Seismic Vulnerability

Forty-three (43) select bridges along the New Jersey Turnpike and Garden State Parkway were evaluated for their vulnerability to seismic events. Conceptual retrofit details were developed with corresponding cost estimates. SJH performed multi-modal response spectrum analysis for eleven (11) bridges using CSiBridge, a 3-dimensional general Finite Element program.

Most of the bridges were multi steel stringer type structures with multiple spans. Some were viaduct type structures with numerous spans. Out of the eleven (11) bridges, eight (8) were classified as “Essential” bridges and three (3) were classified as “Critical” bridges by the Turnpike Authority which required site-specific analysis and consideration of soil and structure interaction. A detailed geotechnical investigation program was conducted with many Standard Penetration Tests (SPT), Seismic Cone Penetrometer Tests (SCPT) and Crosshole Survey tests to augment the existing boring information and to develop the site-specific seismic response spectrum, foundation stiffness and evaluation of liquefaction potential.

The analysis was performed in accordance with the latest AASHTO LRFD Bridge Design Specifications and Turnpike Authority Design Manual. The Essential and Critical bridges were analyzed for 1,000-year and 2,500-year seismic events, respectively. Force demands resulting from the analysis were compared to the member capacities and Capacity/Demand (C/D) ratios were calculated for each component evaluated including, but not limited to, pier cap beam moment/shear, column PMM/shear, footing moment/shear. Detailing requirements such as bridge seat support length, bearing anchor bolt shear, column reinforcement confinement/lap splice length were also evaluated. A push-over analysis was performed for each structure to obtain the displacement capacities in both longitudinal and transverse directions and compared with the displacement demand.

Standard conceptual retrofit details and unit costs were developed to address the deficiencies for all the bridges in the project to maintain consistency. The retrofit details included, steel continuity details, bearing replacements, and substructure details for ductility. The retrofit costs were compared to the 25% of bridge replacement cost as per the Turnpike Authority Design Manual to determine whether or not a replacement design is warranted.