Seismic Fragility Analysis Of Beam String Structure

The research and application of performance-based seismic vulnerability had been started relatively late in China.The objects and results achieved are mainly concentrated in reinforced concrete structures and steel frame structures.It is relatively rare to apply it to the string beam structure,especially for the radiant beam string structure supported by special-shaped steel columns.In the past beam string structure seismic response analysis,the development of structural displacements and internal forces was mainly investigated,so as to carry out the dynamic stability analysis and seismic capacity analysis of the structure.However,it tends to underestimate the plastic energy dissipation of structures and overestimation of structural collapse resistance.There are certain security risks.In this paper,taking the radiant beam string stadium supported by special-shaped steel columns at the Jiulonghu campus of Southeast University as the study object,the ductile displacement,plastic energy dissipation,dynamic failure mode,damage grade,damage index and vulnerability of beam string structures under earthquake are studied.A nonlinear incremental dynamic time history analysis of ductile displacement and plastic energy dissipation was carried out in beam string structure.The damage index and criterion for rational characterization of damage grade of beam string structure are proposed.An effective method for seismic fragility analysis of beam string structure is developed.The paper mainly includes the following five aspects:(1)The finite element software ANSYS was used to establish the radiant beam string structure by special-shaped steel columns model.The on-site inspection results were compared with the results of the finite element tension analysis,which includes vertical displacement from the central rigid ring,the total radial contraction value of the outer ring beam,tension value in the process of tension.The accuracy of the string beam structural model and the reliability of numerical analysis were verified.(2)An improved multiple response full load-range dynamic time history analysis for beam string structure was proposed,On the basis of this method,the criterion of dynamic failure modes of beam string structures based on characteristic response was proposed.Nonlinear incremental dynamic time history analysis of beam string structure was carried out by choosing two seismic waves.The maximum node displacement of the structure was inspected.Ratio of different yield levels of three different components of upper chord member,stay bar and stay cable was investigated.The correlation among the three time history curves of the characteristic nodes was verified.The failure modes of structures under the action of two seismic waves were determined.The similarities and differences of performance-based seismic design codes between China and foreign countries were compared and analyzed from three aspects: waterproof level,performance level and performance objective.Combining with the existing literature on performance and the performance analysis of long-span structures,a qualitative criterion for determining the failure grade of beam string structures was proposed.(3)Combined with the characteristic response under the action of two seismic waves,the qualitative division standard was quantified.Considering the influence of structural and seismic uncertainties on the structure,the Latin hypercube sampling method was used to obtain multiple pairs of structural-seismic samples.Nonlinear incremental dynamic time history analysis was performed on each pair of samples to obtain the characteristic responses of all samples to different intensity earthquakes.According to the characteristic response of failure limit time,the damage index was fitted to describe the damage degree of beam string structure.The accuracy of the damage index was verified by comparing the characteristic response of all samples to the damage index under different earthquake intensity,and the criterion of the damage grade based on the damage index was proposed.The characteristic responses corresponding to the limit values of the damage exponents under four limit states were statistically analyzed,and the rationality of the selected limit values was proved.(4)The deformation and internal force of the structure under various load conditions were analyzed when the initial pre-stress changed.According to the determination criterion of reasonable initial pre-stress,the optimal initial pre-stress value and the reasonable range of initial pre-stress were determined.The seismic vulnerability curves of different initial prestressing forces were drawn by using the theoretical analysis method based on probability and statistics.The influence of initial pre-stressing on seismic vulnerability of structures in a reasonable initial pre-stressing range and given earthquake intensity was studied.The seismic behavior of the structures with different initial pre-stressing force of 7 degrees of fortification intensity at 6 degrees,7 degrees,8 degrees and 9 degrees of fortification intensity was studied.(5)The relevant regulations on Corrosion in China are comprehensively analyzed,and the rust time and corrosion rate of the structure were determined.The degradation law of steel corrosion in existing literatures was analyzed,and the constitutive model considering corrosion suitable for this structural model was selected.From the viewpoint of ductile displacement and plastic energy dissipation,the degradation law of structural stiffness in different service life was studied.Using the theory of seismic vulnerability considering time,the seismic vulnerability curves of different service ages were drawn.It provides reference for structural seismic safety assessment,later identification and reinforcement.The seismic behavior of structures with 7 degree fortification intensity and 6 degree,7 degree,8 degree and 9 degree fortification intensity in different service life was studied.