Study On Eccentric Seismic Response Analysis And Torsion Resistance Performance Of Bottom Frame Structure

The bottom frame structure has the advantages of simple structural form,flexible spatial layout,convenient construction and economical practicality.It has been widely used in southern China since the 1970 s,especially in low-rise civil buildings in cities.However,the bottom frame structure is mainly composed of bottom frames,shear walls and upper masonry structures.The vertical structural system varies greatly and the force is complex.Compared with reinforced concrete structures,its seismic performance is poor.In major earthquakes,there were many cases of damage to bottom frame structures.Especially in high intensity areas,seriously damaged or collapsed bottom frame structures accounted for a high proportion.Meanwhile,in order to meet the spatial function needs,some eccentrically bottom frame structures produce torsional effects,aggravating the damage.The current specifications for the seismic design indexes and requirements of bottom frame structures are insufficient,lacking consideration of torsional control of bottom frame structures.Therefore,analyzing the eccentric torsional mechanism and response laws of bottom frame structures can provide theoretical support for revising specifications,putting forward torsional control indexes and corresponding design requirements,and promoting the improvement of the seismic design level of bottom frame structures.Through elastoplastic time-history analysis of bottom frame structures,this paper studies the torsional response laws of bottom frame structures with different design parameters under seismic action.The main works and achievements are as follows:(1)The relevant clauses of the specifications for structural torsion control and design in various countries are compared and analyzed.The changing trend of the clauses of bottom frame structures in China’s seismic codes is summarized.The equivalent static method and the mode decomposition response spectrum method are used to analyze the elastic torsional response of a single-story eccentric structure,and the influence laws of eccentricity ratio e/r,period ratio Tθ/Ty and aspect ratio b/a on the torsional displacement ratio δmax/δavg are obtained,providing a basis for the design of eccentric bottom frame structures.(2)Traditional theoretical models are used to analyze the factors affecting the yield performance and failure mode of reinforced concrete columns.A PSO-BP neural network prediction model of the yield performance and failure mode of reinforced concrete columns in bottom frame structures is established and validated to have high accuracy and strong generalization ability.The optimization performance of different optimization methods(including GA,SSA and PSO)is compared.The PSO method has higher accuracy and computational convenience,providing the basis for strength and ductility to control the torsional effect of bottom frame structures by changing the design parameters of the bottom columns.(3)A four-stage hysteretic model of reinforced concrete columns including elastic stage,hardening stage,softening stage and residual stage is proposed.Through the analysis of previous test data,the influencing factors of strength degradation are summarized,and the concept of positive and negative effective cumulative hysteretic energy consumption is proposed.The relationship between strength degradation rate and positive and negative effective cumulative hysteretic energy consumption is established.By writing subroutines,the beam element is used to simulate the strength degradation phenomenon of reinforced concrete columns under hysteretic action,and the applicability of the method is summarized,which provides the basis for energy consumption capacity to control the torsional effect of bottom frame structures by changing the design parameters of the bottom columns.(4)Based on YJK interface software and Python software programming,a rapid construction method of the numerical model of the bottom frame structure is realized,and the rationality of the method is verified.The method is used to analyze the torsional response laws of bottom frame structures under different eccentric conditions.According to the relationship between the damage level and the inter-story displacement angle,the limit values of the plastic torsional displacement ratio under the frequent and rare earthquake are given.The design limit values of the eccentricity ratio of bottom frame structures under different fortification intensities are proposed.The eccentricity ratio of the bottom frame layer of the bottom frame structure should be less than 0.13.The eccentricity ratio of the masonry layer of bottom frame structures with 6-7 degrees of fortification should be less than 0.14,and that of bottom frame structures with 8 degrees of fortification should be less than 0.09.In addition,the influence of changing the section size and concrete strength of the bottom corner columns on the seismic capacity of bottom frame structures is analyzed,and design suggestions for controlling the torsional effect of bottom frame structures are proposed.