Study On Seismic Behavior Of Self-Centering Prefabricated Steel Beam-to-Column Connections With Shape Memory Alloys

In recent years,"earthquake resilient structures" have been widely concerned by many scholars at home and abroad because they can reduce or even eliminate the residual deformation in the structure after an earthquake,so that these structures can quickly restore the intended function.Based on the idea of earthquake resilient structures,self-centering steel beam-to-column connections have been developed and studied.Shape memory alloys(SMA)have both superelastic and high damping properties at the material level,and a seismic mechanism of self-centering and energy dissipation can be realized by applying SMA to steel beam-to-column connections.It can effectively improve the post-disaster recovery capacity of steel frame structures and has significant social and economic benefits for the development of earthquake-resistant and resilient cities.In this dissertation,an innovative self-centering prefabricated steel beam-to-column connection with SMA(has been authorized by invention patent)was presented,and its seismic performance was studied by experimental study,finite element simulation and theoretical analysis.The main contents and achievements of the research are as follows:Increased cyclic tensile tests and constant amplitude cyclic tensile tests were carried out on SMA bars subjected to different heat treatments,respectively.A finite element simulation method for SMA bars with residual deformation was proposed.The results show that different heat treatments have a great influence on the mechanical properties of SMA bars.For a SMA(Ti-50.8at.%Ni)bar with a diameter of 16.3mm in parallel section,the heat treatment for 30 min at450℃ can make it obtain excellent mechanical properties with both superelasticity and energy dissipation capacity.The finite element simulation method for SMA bars with residual deformation adopts the idea of material replacement to reflect the residual deformation of SMA bars by equivalent plastic deformation of elastic-plastic metal.It has been proved that it can accurately simulate various mechanical properties of SMA bars including residual deformation.Eight self-centering prefabricated steel beam-to-column connections with SMA bars(hereinafter referred to as the "SMA self-centering connection"),a steel beam-to-column connection with steel bars,and a Kaiser bolted bracket(KBB)beam-to-column connection were designed and pseudo-static tests were performed on them.Then parametric analysis of the seismic behavior of the SMA self-centering prefabricated steel beam-to-column connection was carried out by using finite element method.The results show that the rotational deformation capacity of all SMA connections met the ductility design target,and no plastic damage occurred in the beam and column.Except for the KBB connection,the other specimens belong to semi-rigid and partial strength connections according to Eurocode 3.Except for the connection with standard pre-tension applied to shear bolts,all the SMA connections were repairable within the designed inter-storey drift angle.According to the result of finite element analysis,the pre-tension strain,length and quantity of SMA bars,the pre-tension of shear bolts,the beam section height and ambient temperature all have great influence on the seismic performance of the SMA self-centering connection.On the premise of reasonable design,the SMA self-centering steel beam-to-column connection can meet the requirements of earthquake resilient structures.The mechanical properties of U-shaped dampers under an ideal stress state were theoretically deduced,and the theoretical formulas were modified according to the working constraints in the SMA connection.The finite element analysis of the SMA self-centering prefabricated steel beam-to-column connection with U-shaped dampers was carried out to study the influence of the U-shaped damper on the seismic performance of the connection.The results show that the modified formulas can well reflect the elastic stiffness,yield strength,yield displacement and ultimate strength of the U-shaped damper.The effective length of the straight section,the radius and thickness ratio of the bending section,the steel strength grade,and the width of the U-shaped damper have different effects on the mechanical properties of the damper itself and the connection.In the design of the U-shaped damper,the thickness of the damper plate and steel strength should not be too large,so as to prevent the residual deformation of the connection from increasing obviously and losing the repairability.The theoretical model of restoring force of the SMA self-centering connection was established.Based on the force characteristics of SMA bars and U-shaped dampers,the mechanical model of the energy-intensive SMA self-centering connection was analyzed,and the concrete method of establishing the restoring force model was proposed.The theoretical model of the restoring force of the SMA self-centering connection was proved to be accurate.The results can provide a research basis for the design and analysis of self-centering connections with SMA.On the basis of summarizing the above research results and combining with the current domestic and foreign standards,the design method of the SMA self-centering connection was presented.For the energy-intensive SMA self-centering connection,a graded and phased energy dissipation design method was proposed according to earthquake intensity.The research results of this series of SMA self-centering connections design method can provide a reference for the design and application of such self-centering structural systems or recoverable function seismic structures.