Study On Damping Evolution Law Of Reinforced Concrete Beams After Cracking

Damping is the most complex dynamic characteristic of structure.It has the characteristics of low test accuracy,many influencing factors,large discreteness and difficult theoretical description.Due to the discrete and nonlinear characteristics of concrete materials,its damping characteristics are more complex.Previous studies have shown that cracking damage will have a significant impact on the damping characteristics of reinforced concrete(RC)structures.At present,there is no widely recognized conclusion on the relationship between cracking and damping characteristics of reinforced concrete structures.The fact that the damping ratio of structure is simply taken as a constant according to its material in the current code is inconsistent with the reality.Therefore,it is urgent to study the relationship between cracking and damping characteristics of reinforced concrete structures.With the implementation of the strategy of China’s strength in transportation,the formation of spacing transportation network has accelerated.Reinforced concrete simply supported beam is widely used in bridge structure because of its unique advantages.Due to the characteristics of concrete materials,cracking exists in reinforced concrete simply supported beams in normal use to varying degrees.In this thesis,taking the reinforced concrete simply supported beam as the research object,the theoretical calculation formula of concrete material damping ratio before and after cracking is deduced,and the dynamic tests of 9different simply supported beam specimens under different cracking damage states are carried out.Based on the comprehensive analysis of the test results and theoretical calculation results,the following conclusions are drawn:(1)The resonance frequency of RC simply supported beam decreases significantly at the initial stage of cracking and after the yield of longitudinal reinforcement,and is basically stable in the middle and late stage of cracking until the yield of longitudinal reinforcement.In the whole process of crack damage of reinforced concrete simply supported beam,there is an obvious correlation between its resonance frequency and longitudinal reinforcement ratio.The higher the reinforcement ratio is,the higher the resonance frequency is,and vice versa.(2)The damping ratio of reinforced concrete simply supported beams under different cracking damage is between 1%-3%.(3)The variation law of relative damping ratio in the initial cracking stage of reinforced concrete simply supported beam is related to the reinforcement ratio of longitudinal reinforcement,the thickness of cover and the diameter of longitudinal reinforcement.The smaller the ratio of longitudinal reinforcement,the greater the thickness of cover and the larger the diameter of longitudinal reinforcement,the smaller the rise of damping ratio of simply supported reinforced concrete beams at the initial stage of cracking.When the reinforcement ratio is less than a certain value,the damping ratio decreases at the initial stage of cracking;When the thickness of cover and the diameter of longitudinal reinforcement increase to a certain extent,the upward trend of damping ratio in the initial stage of cracking is not obvious.Different reinforcement ratio in compression zone has different influence on the variation trend of damping ratio after beam cracking.In the middle and late stage of cracking of reinforced concrete simply supported beams,the damping ratio shows a downward trend.(4)In the whole cracking process of reinforced concrete simply supported beams,there is an obvious correlation between the relative damping ratio and the ratio of longitudinal reinforcement.When the relative load is the same,the higher the longitudinal reinforcement ratio of the beam,the greater the relative damping ratio,and vice versa.(5)After the crack of reinforced concrete simply supported beam,its comprehensive damping comes from two parts: concrete material damping and Coulomb friction damping.Coulomb friction damping mainly comes from the friction energy consumption between closed cracks in the tensile zone.When the relative load is 0.3,the Coulomb damping ratio reaches its peak.(6)The ratio of longitudinal reinforcement,the thickness of cover and the diameter of tensile reinforcement affect the Coulomb damping of reinforced concrete simply supported beams by affecting the parameter of closed cracks in the tensile area.When the relative load is the same,the higher the reinforcement ratio,the smaller the thickness of the cover and the finer the reinforcement diameter,the stronger the energy dissipation capacity of the closed crack and the greater the Coulomb damping ratio at the initial stage of cracking.(7)The sensitivity of damping characteristics of reinforced concrete simply supported beams to cracking damage is higher than that of resonance frequency.