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Failure mode identification and shear strength prediction are critical issues in designing reinforced concrete (RC) structures. Nevertheless, specific guidelines for identifying the failure modes and for accurate predictions of the shear strength of rectangular hollow RC columns are not provided in design codes. This study develops hybrid machine learning (ML) models to accurately identify the failure modes and precisely predict the shear strength of rectangular hollow RC columns. For this purpose, 121 experimental results of such columns are collected from the literature. Eight widely used ML models are employed to identify the failure modes and predict the shear strength of the column. The moth-flame optimization (MFO) algorithm and five-fold cross-validation are utilized to fine-tune the hyperparameters of the ML models. Additionally, seven empirical formulas are adopted to evaluate the performance of regression ML models in predicting the shear strength. The results reveal that the hybrid MFO-extreme gradient boosting (XGB) model outperforms others in both classifying the failure modes (accuracy of 93%) and predicting the shear strength (R2 = 0.996) of hollow RC columns. Additionally, the results indicate that the MFO-XGB model is more accurate than the empirical models for shear strength prediction. Moreover, the effect of input parameters on the failure modes and shear strength is investigated using the Shapley Additive exPlanations method. Finally, an efficient web application is developed for users who want to use the results of this study or update a new dataset.

Corrosion significantly affects the structural behavior of members in a connection (i.e., the thickness of steel plates, the preload force of bolts, and the friction factor of steel plates). Safety assessment of corroded steel frames (i.e., beam-to-column connection, beams, or columns) has been a major concern in engineering. In this work, an experiment of accelerated corrosion testing is carried out to obtain corroded specimens connected with high-strength bolts, and the preload force of high-strength bolts (PF-HSB) is monitored throughout the whole stage of the corrosion testing. Before the corrosion testing, the PF-HSB caused by the stress relaxation is also recorded. The PF-HSB decreases rapidly in the first five hours after the final screwing of bolts and it keeps stable after 100 h. The PF-HSB is seriously affected by corrosion, which decreases by 30.0% of the original preload force when the corrosion rate of steel plate reaches 3.5%. A finite element method for predicting the PF-HSB after corrosion is proposed. An estimation model for the PF-HSB considering the stress relaxation is established. A degradation model for predicting the PF-HSB after corrosion is also suggested, and is in good agreement with experimental data. The results of this research are of great significance for the safety assessment of in-service steel structures.