Effect of sustained load on ultimate strength of high-strength composite columns using 800-MPa steel and 100-MPa concrete

To investigate the effect of sustained loads on the ultimate strength of high-strength composite columns using grade 800-MPa steel and 100-MPa concrete, ultimate load tests were performed after long-term loading. Two concrete-encased steel (CES) columns were tested under concentric loading, and a CES column and a concrete-filled steel tube (CFT) column were tested under eccentric loading. The test results showed that stress-relaxation in the concrete and gradual stress-transfer to the steel and longitudinal bars occurred, and the ultimate shrinkage strain and ultimate creep coefficient of the high-strength concrete were estimated as 553 με and 1.44, which are lower than those of normal-strength concrete but belong to the ranges specified in concrete standards. To confirm the tests, a numerical parametric study was performed, considering the age-adjusted effective modulus and relaxation. The results of the experimental and numerical studies showed that, in the case of the high-strength composite columns, the sustained load increased deformation (axial strain and curvature) at service load conditions, but the detrimental effect on the ultimate strength was not significant. The factors for the long-term effect specified in current design codes gave good predictions for the long-term effective flexural stiffness.