Research On Mechanical Behavior And Dynamic Response Of Reinforced Concrete Piles Subjected To Horizontal Impact Loading

When a large-volume vessel docks,due to the large inertia,the improper speed control and the wrong human operation often lead to excessive impact force.As one of the main structural types of wharfs,reinforced concrete high pile foundation has smaller horizontal bearing capacity and horizontal stiffness than gravity wharfs and sheet pile wharfs,which may produce large horizontal deformation under impact loading.Therefore,it will cause the collapse of whole structures.At present,there is little research on the failure mechanism of reinforced concrete pile structures under horizontal low-velocity impact,and the calculation method of equivalent impact force given in the current code is mainly based on the results of ship collision tests,of which the rationality needs to be further discussed for the impact-resistance design of engineering structures.In view of the above background,this study has mainly undertaken on the following research contents:(1)The low-speed horizontal impact test(v=0.4～1.2m/s)of 19 reinforced concrete straight piles was carried out,in which the impact mass,impact velocity,slenderness ratio of specimen,reinforcement ratio and axial compression ratio were designed as the main parameters.The results show that the impact response of reinforced concrete straight piles under horizontal impact can be divided into four stages:oscillation stage,separation stage,stabilization stage and unloading stage.Increasing the impact mass makes the impact force-time curve plumper and improves the impact force of the stabilization stage,while the increase of impact velocity is significant for increasing the duration time.During the impact process,the uniformly retarded motion of straight piles is recognized,and the displacement/strain time-history curves appear to be half sine waveform,while the strain rate-time curve is sine waveform.With the variation of parameters,the maximum displacement shows a good linear relationship.After horizontal impact tests,six specimens with slight damage under different impact energy(two specimens with different slenderness ratio in three groups)were selected for residual capacity static test.The reduction coefficient of ultimate capacity after impact was obtained,which is 7.38%to 14.01%.(2)The low-speed horizontal impact test of 11 reinforced concrete inclined piles was carried out,in which the impact velocity,inclination angle and axial compression ratio were designed as the main parameters.The monotonic horizontal static tests of two reinforced concrete inclined piles and one double straight pile were designed as the control group.The results show that after the initial impact,the separation valley in impact force time histories caused by the separation between the test truck and specimen was obvious for the double straight pile with relatively small lateral stiffness.Compared with the other two inclined piles,the whole impact force-time curve is stable at a lower level,except for the peak value.With the increase of impact velocity,the maximum displacement of inclined piles with different inclination angles increases linearly,of which the slope enhances as inclination angle increases.Three groups of continuous impact tests were carried out(the impact times of each specimen were 3 times).It was found that with the increase of the impact times,the damage state of the specimen increased gradually,but the variation of peak impact force remained similar to that of the intact specimen.From the view of the displacement response,when the specimen has not been destroyed,there is a linear relationship between maximum displacement and impact velocity.But after the specimen fails,the maximum displacement promotes obviously as impact velocity increases,and the slope of the curve is significantly higher than that of the previous one.(3)With the consideration of strain rate effect of materials,the structural behavior of reinforced concrete piles under impact and static loads is numerically simulated by nonlinear finite element software ABAQUS.The numerical results are validated to be in good agreement with the experimental results.(4)The parametric analysis under different impact kinetic energy Ek and momentum P is carried out,which includes several groups of different m and v0.Through the dynamic response of straight pile and inclined pile,the mechanism of reinforced concrete pile members under impact loads is revealed.The results show that under the same impact kinetic energy,when m is small and v0 is large,the local damage of concrete will appear at the impact position,resulting in smaller overall displacement.When m increases to It,the overall deformation of straight pile is no longer affected by the various combinations of m and v0,and the displacement increases almost linearly with the increase of Ek.For the same impact momentum P,the maximum displacement change nonlinearly as the combination of m and v0 changes.Therefore,using impact kinetic energy to describe the impact effect is more benificial to evaluate the deformation of the specimens subjected to impact loads.(5)The equivalent static load formula in the AASHTO-LRFD code was used to calculate the reinforced concrete straight pile and inclined pile test in this paper,and the results were compared with the measured peak value of impact force and static load bearing capacity.It is found that the standard formula is suitable for the RC members with relatively large stiffness.However,for those with relatively small stiffness,the calculation method might lead to over-conservative results.The displacement-based impact-resistance design method of reinforced concrete pile under horizontal impact is put forward,which is concise and can be safely used to calculate the impact bearing capacity of reinforced concrete pile.