This study investigated retrofitting measures for improving the seismic performance of existing multi-column bridge bents. Experimental tests were conducted on 1/4.5-scale footing and column assemblages which incorporated details that were selected to represent deficiencies present in older bridges. Various retrofit measures for the bents were evaluated. The specimens were subjected to increasing levels of cycled inelastic lateral displacements under constant axial load. Specimen performance was evaluated on the basis of load capacity, displacement ductility, strength degradation and hysteretic behavior.
Tests on the as-built specimens resulted in severe cracking in the footings due to insufficient joint shear strength in the column/footing connections. However, due to structural redundancy, the bents continued to resist lateral loads until eventual bent failure occurred as a result of flexural hinge degradation in the columns.
Measures developed previously for retrofitting single-column bent bridges were found to be effective in improving the performance of the footings and columns. When all substructure elements were retrofitted, a ductile bent response was obtained. Retrofitting only some of the substructure elements resulted in incremental improvements in performance according to the number of elements retrofitted. While extensive damage occurred in the unretrofitted elements, the damaged regions continued to transfer forces during testing, enabling a stable bent response until failure occurred within one or more of the retrofitted elements.
The addition of a stiff link beam just above the footings was found to be effective in preventing damage in the footings during testing, and a reasonably ductile bent response was achieved. Because the link beam retrofit may not require retrofitting of the footings, this strategy may be a very cost-effective approach for retrofitting multi-column bents.