were used instead of the as-designed steel plate girders," Holombo says.
The new underpass includes a two-span bridge measuring 171 ft long × 52 ft wide (52 × 16 m). A precast concrete T-wall-based retaining wall system extends a half a mile (800 m) in either direction on both sides of the tracks to elevate the line while meeting right-of-way requirements. Seven-foot-deep (2.1 m) precast concrete fascia beams were also used to support maintenance walkways, railings, and communication conduit on both sides.
"The as-designed fascia beams were to be cast with an architectural formliner to give the exterior surface an Ashlar-stone appearance bordered with granite bands. These beams were redesigned to allow fabrication of 7.5 ft [2.3 m] wide sequentially cast precast concrete panels that were cast in to the fascia beam forms, reducing the cost of the formliner. The result gives the overall structure a pleasing profile that blends well with the adjacent T wall panels and economically meets the intent of the original design," Holombo says.
The high-strength precast concrete units also met stringent American Railway Maintenance-of-Way Association strength and deflection requirements despite limited vertical clearance. These requirements were met by using high compressive strength concrete of 9000 psi (62 MPa) and prestress forces approaching the upper limit of the fabricator's capacity of 2400 kip (10,700 kN).
By choosing high-performance precast concrete, the designers achieved faster delivery of the project, including design and agency review and approval, with minimal effects on traffic. Both the underpass and approaches were constructed in two phases, allowing the busy railroad line to continue in full operation throughout construction.
"The precast design did require some minor modifications to the foundation size due to the added weight of the concrete girders versus the steel, but when all costs were included, the net savings to the project was approximately $900,000," Holombo says. That savings was split between the contractor and the owner. The choice of precast concrete also eliminated the need to paint the structure, which would have cost the city an additional $1.5 million over the lifetime of the bridge, according to data published by the National Steel Bridge Alliance.
"This project demonstrates that high-performance precast concrete is ideal for railroad bridge spans greater than 80 ft [24 m]," Holombo says. "It has better economy, reduced maintenance, and faster delivery than equivalent steel spans."