Investigation On Mechanical Performance Of Self-centering Beam-column Connections With SMA Bars

The beam-column connections of the conventional steel moment-resisting frame generally consist of fully welded and bolt-welded connections.However,lots of brittle fractures were observed in the welded connections of steel structures during the earthquake,which may lead to a low ductility of structures.With the development of the performance-based seismic design concept,the new beam-column connections with a high energy-dissipation capacity and self-centering effect have become a valuable research issue in the structural seismic filed.This paper has proposed a self-centering beam-column connection including shape memory alloy(SMA)bars and non-asbestos(NAO)friction dampers,referred to as NAO-SMA-SC connections,which could make full use of the super-elasticity property of SMA,the simplified construction and stable performance of NAO friction dampers.The self-centering effect of NAO-SMA-SC connections is provided by several large SMA bars,but researches about the material property and constitutive model applicable for the large SMA bars are currently insufficient.At the same time,the parametric study about the hysteretic performance of SMA connections,and research about seismic performa nce of SMA frame are also rare.In this paper,the author has combined the experimental research and finite element analysis,and the main contents are as follows:(1)The material tests of SMA bars with various diameters were carried out,and by adopting several performance parameters,including the martensite phase transformation start stress,residual strain,single hysteretic energy dissipation and equivalent viscous damping ratio,the influences of the loading cycles,strain amplitude,SMA diameters and the loading rates on the material property have been discussed.The analysis results indicated that a certain number of constant amplitude loading could help SMA exhibit a more stable property,and the increase of strain amplitude could improve the energy dissipation capacity,but lower the super-elasticity of SMA.Meanwhile,the SMA diameter and loading rates may have no evident influence on the energy dissipation capacity of SMA,while on the condition of same strain amplitude,the residual strain increases with the SMA diameter.(2)To consider the differences between the loading and unloading path of strain hardening part appearing in the stress-strain curves of SMA,based on the Lagoudas model,a one-dimensional macroscopical super-elastic constitutive of SMA,this paper has proposed a modified strain-hardening constitutive of super-elastic SMA.Further,the relationships of loading cycles and strain amplitude with residual strain were obtained,and the concept of cumulative transformation strain(CTS)was introduced Based on the statistical relationship between CTS and the residual strain,a constitutive model of SMA bars,considering the residual strain,has been proposed.The effectiveness of the proposed model has been confirmed by the fact that the result of experimental research and numerical analysis behave as a high degree of agreement.The SMA constitutive model could be incorporated into OpenSees through the secondary development platform of Opensees.(3)Based on the experimental analysis on the SMA bars and NAO friction dampers,the construction of NAO-SMA-SC connections,including the self-centering beam-column connections and NAO friction dampers,has been introduced.The working principle and mechanical behavior of NAO-SMA-SC connections have been analyzed systematically,and analysis results showed that NAO-SMA-SC connections have taken the advantage of both the super-elasticity of SMA bars and the stable energy dissipation feature of NAO friction dampers,achieving a self-centering structural system with a high energy dissipation capacity.Meanwhile,this kind of connection construction could get over the inconvenience of floor stab layout,and improve the repairability of structural components.Establishing the simplified model of NAO-SMA-SC connections in Opensees,the mechanical behaviors of each component during the rotation of NAO-SMA-SC connections,were described in detail,verifying the effectiveness of simplified model of connections.(4)Based on the simplified model of NAO-SMA-SC connection in Opensees,the influences of several key design parameters,including the friction force Ff,provided by the NAO friction dampers,the diameter DSMA,and the initial strain PSMA of SMA bars,on the hysteretic performance of NAO-SMA-SC connections were discussed.The analysis results indicated that the moment resisting capacity and energy dissipation property could be significantly improved by increasing the Ff,but the residual deformation was also increased.Amplifying the DSMA could improve the moment resisting capacity and self-centering property,and exerting the PSMA could also effectively lower the residual deformation of NAO-SMA-SC connections.Overall,the energy dissipation capacity of NAO-SMA-SC connections is improved at the cost of increasing residual deformation.(5)According to the characteristics of NAO-SMA-SC connections,a 3-story NAO-SMA-SC steel frames was designed.Based on the simplified model of structures established in Opensees,the nonlinear dynamic analysis under a set of ground motions on behalf of maximum credible earthquakes were conducted,discussing the influences of the area ASMA of SMA bars,the equivalent truss area ANAO of NAO friction dampers,and the initial strain PSMA of SMA bars on the main seismic performance index of NAO-SMA-SC frames.The analysis results showed that the increase of ASMA can reduce the maximum story drift and residual drift of each floor,beneficial to improve the self-centering effect of structures and the distribution uniformity of story drift.The increase of ANAO can effectively lower the maximum story drift,but increase the residual drift,which may weaken the self-centering effect and not evidently control the weak story effect of structures.The increase of PSMA can greatly improve the self-centering effect of structures,but when the value of PSMA is too large,the improvement will be small.Meanwhile,the maximum story drift reduces gradually,and the distribution shape keeps basically the same.