Adequate initial bolt preload is necessary to ensure the strength and stiffness of bolted connections. In this study, an experimental torque control method was used to determine the relationship between tightening torque and preload of nine Extended Hollo-Bolt (EHB) blind bolted connections to Concrete-Filled Steel Hollow Sections (CFSHS). In order to obtain the EHB nut factors, which allow to calculate the level of preload for any value of applied torque, torque versus preload curves were drawn based on the experimental results and curve fitting method was carried out. Bolt preload relaxation was also recorded for a period of 7 days while concrete hardening occurred. Additionally, a detailed 3D Finite Element (FE) model of the tightening stage of the EHB was established. The experimental and numerical results show that the nut factor for the EHB is higher than that of standard bolts and bolt relaxation is not affected by the concrete presence during the hardening stage. Adequate friction coefficients were proposed as well as an equation for calculating the residual preload of the EHB.

足够的初始螺栓预紧力对于确保螺栓连接的强度和刚度是必要的。在这项研究中，使用实验扭矩控制方法来确定九个加长空心螺栓 (EHB) 盲螺栓连接到混凝土填充钢空心截面 (CFSHS) 的拧紧扭矩和预载荷之间的关系。为了获得 EHB 螺母系数，允许计算任何施加扭矩值的预紧水平，根据实验结果绘制扭矩与预紧曲线，并进行曲线拟合方法。当混凝土硬化发生时，还记录了 7 天的螺栓预紧力松弛。此外，还建立了 EHB 拧紧阶段的详细 3D 有限元 (FE) 模型。实验和数值结果表明，EHB 的螺母系数高于标准螺栓的螺母系数，并且螺栓松弛不受硬化阶段混凝土存在的影响。提出了足够的摩擦系数以及计算 EHB 残余预紧力的公式。