RC空心墩等效塑性铰长度理论模型的试验研究

既有等效塑性铰长度模型考虑了诸多因素的影响，但对塑性铰形成过程中纵筋拉伸漂移影响的关注不够，针对变截面空心墩塑性铰的研究也较少。为探讨变截面空心墩塑性变形特性，建立空心墩通用的塑性铰模型，在已有试验成果的基础上，开展了5个变截面圆端空心墩的拟静力试验。考虑变截面及墩底实心段的影响，假定了墩身曲率分布，给出了变截面圆端空心墩墩顶位移能力的计算方法；基于塑性位移等效原则，推导出考虑弯曲、纵筋拉伸漂移和黏结滑移影响的塑性铰长度待定参数模型，根据文献中7个等截面和本文变截面空心墩的试验结果，对待定参数进行标定，得到等截面和变截面空心墩等效塑性铰长度的统一模型，并采用另外6个试验结果验证了所给模型的适用性。研究结果表明：由于墩底实心段、倒角和墩身变截面的共同影响，变截面圆端空心墩塑性铰区整体上移，极限状态时钢筋拉断/屈曲及混凝土压溃主要集中在墩底空心倒角上缘附近；塑性铰长度随墩高L、截面宽度h、剪跨比λ和应变渗透参数(f_yd_b/√f'_c)近似呈线性变化；既有模型给出的等截面矩形空心墩塑性铰长度结果中，Priestley、李贵乾和韦旺的估值略高；所提模型的计算结果具有更高的精度和更小的离散性，适用于估算等截面和变截面空心墩的等效塑性铰长度。

Theoretical Model of Equivalent Plastic Hinge Length of RC Hollow Piers Based on Test Results

The existing formulas for equivalent plastic-hinge length consider several factors. However, the tension shift of the longitudinal rebar during the development of the plastic hinge has not been sufficiently considered. Relatively, few studies have been conducted on hollow piers with variable cross-sections. To investigate the plastic characteristics of a hollow pier with a variable section, a novel formula for the plastic-hinge length was established. Based on existing test results, a quasi-static test of five thin-walled round-ended hollow piers was conducted. To determine the displacement capacity of the pier tip, the curvature distribution assumption was proposed, including the effects of the variable cross-section and solid segment at the pier bottom. Based on the equivalent principle of plastic displacement, the general plastic-hinge length model was established, considering flexure, tension shift, and bond-slip. The indeterminate factors of the model were evaluated based on the test results of seven members with uniform sections in the literature and round-ended with variable cross-section. The general theoretical formula of the plastic-hinge length was obtained for hollow columns with a constant/variable cross section. The applicability of the proposed formula was verified by six additional tests in the literature. The analysis indicated that the plastic-hinge region of the round-ended hollow specimens extended and ascended entirely owing to the bottom solid segment, solid-to-hollow part, and variable section. The rebar rupture and concrete crushing were mainly concentrated near the upper edge of the hollow chamfer at the pier bottom. The equivalent plastic-hinge length increased with the pier height L, shear span ratio λ, section width h, and parameters of bond-slip (f_yd_b/√f'_c). The results of the Priestley, Li, and Wei were larger than the measured values of the hollow rectangular members compared with the uniform section. Compared with the existing models, the author's formula is more applicable in assessing the plastic hinge length of hollow columns with higher precision and less discreteness.