Experimental investigation on the initial stiffness of bolted connections in precast RC members

This study investigates the initial stiffness of bolted connections used in assembling precast concrete members under varying preload levels and alignment imperfections. Sixty-three quasi-static tensile tests were performed on three beam assemblies (CBA-16, CBA-20, and CBA-24) across three alignment configurations (perfect, first imperfection, and second imperfection) and seven torque levels (0%, 15%, 20%, 25%, 30%, 35%, and 50% of the bolt bearing capacity). The initial tensile stiffness was determined using the secant modulus method applied to the experimental load–separation responses. Results show that stiffness increases with bolt diameter and tightening torque but decreases with assembly imperfections. The adverse effect of misalignment was most pronounced at low torques and diminished at higher levels. A three-way analysis of variance confirmed the statistical significance of all main effects and two-way interactions, with the bolt–torque interaction showing the largest effect size. The load–separation data were further converted into moment–rotation relationships to determine the initial rotational stiffness, which was compared with values calculated using a proposed empirical formula based on the Eurocode 3 component method. The comparison revealed that increasing preload effectively mitigates the negative influence of geometrical imperfections. For the perfect configuration, 25% tightening torque achieved the empirical stiffness; the first imperfection required 30–40%, and the second at least 50%. Overall, the findings provide essential structural properties and behavioral insights for such bolted connections, offering valuable guidance for accurate design and optimization of precast concrete structures.