Paul Morel, P.E., Kleinfelder|Simon Wong Engineering, San Diego, CA, Nathan Johnson, Ph.D., P.E., Kleinfelder|Simon Wong Engineering, San Diego, CA, Craig Shannon, P.E., Kleinfelder|Simon Wong Engineering, San Diego, CA
It is not every day that a bridge engineer gets the opportunity to design a semi-circular pedestrian bridge. Our opportunity presented itself as part of the 22-mile I-5 North Coast Corridor project located along the scenic I-5 corridor in coastal San Diego, California. The Genesee Avenue Pedestrian Overcrossing (POC) will provide a direct connection across Genesee Avenue for a new class 1 bicycle and pedestrian facility that will run parallel to I-5 on the west side. The two-span, 260-foot-long, 18-foot-wide, cast-in-place reinforced concrete box girder bridge was designed for an extreme horizontal curve (115-foot radius) as demanded by architectural and layout needs. The box girder type was selected for its high torsional rigidity. The bridge will be supported on a single-column bent and diaphragm-type abutments with deep foundations; this serves as a creative solution to handle the torsional demands imparted by the superstructure.
Aesthetics were a major consideration for this bridge being that this is the first significant project on the corridor and will be looked at to set the stage for all future segments. Close coordination with the City of San Diego, Caltrans and the San Diego Bicycle Coalition was required. The horizontal curve of the POC was ultimately chosen not only for the functional purpose of providing an uninterrupted pathway across Genesee Avenue for the benefit of the bicycle users, but also to provide a uniquely shaped signature structure for the coastal corridor. Integral colored concrete, a tapered octagonal column, pilasters behind each abutment, form liners on the approach retaining walls, and lighting integrated into the exterior concrete barrier face were all added to the final design of the project to complete the desired aesthetic theme. The metal railing is another unique architectural feature of the bridge consisting completely of weathering steel. It features a 5-inch-diameter steel tube bent to match the bridge radius, combined with perforated sheet metal components utilized to comply with strict American with Disabilities Act requirements related to pedestrian railings.
The unusual curvature of the POC required a high level of analysis for the superstructure that included development of a grillage model and a 3-D finite element model. The grillage model, used to design the bridge, was developed in accordance with the guidelines included in the NCHRP Report 620, “Development of Design Specifications and Commentary for Horizontally Curved Concrete Box-Girder Bridges”(1). The grillage model consisted of longitudinal beams located along each girder line and transverse beams to model the bridge deck, soffit, and all diaphragms along the span. Since the maximum superstructure torque under dead and pedestrian loads could exceed the section’s cracking torque, two boundary conditions were used to design for shear and torsion: a lower bound where the superstructure torsional stiffness is equal to 5 percent of the gross torsional stiffness, and an upper bound where the superstructure torsional stiffness is equal to 20 percent of the gross torsional stiffness. The shear and torsional reinforcement were designed for the envelope of these two conditions.
The 3-D finite element model was initially developed to confirm appropriateness of the grillage methodology, based on comparison of displacements between methods. In part, this confirmation method was suggested out of a peer review process that was elected by the designers to assist in evaluating the behavior of such an unusual structure. Thin shell elements were used to model the deck, girders, and soffit. The shell finite element method implicitly captures issues related to high degree of curvature including shear-flexure-torsion interaction, shear lag, and twist deformations due to distortion of the cells. The confirmation process not only provided support of the grillage method but allowed the engineers to debug and check parameters of the grillage design model. In addition, the complete independent check for superstructure design was then performed using the shell model.
The project was greatly accelerated in order to take advantage of additional state funding sources that became available as the planning phases of the project progressed. Due to the extreme horizontal curvature, this structure deviated significantly from what standard design methods can handle. A three-pronged approach that included an elective peer review process, varied analytical methods, and a full independent check enabled the design team to be successful in meeting this challenging deadline, while gaining confidence that adequate engineering was applied to the unique situation. The design of the Genesee POC was completed in July 2012. Construction is anticipated to start in 2015 with an expected completion early 2016.
Further Information
For more information, please contact the primary author Paul Morel at [email protected].
References
(1)Nutt, Redfield and Valentine in Association with David Evans and Associates, and Zocon Consulting Engineers, 2008. Development of Design Specifications and Commentary for Horizontally Curved Concrete Box-Girder Bridges, NCHRP Report 620. Transportation Research Board, National Research Council, Washington, D.C.