Experimental Study On Behavior Evolution Of Concrete Beams Under The Step-variable Amplitude Fatigue Load

In China,reinforced concrete(RC)girder bridges have been widely used,including a large number of passenger and freight collinear old railway bridges.In the early stage,the speed and capacity of the passenger freight railways was low,and the train acted on the bridge with low stress amplitude.With the completion of six times of speed increase and the stress release caused by rapid development of high-speed railway,the stress amplitude of the old railway bridges continued to increase in a step-by-step trend.Based on the above situation and investigation,it is shown that the passenger and freight collinear old railway bridges bears stepvariable amplitude fatigue load in practice,and the problem of fatigue damage accumulation of the bridge body cannot be ignored due to long-term service.In this paper,the static and step-variable amplitude fatigue loading tests are carried out on five RC test beams with the background of the passenger and freight collinear old railway bridges,which are loaded synchronously by MTS double actuators.The information of force,displacement,strain,crack,residual deformation and fatigue cycles are collected.Taking each loading stage(before and after cracking,after different fatigue cycles,after fatigue fracture)of the test beams as the time node,through modal test and analysis,the vibration mode,natural frequency,damping ratio and other natural vibration characteristics are collected.Through experimental research and analysis,the following results have been achieved:1.Explored the Evolution Characteristics of Static and Dynamic Behavior of RC Beams under the Step-Variable Amplitude Fatigue Load(1)During the whole process of fatigue loading,the static and fatigue behavior of the beams decreases with the decline of the natural vibration characteristics,and the stiffness and natural frequency show the three-stage development characteristics.(2)The initial stage of the increase of stress amplitude is the stage of rapid degradation of static and dynamic behavior of the beams,and there are abrupt changes in the behavior of the beams in the stage of "cracking" and "fatigue fracture".(3)There is a dynamic amplification effect between the tension steel bars strain and the concrete strain under the step-variable amplitude fatigue load,and the dynamic amplification coefficient corresponding to the tension steel bars is significantly greater than that of the concrete.(4)The frequency of natural vibration of the beams decreases greatly during fatigue fracture,which not only clearly reflects the deterioration of behavior,but also provides a simple method for judging whether the internal steel bars of the beams have fatigue fracture.(5)The damage accumulation of beams under fatigue loading has random difference.2.Reveals the Influence of Early Low Stress Amplitude Fatigue Loading on the Static and Dynamic Behavior of RC Beams(1)The early low stress amplitude fatigue loading makes the RC beam produce low stress interface damage,which has no effect on the ultimate bearing capacity of the beam(the absolute maximum error is only 1.8%),but it greatly reduces the ductility of the concrete in the top compression zone and makes its static failure mode develop in the direction of brittleness.(2)The early low stress amplitude fatigue loading makes the RC beam produce low stress fatigue strengthening(The fatigue life of BF1 and BF2 beams were 1 903 451 cycles and 4 268 798 cycles,respectively).This phenomenon is mainly caused by the strain strengthening of tension steel bars,which improves the fatigue behavior and fatigue life of beams to a certain extent.(3)The early low stress amplitude fatigue loading makes the natural frequency of RC beams unstable degenerate.The natural frequency of a certain stage is even slightly higher than that of the previous stage,and the overall situation is high-low alternation,up-down fluctuation.To a certain extent,the low stress fatigue strengthening was verified again.The above research results can provide a reference for the safe operation and scientific management and maintenance of the passenger and freight collinear old railway bridges.The details are as follows: in the initial stage of the increase of the stress amplitude caused by the acceleration and weight increase of the passenger and freight railway,the behavior degradation speed of the bridge is faster,which needs to be warned;the fatigue resistance of the passenger and freight collinear old railway bridges is safe and reliable,but it is need to be aware of the sudden damage caused by the reduction of ductility in the compression area of the upper part of the bridge,and it shall be one of the main contents of operation and maintenance inspection,and corresponding reinforcement measures should be taken if necessary.