Cold-formed steel (CFS) is a typical green building material with the merits of being low-cost, lightweight, high-strength, and recyclable. CFS built-up section beams are widely used in CFS frames owing to their outstanding mechanical properties. However, a simplified and accurate method for calculating the flexural moment capacity of multi-limb built-up beams is missing in specifications. In this study, the flexural behaviors of CFS four-limb built-up beams with closed and open sections are investigated via experiments and finite element (FE) modeling. Firstly, the flexural moment capacities and failure modes of the beams are obtained by four-point bending experiments. The ultimate load capacity of the new open section beam is found to be higher than that of the closed section beam, and the failure mode is local buckling of the web and upper flange. Then, the FE models validated by the tests are developed to conduct an extensive parametric study. Numerical results show that the flexural moment capacity increases with the thickness and web depth. Finally, a simplified calculation method for the flexural moment capacities of the closed and open section beams is proposed by considering the reduction factor of gross section modulus of the built-up section.

Offshore wind turbines (OWTs) operate under harsh marine conditions involving strong winds, waves, and salt-laden air, which increase the risk of excessive vibrations and structural failures such as tower collapse. To ensure structural safety and achieve effective vibration control, accurate modal parameter identification is essential. In this study, a vibration monitoring system was developed, and the Bayesian Spectral Decomposition (BSD) method was applied for the operational modal analysis of a 5.5 MW monopile OWT. The monitoring system consisted of ten uniaxial accelerometers mounted at five elevations along the tower, with two orthogonally oriented sensors at each level to capture horizontal vibrations. Due to continuous nacelle yawing, the measured accelerations were projected onto the structural fore–aft (FA) and side–side (SS) directions prior to modal analysis. Two days of vibration and SCADA data were collected: one under rated rotor speed and another including one hour of idle state. Data preprocessing involved outlier removal, low-pass filtering, and directional projection. The obtained data were divided into 20-min segments, and the BSD approach was applied to extract the primary modal parameters in both FA and SS directions. Comparison with results from the Stochastic Subspace Identification (SSI) technique showed strong consistency, verifying the reliability of the BSD method and its advantage in uncertainty quantification. The results indicate that the identified modal frequencies remain relatively stable under both rated and idle conditions, whereas the damping ratios increase with wind speed, with a more significant growth observed in the FA direction.

The eddy current damper (ECD), as a high-performance seismic protection device, has been increasingly applied in the field of civil engineering. However, most existing finite element software lacks a nonlinear constitutive model for this type of damper, and an equivalent simplified model capable of accurately capturing its real damping characteristics under complex seismic excitations has not yet been established. To address this gap, this paper proposes a stochastic optimization framework for calibrating a bilinear ECD model, using the Wouterse model as a reference. Specifically, the core of this framework employs a Monte Carlo Simulation (MCS) approach, in which a suite of spectrum-compatible seismic excitations is generated and analyzed to evaluate the corresponding stochastic structural responses. The Grey Wolf Optimizer (GWO) is then utilized to minimize the discrepancy between the dynamic responses predicted by the proposed bilinear model and those of the reference model. A key outcome of this study is a practical bilinear model in which these parameters can be treated as constants, enabling designers to focus exclusively on the physical parameters $f_{\mathrm{m}\mathrm{a}\mathrm{x}}$ and $v_{\mathrm{c}\mathrm{r}}$. This study bridges the gap between high-fidelity simulations and practical engineering design, providing a feasible pathway for integrating ECDs into existing finite element software for dynamic analysis.