Under Project TN-55, different rehabilitation and replacement options have been studied for the Bronx and Queens approach viaducts. One of the alternatives was to retain existing piers and foundations. CHI performed structural modeling and seismic analysis to evaluate the adequacy of existing piers and pile foundations with the addition of new piers and drilled shafts.

Five pier foundations were selected for evaluation. In each foundation, all existing piles, pile caps and new concrete drilled shafts were explicitly modeled in a local soil-structure interaction (SSI) finite element model, as shown in the figure below. The interaction between piles and soils were captured by connecting them together at common nodes. Foundation stiffness values were obtained from these foundation models and incorporated into the global model. Seismic analysis was first performed on the global model for the 2500-year upper level seismic events. Static and seismic foundation reactions were then applied on top of the SSI model for checking the adequacy of existing piles.