Study On Strengthening And Seismic Performance Of Reinforced Concrete-Steel Irregular Hybrid Frame Structures

Because of the limitation of the horizontal space of the surrounding area,adding a new frame on top of the pre-existing structure is one of the commonly used methods for extension and reconstruction projects.Reinforced concrete(RC)frame structures are frequently used in the previous period of service,and the steel frame is preferred as the newly adding structure given its light weight and constructability.Thus,a RC-steel hybrid frame structure with a vertical combination of a RC frame located in lower part and a steel frame located in upper part is generated.Due to the complex mechanical calculation model and ambiguous internal force transmission for this category of structure system,the seismic design and analysis of such a structure system based on traditional method cannot meet the requirement of performance-based design.The prototype structure in this paper is based on the practical retrofitting project for denitrification in power-generation plants,a variety of possible strengthening and retrofitting schemes at member level and/or structurallevel are conducted for the pre-existing RC frame.According to the structural characteristics,seismic analysis methods suitbale for vertical irregular structures areproposed.The nonlinear seismic performance analyses are carried out by creating numerical models of the hybrid frame structures retrofitted with different strengthening strategies on the RC frame.Based on the nonlinear response parameters the effects of vertical irregularity features caused by different strengthening strategies on the seismic performance of the hybrid frame structures are systematically studied.It lays a solid theoretical foundation for the strengthening design and seismic performance evaluation of the RC-steel irregular hybrid frame structures.The main contents of the research works can be summarized as follows:(1)Investigation on load-carrying capacity and failure mode of RC beams strengthened with steel plate jacketing.Using the finite element software ABAQUS,the numerical models consistent with the test specimens in a reference paper are established,and the accuracy of selected material constitutive model,parameter values in software,and the finite element analysis results are validated.The proposed material constitutive model and the parameter values in software are used to create the finite element numerical models of simply-supported RC beams and RC frame beams strengthened with steel plate jacketing,respectively.The load-carrying capacity and the failure mode of the strengthened RC beams are studied,the influence of batten plate spacing on the strengthening effect of RC beams are investigated,and the rational values of shear strength reduction factor of batten plates are proposed.(2)Experimental study on seismic performance of structural-lev el strengthened RC frame structures.The test specimens of RC frame strengthened with strong steel bracings(X-bracing and inverted V-bracing),weak steel bracings(X-bracing and inverted V-bracing),and concrete shear walls are designed and manufactured.The cyclic loading tests are carried out on the test specimens.Using finite element software ABAQUS,the numerical models of the corresponding test specimens are developed and the numerical simulation analyses are implemented.The seismic performance of the test specimens retrofitted with different strengthening strategies are comparatively analyzed on aspects of horizontal ultimate load-carrying capacity,hysteretic energy dissipation,deformation capacity,and failure mode.The rational design method of strengthening RC frame structure with strong steel bracings is studied.(3)Study on static pushover analysis of vertical irregular hybrid frame structures.The applicability of the static pushover analysis on seismic performance evaluation of irregular hybrid frame structure is studied.Based on the dynamic characteristics of the structure,a lateral load pattern for static pushover analysis of vertical irregular structures is proposed.Six hybrid frame structures with different degrees of vertical irregularity are generated by changing the strength and stiffness of the frame.The numerical models of irregular hybrid frame structures are established using the software Performa-3D,and the static pushover analyses with a variety of lateral load patterns are carried out on the structural models.The accuracy of the lateral load pattern based on multi-modes is verifed by comparatively analyzing the results of static pushover analysis and dynamic time-history analysis.The formation mechanism and distribution characteristics of plastic hinges are obtained from static pushover analyses,and the effect of vertical irregularity degree on the seismic performance of hybrid frame structures are evaluated.(4)Strengthening and static pushover analysis of RC-steel irregular hybrid frame structures.The prototype structure of RC-steel irregular hybrid frame originates from a practical retrofitting project of power-generation plants,and a variety of possible retrofitting schemes on the RC frame are proposed according to the strengthening methods at member level and structural level.Using the software Performa-3D,the numerical models of irregular hybrid frames retrofitted with different strengthening strategies on the RC frame are developed and the static pushover analyses are carried out.The pushover curves with base shear force along vertical axis and roof displacement along horizontal axis are obtained.By comparing and analyzing the nonlinear structural responses such as ultimate load-carrying capacity,initial lateral stiffness,distribution of shear force,formation mechanism and distribution characteristic of plastic hinges,the seismic performance of irregular hybrid frame structures retrofitted with different strengthening strategies are evaluated.(5)Dynamic time-history analysis of RC-steel irregular hybrid frame structures.A series of ground motion records are selected based on the earthquake magnitude and soil classification.The selected ground motion records are scaled to meet the seismic analysis requirements at different earthquake intensity levels and the adjusted input ground motions are employed in the dynamic time-history analyses of the structural models.The story capacity factors are adopted to preliminarily characterize the strength and stiffness irregularity along the height of the hybrid frame structures retrofitted with different strengthening strategies.The roof displacements and the story drift ratios of the hybrid frames are calculated using response spectrum method and comparatively analyzed with the results attained in dynamic time-history analyses.The applicability of empirical formulas to the calculation of structural deformation of vertical irregular hybrid frames is studied.A systematic investigation is conducted on the structural seismic responseparameters of vertically irregular hybrid frames,and the seismic performance of the hybrid frame structures retrofitted with different strengthening strategies are analyzed.(6)Study on energy dissipation and inelastic damage of vertical irregular hybrid frame structures.Dynamic time-history analyses at incremental seismic intensity levels are conducted on the hybrid frame structures with different degrees of vertical irregularity.The effect of accumulated hysteretic energy on the seismic performance of irregular hybrid frame are studied according to the earthquake input energy and hysteretic energy dissipation.The plastic damage of irregular hybrid frame at incremental seismic intensity levels are calculated and analyzed based on the modified Park&Ang double parameter damage model.The influence of changes in stiffness and strength on the overall damage index and story damage index of the hybrid frames are studied,and the effect of seismic intensity levels and irregularity features on energy dissipation damage and deformation damage are reveled.It lays a solid foundation for evaluating seismic performance of irregular hybrid frame structures using accumulated hysteretic energy and damage index.