An efficient framework of nonlinear hysteresis analysis for multi-bolt connections via improved Iwan model and CG-FFT

Multi-bolt connections are widely employed in structural systems. Under alternating loads, the nonlinear stick, micro-slip, and macro-slip behaviors at the bolted connection significantly affect the system's mechanical response, leading to hysteresis in the structure. However, achieving efficient and accurate nonlinear analysis of hysteretic behavior in multi-bolt structures remains challenging. Therefore, this paper proposes an efficient framework integrating both the nonlinear interface mechanics of multi-bolt connections and the global hysteresis characteristics of complex assembled structures. First, the bolt connection interface is characterized as a rough surface with microscale asperities, wherein individual micro-protrusions exhibit friction-governed stick-slip transitions. Subsequently, a computationally efficient contact pressure solver is developed through the Conjugate Gradient-Fast Fourier Transform algorithm, followed by the formulation of a tangential nonlinear mechanical model for rough interfaces based on Mindlin's contact theory. Then, an improved Iwan equivalent model is used to establish a constitutive relationship describing the progressive transition from localized stick to macro-slip behavior in bolted connections. The finite element iterative framework incorporates the nonlinear hysteresis response of local bolt connections to resolve the global structural hysteresis deformation. Finally, the proposed method is validated by both experimental tests and finite element simulations.