Study On Mechanical Behaviors Of Anchored Blind Bolt And Its Application In Moment-Resisting Frame

Concrete-filled steel tube(CFST)columns have been widely used in building structures primarily due to their high structural efficiency,low cost,and fast construction time.The majority of existing CFST columns are connected to steel beams via welding processes,but this connection installation is complex,time consuming and costly.Moreover,it is quite difficult to ensure the quality of the site construction,and welding beam-to-column connections are prone to severe damage and brittle fracture during earthquake disasters.A bolted connection between the CFST column and steel beam can solve these problems.However,two restrictive factors hinder the popularization and application of such joints.First,the bolted connection between square steel tube column and steel beam is limited by the closed column section.Second,premature yield and excessive outward deformation of the square steel tube wall are typical controlling factors of joint failure.Anchored blind bolt can solve the connection problem between the closed square steel tube and the steel beam and reduce or even avoid the premature yield and excessive outward deformation of square steel tube wall.However,Study on performance on either the anchored blind bolt or the square CFST column joints with anchored blind bolts are lacking.This work is carried out from the three levels of anchored blind bolt,anchored blind-bolted connection,and composite frame from bottom to top.The application of anchored blind bolt in moment-resisting frame is systematically investigated using experimental research,numerical simulation,and theoretical analysis to establish the connection of the three levels.(1)Twenty-nine blind bolts were tested under monotonic loading with test parameters of width-thickness ratio of square steel tube,anchoring length,anchoring mode,edge distance,group anchoring effect,wall restraint of square steel tube,bolt grade,and bolt diameter.Failure modes,anchorage performance,nonlinear force-displacement curves,and load transfer mechanism were analyzed.Effects of different test parameters on tensile performance of anchored blind bolts were explored.The axial tensile linear mechanical model under two failure modes(rupture and pull-out)and the design and construction measures advices were proposing.(2)Cyclic loading tests of eight novel anchored blind-bolted end-plate connections to square CFST columns were carried out.Failure modes,stiffness,flexural resistances,rotational capacity and ductility,stiffness and strength degradation,energy dissipation,and anchorage reliability were investigated using experimental observation and result analysis.Effects of test parameters,such as steel tube wall thickness,end-plate thickness,local strengthening connection method,anchorage method,beam section,and stiffener,on mechanical properties of such joints were analyzed.(3)Finite element analysis model of anchored blind-bolted square CFST column joints is established using ABAQUS software and then verified against the experimental results.Working mechanism and stress characteristics of joints were analyzed;main components of joints were extracted for detailed stress analysis;and finite element parameter analyses with different load,geometric,and material parameters were carried out using numerical simulation.(4)Formulas for calculating related components of anchored blind-bolted end-plate connections to square CFST columns were derived,and the mechanical model and formulas for calculating the initial rotation stiffness of such connections were proposed on the basis of the component method.Calculation formulas for the joint moment were put forward on the basis of failure modes of such connections when the blind bolt,end-plate,and concrete-filled square steel tubular column reach their ultimate bearing capacity and the minimum value is taken as joint flexural resistance.Simplified trilinear and bilinear design models with simple calculation and good reliability were proposed for application convenience in practical engineering.(5)Dynamic time history analysis of three different types of CFST composite plane frames with anchored blind-bolted connection is carried out using OpenSees platform,and the influence of joint stiffness on the dynamic performance of the composite frame is examined.Various judgmen methods were compared,and then reasonable suggestions were offered to assess the semirigid interval of joints in terms of different types of composite frames.The design method suggestions,detail requirements,and construction method of anchored blind-bolted end-plate connections to CFST columns were put forward to provide guidance and reference for the application of such joints in engineering practice.