Dynamic response of RC structural elements under impact loading

Abstract

The effect of loading rate on the dynamic response of reinforced concrete (RC) beams under impact loading is investigated experimentally, via drop-weight testing, and numerically, through the use of three-dimensional (3D) dynamic nonlinear finite element analysis (NLFEA). During drop-weight testing, the behaviour of each specimen is established through the combined use of conventional instrumentations and a high-speed (HS) camera. The primary objective of the experimental work is to investigate the reasons that trigger the observed shift in specimen behaviour (compared to that established during static testing), once certain thresholds of applied loading rate and intensity are surpassed. The analysis of the test data suggests that the observed shift in specimen behaviour is largely attributed to the nature of the problem at hand (i.e. a wave propagation problem within a highly nonlinear medium) as well as the inertia forces developing along the element span (during the application of the impact load) and the ensuing localised response. The strain-rate sensitivity of the material properties of concrete does not appear to have a significant effect on the behaviour of the specimens tested as high values of strain-rate appear to be associated with the development of cracking along the element span. The data obtained from the drop-tests conducted on slender and short beams reveal that the response exhibited under impact loading differs significantly from that established during equivalent static testing. This shift in structural response predominantly takes the form of an increase in the maximum sustained load as well as a reduction in the portion (span) of the beam reacting to the imposed action which tends to concentrate around the area of impact. However, measurements obtained from the drop-weight tests, concerning certain important aspects of RC structural response (e.g. maximum sustained load or deflection) often correspond to a specimen physical-state characterised by high concrete disintegration in combination with low residual load-bearing capacity and stiffness. This stage of structural response has little practical significance as it depends heavily on post-failure mechanisms for transferring the applied load to the specimen supports. In view of the above, the available test data cannot provide insight into the mechanisms underlying RC structural response nor can it identify the true ultimate limit state of the specimens when subjected to impact loading. To achieve further insight into the mechanics underlying RC structural response under impact loading a well-established structural analysis packages (ADINA version 9.3.1) is employed which is capable of carrying out three-dimensional dynamic nonlinear finite element analysis while realistically accounting for the nonlinear behaviour of concrete and steel. The numerical predictions obtained are validated against available data obtained from the drop-weight tests. The validated models are then used to conduct a parametric investigation to study the dynamic response exhibited by RC beams when subjected to different rates and intensities of impact loading. The latter investigation reveals that ‘true’ load-carrying capacity is often significantly lower than the maximum sustained load recorded experimentally. In fact, the higher the loading rate and intensity characterising the impact load imposed the larger the latter difference becomes. Based on the available test data and the numerical predictions obtained, a simplified model is proposed aiming to describe the behaviour of the RC beams under impact loading. The model attempts to link the observed shift in structural response to the localised behaviour exhibited by the beams with increasing rates of applied loading. A comparison of the predictions obtained with the relevant test data reveals good agreement.