Seismic Soil-Foundation Investigation of the Brooklyn Bridge

A comprehensive seismic investigation of the Brooklyn Bridge was completed to assess its potential retrofit needs. The Brooklyn Bridge, built in 1883, has become a national treasure and architectural and engineering marvel. To ensure that the seismic retrofit needs of the bridge were based on a rational framework, avoiding overconservatism that would potentially lead to unnecessary retrofit and impacting negatively on the architecture of the bridge, advanced engineering investigations of the condition of the bridge and its seismic response were made. Specifically, the seismic investigation of the main bridge was performed following two approaches, referred to as the global and local analyses. In the global analysis, the entire main bridge with its foundation caissons was modeled, and the effects of soil-foundation interaction were incorporate through the use of foundation impedances. In the local analysis, each bridge tower with its caisson and the surrounding soils was investigated with a model using solid finite difference and slip and gap interface elements. The local analyses of the towers were performed to confirm quality of the motions and foundation impedances used in the global analysis, and to ensure that the conclusions regarding the potential need for foundation retrofitting was realistic and essential. This paper presents the details of the two seismic evaluation approaches, and compares the bridge foundation responses from both analyses. It also demonstrates the benefits of local analysis in the seismic evaluation of long-span bridges.