Research On Normal Section Bearing Capacity Of RC Vertical Members Strengthened By Enclosure Method And Unbraced Local Reinforcement Replacement Method Considering Stress Lag

With the development of economy and urbanization in China,the reinforcement and reconstruction of buildings in the field of urban renewal and engineering construction are gradually increasing.There are many researches on structural reinforcement at home and abroad,and the relevant national codes and standards are becoming more and more perfect.So far,most of the research results of structural strengthening are based on engineering experience and theoretical analysis,and the relevant experimental studies are relatively few,especially the stress lag effect in structural strengthening has not been well reflected in the relevant codes,and the code formulas are unsafe.The development and application of high-performance building materials continue to promote the innovation and development of building structure strengthening technology.Unbraced unloading and local strengthening replacement method is effective in strengthening RC shear walls,and the stress lag effect also affects the stress of the whole section of the member,so the bearing performance of RC members strengthened by this method needs to be further studied.In This paper,based on the simulation analysis and method verification of the existing literature RC strengthening tests by using the efficient and refined nonlinear finite element analysis program VFEAP,the axial compression analysis models of 18RC columns strengthened by encircling method and 21 RC shear walls strengthened by unbraced unloading local strengthening replacement method are designed and established respectively.Parametric nonlinear simulation analysis is carried out considering the factors such as the original RC member parameters,the initial vertical stress level,the increased section size（or the area ratio and batch of the replacement area）,the strength index of the strengthening（or replacement）materials（concrete,steel bar or section steel）,the stirrup configuration and its form,etc;Based on the simulation results and the test data of the existing literature,considering the influence of different initial stress levels and stress lag,the confinement effect of stirrups and peripheral concrete,and the sequential conditions of replacement in batches,through theoretical analysis and regression fitting,the results are obtained.The calculation formulas of axial compression bearing capacity of RC columns strengthened by encircling method and RC shear walls strengthened by unbraced unloading and local strengthening replacement method are established respectively.Compared with other formulas,they have good safety,high accuracy and small discreteness,and point out the insecurity when the initial stress level of the Code is high;Nine eccentric compression analysis models of RC shear walls with T-shaped sections strengthened by local strengthening and replacement method without bracing unloading are designed and established.The influences of initial stress level,replacement area,material（RC or SRC）parameters,vertical force eccentricity and other factors are discussed through nonlinear simulation analysis.The eccentric compression mechanical behavior of RC shear walls with T-shaped sections strengthened by local strengthening and replacement method is preliminarily studied.The main conclusions are as follows:（1）Increasing the stirrup ratio,using composite stirrups and increasing the thickness of the enclosure can enhance the restraint of the original column concrete and increase the bearing capacity of the original column.When the initial stress level is greater than 0.7,the utilization coefficient of the newly added concrete strength is generally lower than the code value of 0.8,especially when the initial stress level reaches 1.0,the utilization coefficient is far less than 0.8.It is shown that when the initial stress level is greater than 0.7,it is unsafe according to the current code,and the safety decreases with the increase of the initial stress level.（2）According to the concrete strength theory,the paper respectively introduces the concrete strength improvement factors K₁ and K₂ considering the confinement of the original column and the newly added confinement stirrups,and deduces the formula of the axial compression bearing capacity of RC columns strengthened by enclosure method through theoretical analysis.Compared with other formulas,it has good security,high accuracy and small discreteness.（3）When the initial stress level is higher than 0.4,the one-batch replacement method is not suitable,and the two-batch replacement or steel reinforced concrete replacement is recommended;when the initial stress level is higher than 0.7,the three-batch replacement or enclosure method is recommended;when the initial stress levels is higher than 0.8,it is not recommended to use the unsupported replacement method.（4）The utilization coefficient of concrete strength is related to the replacement batch.The later the replacement batch is,the lower the utilization coefficient of concrete strength is than the standard coefficient 0.8.By introducing the concrete confinement strength improvement coefficient K and considering the stress lag effect,the calculation expression of the axial compression bearing capacity of RC shear walls strengthened by the first,second and third batch replacement method without bracing and unloading local strengthening is derived through theoretical analysis,and the calculation formula of the axial compression bearing capacity is fitted based on the results of parametric nonlinear simulation analysis.Ood safety,high accuracy and small discreteness.（5）When the unbraced unloading and local strengthening replacement method is used to strengthen RC eccentric compression shear walls,the wall segments on the tension side may not be replaced,but the wall segments on the compression side must be replaced and strengthened,and the wall sections on the compression side far away from the neutral axis of the section should be replaced in the first batch to improve the utilization rate of concrete strength.The influence degree of each factor on the eccentric compression bearing capacity of the shear wall strengthened by replacement is as follows:the strength of concrete for replacement,the strength of steel,the initial eccentric stress level and the eccentricity.