Study On The Mechanical Behaviour Of Existing RC Frame Structure Subjected To Seismic Excitation

Taking the advantages of simple structural style, flexible layout and explicitforce-transmission path into account, RC frame structures play an important role in thecivil engineering construction in China even all over the world. Compared with other newstructure systems, a large number of RC frame structures have been entered the phases ofmiddle age, old age and even over age. Besides, large-scale seismic activities in Chinaand neighboring countries in lately years indicate that earthquakes with high intensity willhappen to China mainland probably in the next few years. Thus, it is necessary to studythe multi-scale damage behaviors of existing RC frame structure subjected to seismicexcitation, and then propose some suggestions for optimal seismic protection.Theoretical analysis and numerical simulation are adopted to study the mechanicalbehaviour of existing RC frame structure. The degradation mechanisms and time-varyinglaws of main building material mechanical properties in common atmosphereenvironment are explored and damage evolution laws between closely scales arerevealed, then structure seismic collapse resistant capacity is evaluated quantitatively andthe best balance between seismic investment and earthquake damage loss are obtainedfinally. The main research works and conclusions are shown as follows:1. Four stages of RC structure performance degradation due to the reinforcementcorrosion are defined and the calculating model for covered concrete carbonated residuecontent is proposed by the regression of field test data, and the calculating formulas ofreinforcement corrosion rate when protective layer of concrete cracks is deduced bydawing the support of elasticity theory, then the corrosion sensitivities of concretetensile strength, concrete cover depth, reinforcement diameter, rust poisson ratio andrust corrosion expansion rate are revealed subsequently. The formulas for calculatingthe service time of structure with no corrosion and with covered concrete cracking areproposed, and the time-varying constitutive models for reinforcement, concrete and thebond-slip between reinforcement and concrete of multi-age RC frame structure areselected to provide basic theory support for subsequent numerical analysis.2. The numerical modeling theory for existing RC frame structure in componentscale with ABAQUS program is put forward, and the ralated research works contain selection and combination of units, concrete failure criteria, materials time-varyingconstitutive model, model reliability verification and so on. According to the suggestedrules for selecting damage characterization of RC components, the damage evaluationindexes for RC beam and column are proposed. By introducing the cohesive forcecorrection function into calculation formula of RC beam component bending stiffnesswithout corrosion to adjust the average tensile strain of concrete rounded tensile steel bar,the formula for calculating corroded RC beam bending stiffness is estabilished. Thedamage sensitivity analyses for main factors which contain section dimension, concretestrength grade, concrete cover depth and reinforcement ratio are conducted, and then theinfluence laws of main design parameters on component damage evolution are revealed,which will provide theoretical support for establishing damage model of existing RCframe structure subjected to seismic excitation.3. The numerical modeling theory for existing RC frame structure in structure scalewith OpenSees program is established by studying the characterization of RCcomponent mechanical properties in the fiber level. Damage evaluation index for RCframe structure in macroscale is given out and the influence laws of main designparameters on storey damage evolution are revealed, then the storey damage model ofexisting RC frame structure subjected to cyclic loads is proposed. According to theabove achievements, the influences of damaged storey location, damaged storeyquantity, storey damage degree and structural service age on structural seismic damageare investigated, and then the seismic damage model for existing RC frame structure isestablished ultimately.4. By judging the advantages and disadvantages of existing rules, ground motionrecords and ground motion intensity index which are more suitable for the numericalanalysis of RC frame structure elastoplastic seismic response are provided and therelated problems involved in vulnerability analysis of seismic collapse resistant forstructure by means of IDA are solved in this paper. The influences of main factors suchas storey height, structure aspect ratio, column axial compression ratio and floorconstraint on seismic collapse resistant capacity of RC frame structure with typicalreinforcement corrosion ratio are revealed, and then the existing RC frame structureseismic collapse resistant capacity is evaluated quantitatively.5. Drawing support from the optimization thought of the Complex Method,optimization scheme of RC frame structure in the elastic stage is obtained with themethod of divisional optimization and the optimization design for elastic-plastic phase isrealized by limiting maximum interlayer displacement angle, and then the optimizationdesign for minimum seismic investment of RC frame structure under specifiedfortification intensity is achieved. Combined the calculating results of failure probability of structure in different damage degree with quantitative results of single buildingearthquake damage loss, the loss expectation of frame structure with specifiedfortification intensity is obtained finally. Taking the minimum of seismic investment andearthquake damage loss expectation as optimization targets, the fortification intensitycorresponding with minimum objective function value is considered as the optimal one,and then a perfect balance between seismic investment and earthquake damage lossunder optimal fortification intensity is sought finally.