Seismic Fragility Analysis Of Steel Frame With Initial Connection Damage

The structural damages mainly concentrated on connections after the Northridge and Kobe earthquake,especially the lower flanges of the beam-to-column connections.The reasons for the failure of connections were discussed by series of researches,and were generalized as:(a)weld defects including cracks,blowholes,slag and other inevitable defects.(b)low impact toughness of the welded metal.(c)The decrease of yield strength in heat affected zone.(d)Man-made cracks due to backing plate.(e)Residual stress and stress concentration resulted from welding.The existence of connections initial damage affects the seismic performance of the structure,which makes the study of the seismic fragility considering the initial damages necessary.The seismic fragility of the main component in the seismic hazard analysis.The uncertainty of the structure and the earthquake have been considered in seismic fragility,and the failure probability corresponding to different damage levels is also studied under various earthquake intensity.Until now,the researches concerning with connection accompanied with initial damage are limited.Only some literatures have mentioned about the mechanical properties of connections with initial cracks,but no detailed researches have been carried out to compare the seismic fragility of the structure with initial damage.It is of benefit to study the structure with initial connection damage for the future development of controlling the damage of the structure,as well as reducing the economic loss and human unsafety to lowest extent.(1)Different dimensions of initial cracks were firstly considered in this paper.Different finite element models were established using ABAQUS in order to compare the impact toughness.(2)Then,different dimensions of initial cracks and the decrease of strength in the welding heat affected zone were taken into consideration in this paper.Latin hypercube sampling was adopted to generate 150 samples.The dynamic time-history analyses were conducted to obtain the relationship between the seismic demand parameters and seismic strength parameters,where the maximum drift angle(RDA)and the displacement ductility ratio(?d)were taken as seismic demand parameters and the peak ground acceleration(PGA)was taken as the ground motion parameter.(3)The probabilistic seismic capacity analysis was then conducted by Pushover.The average equivalent yield displacement was obtained using geometric mapping method,equivalent energy method and Park method from the load-displacement curves.The different performance levels were defined using Pushover curves according to 2010 seismic standard,relevant documents and self-definition.Then,the different limit values were defined corresponding to different performance levels.(4)Finally,on the basis of the probabilistic seismic demand analysis and probabilistic seismic capacity analysis,the probabilistic seismic fragility analysis of different initial cracks,including a0=0mm,a0=8mm,a0=16mm,and decrease of strength was conducted with the help of reliability theory.The fragility curves were pictured with performance levels including the maximum drift angle(RDA)and the displacement ductility ratio(?d).The failure probabilities of structures with different initial cracks and the decrease of the strength were compared under various earthquake intensity.The difference of different fragility curves was also analyzed.The difference of different finite element models such as plane model,wire model and the multi-dimension model was also compared to provide comments on establishing models for seismic fragility analysis of structures with initial damage.The analysis of the seismic fragility of the frame with initial connection damage can be used as the theoretical basis for structural design of this type of steel frame.