Behavior of RC Beams Subjected to High Rates of Concentrated Loading

The work described in the present article is concerned with the numerical investigation of the dynamic response of reinforced concrete (RC) beams subjected to high rates of transverse loading. Of interest herein is the case of localized impact loading, such as that encountered in the case of contact, impact, and ballistic problems, rather than the case of loads distributed over the entire span of RC structural elements arising particularly from far field explosions. The investigation is based on the use of a commercially available general purpose finite-element (FE) package for nonlinear static and dynamic analysis of three-dimensional FE models. A key feature of our study is the hypothesis that the material properties of concrete and steel reinforcement are independent of the loading rate. Based on this assumption, the effects of the applied loading rate on the exhibited structural response are primarily attributed to the inertia forces that develop within the beam and not to the loading rate sensitivity of the mechanical characteristics of the materials involved. This hypothesis constitutes a major departure from currently accepted design and numerical modeling practices, which adopt exactly the opposite view, thus, providing an alternative explanation as to the causes that affect the complex inelastic response of RC structural elements under high loading rates, as well as the cracking patterns, observed during testing. The results obtained correlate closely with the experimental observation that structural performance, in the form of stiffness, load carrying capacity, and deformability, depart significantly from those recorded under quasi-static loading as certain thresholds of applied loading rates are surpassed, with these changes becoming more pronounced as the rate of loading increases. From the analysis of numerical and experimental predictions, the causes that lead to the above change in behavior are established and a relatively simple design model is proposed, which is able to quantify the observed increase in load-carrying capacity exhibited by RC beams with increasing rates of applied loading as well as the concentration of damage.