Research On Seismic Vulnerability Of Cold-formed Steel Structures Considering Nonlinear Mechanical Properties Of The Hold Downs

Hold downs are one of the key structures of cold-formed thin-wall steel structures,and their mechanical properties have significant effects on the shear properties of cold-formed thin-wall steel composite walls and even the collapse ability of the whole structure.However,it is difficult to accurately predict the real seismic performance of cold-formed thin-wall steel structures because the existing simplified calculation models do not consider the nonlinear characteristics of the hold downs.Therefore,it is necessary to establish a simplified calculation model of cold-formed thin-wall steel structures which can take the nonlinear effects of the hold downs into account,so as to improve the reliability and accuracy of the numerical analysis of such structural systems.In this thesis,the mechanical properties of hold downs of cold-formed thin-wall steel structure were tested,and the results were obtained.On this basis,the existing simplified calculation model of cold-formed thin-wall steel composite wall was improved,and the finite element analysis model of cold-formed thin-wall steel structure was established,which could consider the nonlinear characteristics of hold downs.Based on the results of additive dynamic analysis,seismic vulnerability analysis of such structures is conducted.The main contents and conclusions are as follows:1.Monotonic loading and cyclic loading test studies were carried out on 8 hold down specimens.The effects of bolt number,load type and cross section stiffness of side column on mechanical properties of hold downs were investigated.The results show that:(1)The failure characteristics of all specimens are shear fracture of the through-length bolt on the tensile side of the column body.The bearing capacity of the hold down mainly depends on the shear strength of the through-length bolt.The strength and stiffness of the hold downs can be improved by 46% and 37% by increasing the number of pulling bolts,respectively.(2)The load type has significant influence on the strength and stiffness of the extruded parts.The strength and stiffness of the specimen deteriorated rapidly under cyclic action,and the longer the cyclic time,the faster the stiffness degradation.(3)The concrete-filled steel tube column has no obvious damage,and changing the section stiffness of the side column has little effect on the performance of the hold downs.2.Use Open Sees finite element software to improve the simplified calculation model of the existing composite wall,and explore the influence of mechanical property degradation of hold downs on the shear capacity of composite wall.The results show that:(1)The calculation results of the simplified model of composite wall established in this thesis are basically consistent with the shear test results of composite wall in the literature,and the relative errors of each shear performance index are about 15%,which confirms the correctness and rationality of the simplified model in this thesis.(2)Compared with the existing simplified calculation model,the improved simplified calculation model can more accurately reflect the influence of the nonlinear characteristics of the hold downs on the mechanical properties of the wall.(3)Compared with the existing simplified calculation model of wall,the improved simplified calculation model can accurately reflect the influence of different cyclic loading modes on the mechanical properties of composite wall,and can better highlight the influence of multi-stage cyclic loading on the deformation and energy dissipation capacity of wall,and can provide a more accurate simplified analysis model for the collapse resistance evaluation of cold-formed steel structures.3.The finite element analysis model of cold-formed thin-wall steel structures with and without the nonlinear characteristics of hold downs is established,and the seismic vulnerability analysis of such structures is carried out based on the results of additive dynamic analysis.The results show that the nonlinear characteristics of the hold downs can significantly reduce the seismic performance of the structure.Compared with the structural model without considering the nonlinear characteristics of hold downs,the IDA curve cluster of the model with considering the nonlinear characteristics of hold downs inclines to the right as a whole,and the probability of the structural model with considering the nonlinear characteristics of hold downs exceeds the limit state of IO,SD and CP is higher than that of the model without considering the nonlinear characteristics of hold downs.(2)When analyzing the seismic performance of cold-formed steel structures with obvious strength and stiffness degradation characteristics during ground motion holding,the simplified calculation model improved in this thesis can more accurately reflect the real collapse resistance of the structures,and provide a theoretical basis for the collapse resistance design of such structures.