Long-term Behavior Of RC Columns Under Sustained Eccentric Axial Loads

In order to study the long-term mechanical properties of reinforced concrete columns under eccentric compression,numerical and experimental studies are carried out.At present,scholars at home and abroad mostly focus on short-term mechanical performance of reinforced concrete column concentric compression,and this study first pay attention to long-term mechanical properties,core is reinforced concrete column of creep and shrinkage effects on its performance,the second pay attention to the design of eccentric compression,more in line with the actual usage of reinforced concrete column,the third with high-strength materials,fill the gaps in research at home and abroad.The concept of minimum reinforcement ratio was first introduced in 1936 to prevent longitudinal reinforcement from yielding under load.In reinforced concrete columns,due to creep and shrinkage effects,the actual concrete stress gradually relaxes over time and is transferred to the reinforcement.In low strength reinforced concrete columns,the redistribution of pressure from concrete to reinforcement may cause longitudinal reinforcement to yield,and this premature yield can adversely affect durability and ultimate performance.In many practical design cases,concrete strength is determined by stiffness(flexure and vibration),architectural aesthetics and function,rather than strength requirements.In these cases,the column section may be greater than the strength requirement,resulting in longitudinal rebar quantities determined by the minimum reinforcement ratio.However,since the current 1% minimum reinforcement ratio provision is based on test results using concentrated loading of low-strength materials,the minimum required amount of reinforcement may be less than or greater than 1% of the actual required amount of reinforcement(i.e.,1% May be unsafe or uneconomical in some cases).In the case of high eccentricity or high slenderness ratio,the premature yield of longitudinal reinforced bars is more obvious,which is caused by excessive internal stress of concrete and additional second-order effect.Therefore,in order to provide safer and more economical design standards,current design specifications require minimum reinforcement ratios to be improved.In reinforced concrete columns,longitudinal reinforcement provides flexural strength(or concrete tensile strength).In principle,the ratio of longitudinal reinforcement is to ensure that the bearing capacity of the column is greater than the load requirement(based on the P-M criterion)and not less than the minimum ratio(1% in ACI 318-14 and KBC 2016 and 0.5-0.6% in GB 50010-2010).In order to carry out a more reasonable design and ensure that reinforced concrete columns meet all performance requirements,the current design specification needs to improve the ratio of longitudinal reinforcement,and further consider durability,ductility and flexural rigidity in the design.In order to study the influence of creep and shrinkage of concrete columns on its performance,various design parameters,including loading type,material performance,eccentricity,service life,load level and slenderness ratio,were considered in the numerical study.Numerical studies are compared with those of the existing test results,verify the correctness of the model,the numerical results indicate that various circumstances,the design parameters on the minimum reinforcement ratio(to prevent the use of longitudinal reinforcement to yield,the yield may be caused by creep and shrinkage effects),long-term effective bending stiffness,ultimate strength has a significant effect for a long time.In order to study the long-term behavior of reinforced concrete columns under eccentric compression,10 specimens of reinforced concrete columns were tested by eccentric axial loading.The test parameters include concrete strength,reinforcement ratio,longitudinal reinforcement yield strength,eccentricity ratio,slenderness ratio and loading method.The test results show that the stress and curvature increase with time,and the concrete stress gradually transfers to longitudinal reinforcement due to shrinkage and creep of concrete.Long-term performance depends on test parameters: long-term effects are more important for components that use low strength concrete,low strength reinforcement,low reinforcement ratio,less loading times,higher eccentricity,and higher slenderness ratios.However,in all reinforced concrete column samples,longitudinal reinforcement did not yield under the service load on the final measurement day.In order to make the design more reasonable,the formulas of minimum reinforcement design for concentrated and eccentric compression reinforced concrete columns are presented.This simple design calculation formula(where the long-term effective flexural rigidity is approximately the short-term effective flexural rigidity of reduction)reflects the longterm effect well.The reduction in strength due to long-term effects is not serious in general,but nonlinear creep and second-order effects should be paid attention to in the design.Through the comprehensive analysis of numerical simulation and experimental research,the long-term performance of reinforced concrete columns can be predicted reasonably and accurately if the experimental results of concrete test blocks(mainly including creep and shrinkage parameters)can be obtained.