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.