Seismic Retrofit of Octagonal Columns with Pedestal and One-Way Hinge at Base

Abstract

Recent earthquake experience has shown that the response of retrofitted bridges is generally satisfactory. However, many seismically-deficient bridges remain, notably those that are supported on piers that lack proper detailing to resist strong earthquakes. This article reports on a study of seismic retrofit of bridge columns that are irregular both in the cross section and along the height, with the latter caused by the presence of a pedestal and a one-way hinge connection between the footing and the pedestal. Three identical quarter-scale column specimens were built for shaketable testing. One was tested as-built, the others were enhanced with seismic retrofits. The columns were subjected to the 1994 Northridge Sylmar earthquake in the strong direction until failure. Results showed that lateral steel of the as-built pedestal was highly deficient, resulting in an undesired pedestal failure. The first of the retrofitted columns was tested with a pedestal extension and a glass fiber-reinforced polymer (GFRP) pedestal jacket. The second was tested with the same retrofit as the first but several of the column bars were severed at the base of the column to reduce plastic shear demand. Results showed that the pedestal retrofit was successful in strengthening the pedestal sufficiently to shift the plastic hinging into the column. The severed column bars lowered shear demand and increased the plastic deformation. The authors also used analytical models to determine the shear strength and push-over behavior. The authors suggest that bond slip, shear deformation, and strain rate effects be accounted for in any effort to obtain a reasonable estimate of the push-over response of piers.