Seismic behavior of FRP-high-strength concrete–steel double-skin tubular columns

This paper reports on an experimental study on the seismic behavior of fiber-reinforced polymer (FRP) – concrete – steel double- skin tubular (DST) columns. Nine DST and one concrete-filled FRP-tube (CFFT) columns that were made of high-strength concrete were tested under constant axial compression and reversed-cyclic lateral loading. The main parameters of the experimental study were axial load level, amount and type of FRP confinement, concrete strength, sectional shape and thickness of the inner steel tube, and provision (or absence) of a concrete filling inside the steel tube. Of primary importance, the results indicate that DST columns are capable of developing very high inelastic deformation capacities under simulated seismic loading. The results also indicate that the presence of a concrete filling inside the inner steel tube significantly and positively influences the seismic behavior of DST columns. It is found that the performance of the void-filled DST column is superior to that of a companion CFFT column having identical geometric and material properties apart from the main internal steel reinforcing bars. These early observations suggest that DST columns may provide an attractive alternative to CFFT columns in the construction of new earthquake-resistant columns. No detrimental effect of the increased concrete strength on the displacement capacity of the DST columns is observed, provided that the amount of confinement is increased proportionally with the concrete strength. The influence of the cross-sectional geometry of the inner steel tube is found to be significant, whereas the thickness of the steel tube is found to have only a minor influence on the behavior of DST columns. It is observed that the FRP-tube material has some influence on the lateral displacement capacity of DST columns, with the specimens confined by FRP tubes manufactured using fibers with higher ultimate tensile strains developing slightly higher displacement capacities. Examination of the test results has led to a number of significant conclusions with regard to the influence of the important column parameters on the performance of DST columns. These results are presented together with a discussion of the influence of the main parameters on the seismic behavior of DST columns.Togay Ozbakkaloglu, and Yunita Idri