Seattle's Bridge Seismic Retrofit Program: Ballard Bridge Case Study

This paper presents an overview of the site seismicity and conceptual seismic retrofit design for the Ballard Bridge. It covers the pre- and post-retrofit analysis methods which included a displacement-based methodology for the approach spans and force-based methodology for the bascule span. The development and selection of the recommended retrofit concepts for the final design is also described. Retrofit concepts are presented to address seismic deficiencies of foundation overturning, inadequate shear and flexural strengths of superstructure and substructure members, weak axis bending of floor beams, inadequacy in bearing anchorage and roadway slab joints, and lateral spreading. Retrofit alternatives include additional piling, strengthening pier diaphragms and longitudinal girders, enlarging footings, adding partial column jackets, encasing bearings, adding girder stops and shear blocks, reinforcing roadway deck joints, foundation post-tensioning, trunnion strengthening, and soil improvement.