Dynamic characteristics identification of RC beams subjected to impact damage

Structural dynamic characteristics are essential parameters for detecting structural damage. This study conducted impact and static loading tests on six reinforced concrete (RC) beams, and the plastic energy dissipation of the reinforced concrete beams under both loading conditions is be essentially consistent. The random decrement technique and Hilbert-Huang transform (RDTHHT) combined approach is used to identify the first three mode dynamic characteristics (frequency and damping ratio) of impact-damaged reinforced concrete beams. An approach based on finite element (FE) plastic damage distribution is proposed to obtain the continuous flexural stiffness of damaged reinforced concrete beams. Both the global stiffness and Rayleigh method are used to predict the natural frequency. The results show that the first three mode frequencies of the damaged reinforced concrete beams decrease, whereas their damping ratios increase. Specifically, the first mode frequency reduction of reinforced concrete beams under impact damage is slightly lower than that under static damage, while the second and third mode frequencies exhibited a greater decrease. The increase in damping of the damaged reinforced concrete beams is mainly related to the damage extent, damage distribution, and damage location. Furthermore, the first and third mode damping ratios of reinforced concrete beams increase more significantly than the second mode damping ratio after damage, primarily because the most severe damage occurred at the location (mid-span) with the maximum curvature of the first and third mode modal shapes. Due to the more concentrated distribution and more severe local damage of impact damaged reinforced concrete beam, the increase in the first and third mode damping ratios after impact damage is more pronounced compared to static damage. The global stiffness method overestimates the frequency of damaged reinforced concrete beams. Whereas the Rayleigh method utilizes a reasonable first mode modal shape function and the continuous flexural stiffness to predict the structural frequency after damage, the first mode frequency predicts after damage with an error of less than 3%. It is recommended to consider changes in both frequency and damping to assess the extent of impact damage in structural components.