Impact of D/t on seismic behavior of reinforced concrete filled steel tubes

• D/t ratio was shown to impact the onset of local buckling for RCFSTs.
• Equivalent damping expression for DDBD was developed for RCFSTs as a function of D/t.
• Equilibrium and strain compatibility apply for obtaining flexural strength.
• Local buckling governs over the theoretical confined concrete compression strain.

Reinforced concrete filled steel tubes (RCFSTs) are commonly used for bridge substructures in high seismic regions where the steel tube is used as a permanent casing which eases construction. Concrete confinement is provided by the steel tube, increasing the compressive strength and strain capacity. Tests were performed on twelve large scale RCFSTs, seven of the tests focused on varying D/t ratio and the remaining five focused on varying internal reinforcement. The tubes were subjected to reversed cyclic four-point bending with a constant moment region centered in the pile. The large scale specimens consisted of outer diameters of 20 to 24 in. (508 to 610 mm) and diameter-to-thickness ratios between 33 and 192. Strain limit states for the onset of tube wall local buckling and fracture are developed, as is an expression for equivalent viscous damping for direct displacement-based design. The impact of the tubes on confinement and analysis methods is also discussed.