Study On Experiments And Analytical Methods Of Axially-loaded Circular RC Column Under Lateral Impact Loading

As one of the typical forms of bridge columns,circular reinforced concrete(RC)column structures have been extensively used as bridge piers in engineering practices.During the service life of the bridge structure,besides these static and dynamic loads,the RC column may also suffer from various lateral impact loads.For example,bridge piers located close to a navigable waterway may be vulnerable to vessel collisions,bridge piers in mountain areas may be hit by falling rocks and the viaduct piers may be impacted by a vehicular.In the past few decades,the static compression strengths and seismic performances of circular RC columns have been widely investigated.However,few reseaches have been performed with an emphasis on clarifying the dynamic behavior of the axially-loaded RC column subjected to lateral impact loading.On the other hand,because that these columns are all mainly designed to support the axial loads,it is of great importance to quantify the residual axial capacity for predicting the damage extent and collapse risk of the impact-damaged RC column.To this end,low-velocity impact tests,compression after impact tests,develop the detailed FE modeling method and simplified methods on axially-loaded circular RC columns are designed and conducted to evaluate the impact responses of axially-loaded circular RC columns and the residual strengths of impact-damaged RC columns in this paper.The main contents of this study are summarized as follows:(1)Twelve circular RC column specimens are tested under impact loading through the drop hammer test system.Through the test,the impact force,the displacement of the RC column and the high-speed image data of the whole impact process were obtained.The effects of axial compression ratio,reinforcement ratio and height-diameter ratio on the impact reponse of the RC column were discussed.It is found that the applied axial load have a great effect on the impact responses of the circular RC columns.Generally,the axial load plays a positive effect when the deformations of the column specimen are relatively small.On the contrary,the presence of the axial load could have catastrophic effects,as the ductility of the RC column decreases as the axial force increases.In addition,the axial force and the span-depth ratio are the key factors cause the transformation of the RC column failure mode(such as from the flexural failures to the shear failures).(2)The corresponding finite element(FE)model is established for the impact simulation of axially-loaded RC columns.Comparison with the test date shows that the conventional modeling used in RC beams is shown to have some disadvantages in the predictions of the impact-induced responses of the axially-loaded columns.On this basis,an improved FE modeling are proposed for the simulation of the axially-loaded RC columns subjected to impact loading.In the improved FE model,the concrete material model is modified to accurately consider the confinement effect of the spiral stirrup.In addition,the influences of the bond-slip behavior and the behavior of crack opening and closing on the residual deformation after impact are considered in the improved FE models.According to the research,it can be concluded that the improved FE model are far superior to the conventional model in simulation accuracy,indicating the necessity and effectiveness of the above improvement.Considering calculation accuracy cannot be guaranteed in shear analysis because there are still some limitations for concrete material models.The two-dimensional reinforced concrete membrane code that based on the Modified Compression Field Theory(MCFT)was employed to estimate impact analysis model.The research shows that the established two-dimensional impact analysis model not only can better predict the impact response of axially-loaded RC column,but also can better identify different types of impact failure modes.(3)A total of 9 impact-damaged circular RC column specimens with different axial compression ratio and reinforcement ratio as well as height-diameter ratio are tested under quasi-static axial loading to evaluate their residual compressive strengths after they suffered from different drop-hammer impact tests(or events).The test results shows that the residual axial strengths of these RC columns usually decreased with increasing the residual deformations;Damage modes caused by impact loadings have an significant impact on the residual axial strength.The specimen with shear-dominated failure mode exhibit lower residual capacity than that with flexural-dominated failure mode.(4)Since the maximum load imposed during a collision is usually unknown,an assessment method based on post-impact state(deformation and damage mode)is proposed and demonstrated to be capable of predicting post-impact residual axial capacity.The results obtained from the developed high-resolution FE models are in good agreement with the CAI test data.On this base,an extensive parametric study was conducted by using the validated FE models to investigate the influences of column aspect ratio,reinforcement ratio and axial load ratio on the post-impact strengths of circular RC column.An empirical formula was derived by a multiple regression fit to the FE results for providing an easy estimation of residual strength of circular RC columns after lateral impact.(5)For circular RC columns with flexural-dominated failure modes,considering the characteristics of circular RC columns(such as the initial axial load and annular reinforcement),a two-degree-of-freedom(TDOF)simplified analysis model suitable for axially-loaded circular RC columns is established.The methods based on section analysis and fiber beam element model to calculate the resistance curve of circular RC column were proposed.The method for reasonably determining the equivalent unloading stiffness of the resistance curve is discussed.The validity of the established TDOF analysis model was verified by experiments.Based on this,the effects of impact velocity and mass on the impact damage of RC columns were extensively studied.(6)An efficient modeling method based on fiber beam element is proposed to capture both flexural and shear behaviors of RC beams and columns under impact loading.The method includes a macroelement-based contact model,fiber beam element for simulating nonlinear behavior and two types of shear springs for capturing shear behavior.Nearly fifty impact tests on RC beams and columns reported in the literature were employed to validate the proposed modeling method.Compared with the existing models or methods,the proposed efficient modeling method has the advantages of high computational efficiency,easy application in practical problem analysis,and can be readily implemented without coding in any FE software as long as traditional fiber-section elements,and discrete macroelements are available.