Response Characteristics And Design Method Of Reinforced Concrete Beams Under Impacting Load

Reinforced concrete(RC)structures may be subjected to various kinds of impact loads during the service period.As the most fundamental component in RC structures,the anti-impact performance of RC beams is an important subject in the field of disaster prevention and mitigation.In this study,dynamic behaviors and design methods of RC beams under impact loads are investigated by experiments,Finite Element(FE)simulation and simplified model calculation.The main contents of this study are summarized as follows:(1)Drop hammer tests are performed for small-scale RC beams(Sectional dimension 100mm x 250mm)and the influence of impact mass and velocity on dynamic behaviors of flexural-failure-type RC beams are investigated.Impact force,reaction force,mid-span deflection,strain of longitudinal bar and concrete are measured.The crack propagation is recorded using a high-speed video camera.The specimens reveal two different failure patterns of the beam-hammer contact zone.With increase in impact velocity and decrease in impact mass,the proportion of the hammer kinetic energy to be converted into strain energy of the beam reduces,and more energy is consumed for the local damage of the beam-hammer contact zone.Based on the statistical analysis of test results and those of the tests conducted by previous researchers,an empirical formula to estimate the maximum deflection of RC beams under impact is derived.(2)Numerical simulation to impact tests of small-scale RC beams is conducted using non?linear FE dynamic analysis software LS-DYNA and the energy conversion during impact process is investigated based on simulation results.The impact process is simplified as a two mass model and numerous cases are calculated based on the model considering extensive impact conditions.An iso-damage curve is derived to evaluate the damage of flexural-failure-type RC beams under impact.Through nondimensionalization of the iso-damage curve,it is generalized to the damage evaluation of RC beams with different geometric dimensions and material properties.(3)Drop hammer tests are performed for large-scale RC beams(Sectional dimension 200mm x 500mm)and test variables include beam span,transverse reinforcement ratio,impact mass,and impact velocity.The shear failure patterns and crack propagations are well described based on test results.And impact response characteristics of shear-failure-type and flexure-failure-type RC beams are analyzed and compared based on experiment data such as impact force,reaction force,mid-span displacement and strain of steel bar.The dynamic shear force distribution is derived through measured acceleration data and the influence of stress wave propagation on impact responses of RC beams is investigated.(4)FE models of drop weight tests of large-scale RC beams are established with LS-DYNA.The distribution of shear force and moment,and characteristics of loading on the shear critical section of RC beams under impact are investigated based on simulation results.The influence of beam span on the impact responses is also investigated based on verified FEM models,and the influencing mechanism is discussed using a Timoshenko beam model which considers the contribution of high-order vibration modes.An improved two mass model,which take the effects of stress wave propagation into consideration,is proposed and by comparing its results with those calculated by the traditional two mass model which ignores the effects of stress wave propagation,the influence of stress wave propagation to the impact response is evaluated quantitatively.(5)Based on characteristics of loading on the shear critical section derived from FEM models,a conception of "effective span" is adopted to establish a static model to calculate the shear resistance of RC beams in the early stage of the impact,and the shear mechanisms of RC beams under impact are further discussed.An simplified three degrees of freedom(TDOF)model is proposed to assess type I shear failure of RC beams.In the model,shear and flexural deformation modes are equivalent to different single-degree-of-freedom mass points respectively,and the local and overall response characteristics of RC beams under impact can be represented reasonably.A m-v diagram is proposed to conduct shear damage evaluation of RC beams under impact.