Behavior of oblong and rectangular bridge columns with conventional tie and multi-spiral transverse reinforcement under combined axial and flexural loads

This study investigates the behavior of transversely reinforced oblong and rectangular bridge columns under combined axial and flexural loading, including eccentric compression and lateral cyclic loading under constant axial load. The transverse reinforcement schemes include conventional tie and multi-spiral reinforcement. The multi-spiral reinforcement for the oblong column comprises two interlocking spirals and, for rectangular columns, comprises two interlocking large central spirals interlocked with four small spirals at the corners. The amount of transverse reinforcement for all of the columns conforms to the current seismic bridge design specifications. Test results indicate that all of the columns exhibit ductile behavior with ductility capacities significantly higher than the ductility capacity required by the design code. The oblong spiral column with an amount of transverse reinforcement 43% that of the corresponding tied column shows strength, ductility, energy dissipation, and over-strength similar to the tied column. Additionally, the rectangular spiral column with an amount of transverse reinforcement 59% that of the corresponding tied column exhibits strength, ductility, energy dissipation, and over-strength superior to the tied column. Moreover, the code P-M interaction analysis method can provide a conservative means of estimating nominal moment strength. The two code methods to determine the maximum probable moment strengths may not provide conservative estimates. Results of this study demonstrate that the maximum probable moment of the columns examined can be estimated conservatively as 1.4 times the nominal moment strength.