Experimental investigation on plastic hinge length of RC round-ended hollow columns with a variable section

In the ductile design, it is an essential task to accurately estimate the equivalent plastic hinge length of a reinforced concrete (RC) column. The plastic behavior of a RC hollow column is influenced significantly by tension shift and bond-slip according to the test results, which hasn't yet received adequate attention in the existing models of equivalent plastic hinge in the literatures. Few publications and even no explicit provision in the current codes provide the method to compute the plastic hinge of hollow RC columns, especially those with a variable section. To bridge this knowledge gap, in this study, based on the equivalent principle of plastic displacement, an innovative formula of equivalent plastic hinge length is derived for the hollow columns, which considers both the effects of plastic tension shift and bond-slip based upon the authors' previous research on solid columns. The quasi-static tests on two rectangular and two round-ended hollow columns were conducted to calibrate the undetermined parameters of the proposed formula. The accuracy and applicability of the proposed formula were then investigated based on the available test results of 30 rectangular and 3 round-ended hollow columns in literature. The results show that different slip behavior leads to distinct plastic deformation between these two types of hollow columns. Compared to the rectangular columns with a uniform section, the plastic hinge length of round-ended columns is longer due to the presence of the bottom solid segment, chamfer, and variable section. For both types of hollow columns, the equivalent plastic hinge region increases with the specimen length, section height, and strain penetration. The comparison suggests that the proposed formula is more appropriate to calculate the plastic hinge length of hollow columns with a uniform or variable section than the existing models.