Seismic Performance of Concrete Columns Reinforced with Weakly Bonded Ultrahigh-Strength Longitudinal Bars

This paper presents an investigation of the seismic performance of concrete columns reinforced longitudinally with steel bars (referred to as UHS-WB bars) that have an ultrahigh strength and a low bond strength. Nine cantilever columns were tested with variables including the type of longitudinal steel reinforcement, the anchorage method for the UHS-WB bars, the axial load ratio, and the shear-span ratio of the columns. The columns with securely anchored UHS-WB bars and shear-span ratios of 2.5 and 3.0 exhibited very stable hysteretic behavior and a mild hardening postyield behavior up to drift ratio of 4%, whereas the lateral resistance of the column with a shear-span ratio of 4.0 had a slight decline beyond a drift ratio of 1.5% due to significant P-δ effect. However, all the columns with securely anchored UHS-WB bars had a residual drift less than 0.5% after unloading from the drift of 3%. The stable response and low residual drift can be attributed to the low bond strength, which delayed the yielding of the UHS-WB bars.