ABSTRACT
Monmouth County of New Jersey retained Greenman-Pedersen, Inc. to perform preliminary and final design of a new four-span continuous (90′-100′-80′-80′) steel multi-girder horizontally curved bridge that would replace a nearly 100-year old existing thru-girder bridge over Matawan Creek. Organic clay soils throughout the project site resulted in a Site Class F soil classification and the need for a site-specific ground motion response spectrum analysis. Due to the highly compressible soil of about 50 to 70 feet thick in some areas, deep foundations were required and extended into the stiff clay and dense sand layers below, ending nearly 120 feet below mudline. To minimize the size of the deep foundations, seismic isolation bearings were used to reduce the seismic demand on substructures. Due to the unique nature of the isolated system involving tall piers, a 3D time history seismic analysis was performed in addition to response spectrum analysis. The focus of this paper is to discuss how isolators can reduce the foundation size of a bridge situated in a weak soil site. It also describes the subsurface conditions, the development of a site-specific response spectrum for a 1000-year return period earthquake, isolation bearing preliminary design, the benefits of using isolation bearings and seismic analysis.
