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.

Finite‐element models of reinforced‐concrete beam‐columns often fail to behave in a stable manner near the point of maximum resistance. Simple numerical examples presented indicate that the conventional displacement formulation when used at the section or member level is unable to establish solutions associated with softening behavior. Thus, such analysis models are often limited in their applicability or necessitate the use of unrealistic material laws. A mixed finite‐element scheme is proposed herein that exhibits stable numerical behavior even when critical regions become ill‐conditioned. It is demonstrated that enforcing equilibrium within the member during state determination provides the necessary constraint to obtain a stable solution. Examples are presented to demonstrate the reliability and realism of this approach for members subjected to generalized excitations even where severe deformation softening occurs.

The nonlinear behavior of reinforced concrete columns under biaxial excitation is complicated by a variety of factors. A finite‐element model is presented for the analysis of such members under generalized biaxial loading. The formulation accounts for most aspects of axial‐flexural behavior and is capable of handling numerically ill‐conditioned problems associated with softening behavior. The element formulation is initially outlined and the ability of the element to predict experimental data is demonstrated. Numerical simulations are presented of cantilever columns under static load and of an entire frame in bidirectional seismic excitation. The results demonstrate the complexity of the biaxial resistance mechanism and its influence on structural response, particularly related to the concentration of local damage in columns.

The results of a parametric study are presented, concerned with the evaluation of the structural overstrength, the global ductility and the available behaviour factor of existing reinforced concrete (RC) buildings designed and constructed according to past generations of earthquake resistant design codes in Greece. For the estimation of these parameters, various failure criteria are incorporated in a methodology established to predict the failure mode of such buildings under planar response, as described in detail in a companion publication. A collection of 85 typical building forms is considered. The influence of various parameters is examined, such as the geometry of the structure (number of storeys, bay width etc.), the vertical irregularity, the contribution of the perimeter frame masonry infill walls, the period of construction, the design code and the seismic zone coefficient. The results from inelastic pushover analyses indicate that existing RC buildings exhibit higher overstrength than their contemporary counterparts, but with much reduced ductility capacity. The presence of perimeter infill walls increases considerably their stiffness and lateral resistance, while further reducing their ductility. Fully infilled frames exhibit generally good behaviour, while structures with an open floor exhibit the worst performance by creating a soft storey. Shear failure becomes critical in the buildings with partial height infills. It is also critical for buildings with isolated shear wall cores at the elevator shaft. Out of five different forms of irregularity considered in this study, buildings with column discontinuities in the ground storey exhibit the worst performance. Furthermore, buildings located in the higher seismicity zone are more vulnerable, since the increase of their lateral resistance and ductility capacity is disproportional to the increase in seismic demand.