Design criteria for post and beam bents with drilled shafts.

2006-15
The research work presented in the report addresses the potential areas of conservatism in the current practice related to bridge
bents supported by drilled shafts and piles. The research encompasses modeling efforts and an experimental program. Modeling
includes analysis of existing bridge case studies with drilled shaft foundations to characterize the impact of the current
assumptions on sizing the various components of a bridge bent. An experimental program is performed to evaluate the
connection stiffness, and to determine if a given connection has sufficient capacity to develop such stiffness and transfer moment
The experimental program includes testing bearing pads in compression and shear to define their index properties. The bearing
pads are then tested in a prototype bridge set up to measure their performance under simulated loading.
An improved set of design guidelines and recommendations regarding the superstructure-substructure connection’s rotational
stiffness, shaft/pile top boundary conditions, and resistance factors are proposed along with characterizing the impact of the
current practice and various design assumptions on sizing components of bridge bents. Resistance factors are developed for
drilled shafts, based on load test data from the state’s geologic regions, for compatibility with LRFD implementation. Work in
the report serves to provide a better understanding of the performance of bents supported by drilled shafts under AASHTO
loading conditions. Such understanding serves as a tool that provides NCDOT engineers with the flexibility of specifying the
level of conservatism to be built into a specific bridge bent design.