Seismic Performance Research Of Low Yield Point Circular Steel Pipe Concentrically Braced Frames

Low yield point concentrically braced frames have high lateral stiffness,strength and ductility.At the same time,hysteretic curve is full and there is a lot of plastic energy dissipation capacity,which can effectively reduce the earthquake disaster.Low yield point concentrically braced frames can be in the elastic state under small earthquakes,and have good energy dissipation under medium and major earthquakes,which effectively protect the beam and column,so,this structure has wide engineering application value.In this paper,by studying a large number of relevant literatures,low yield point concentrically braced frames were studied systematically,and the main research work and achievements are as follows:(1)Through the tensile test of materials,material constitutive relation of low yield point steel and the related mechanical properties can be obtained.Design different width-thickness ratio and slenderness ratio of circular steel pipe.The axial load cycles are applied in ANSYS to observe hysteresis curve,the maximum compressive bearing capacity,at the same time systematically summarize buckling performance and energy consumption situation of circular steel pipe of the different slenderness ratio and width-thickness ratio,which provides the necessary theoretical basis for the design of the steel brace.In addition,through the simulation of a two layer concentrically braced frames,the influence of brace with different section parameters on the bearing capacity and ductility of the overall structure are compared.It is concluded that more suitable range of slenderness ratio and width-thickness ratio can be adopted in design.Finally,it can be found that slenderness ratio is too big and too small are not good,general advice to the slenderness ratio is 80-120,because too small slenderness ratio has low bearing capacity and too big slenderness ratio has a bad influence on the beams and columns.At the same time,width-thickness ratio cannot be too big,otherwise local buckling happens,but with the increase of slenderness ratio can relax the limitation on width-thickness ratio,when the slenderness ratio is 50,width-thickness ratio had better not more than 15,when slenderness ratio is 80,width-thickness ratio had better not more than 20,and when the slenderness ratio is 100,width-thickness ratio had better not more than 25.In general,no special requirements,for low yield point(LYP100)circular steel pipe,width-thickness ratio had better not more than 30.(2)This paper does not adopt the traditional design method based on the strength,but the plastic design based on performance.The primary target lateral displacement and the yield mechanism are used as a performance limit state.According to the energy equivalent principle,namely,the energy required to achieve the same state as the equivalent elastic and plastic single degree of freedom system(EP-SDOF)is equal to the work required to achieve the target side shift,which is to calculate design base shear under a given earthquake level.Reasonable shear distribution formulas are used to calculate the shear force of each layer.For the sake of conservatism,assuming the storey shear forces are supported by the braces.After buckling of brace,the bearing capacity is 0.3 times as much as before.Due to the effect of concentrated force,the beam section during the braces need to be strengthed,columns are designed by the traditional methods,and then checked by SAP2000.(3)Finite element software SAP2000 is used to design a 10 layer three spans concentrically braced steel frames,then seismic performance of the designed concentrically braced frames are evaluated through the Pushover analysis method and dynamic time history analysis method.The performance of low yield point(LYP100)circular steel pipe concentrically braced frames were compared with the ordinary(Q345)circular steel pipe concentrically braced frames under different level earthquake,such as base shear,storey shear forces,top displacement.Through the analysis in Pushover,the plastic hinge of beam of low yield point concentrically braced frames are later than the ordinary concentrically braced frames.It also suggests that the low yield point concentrically braced frames consume more energy,lead to the energy dissipation of beam is less,and also illustrates the beam and column of low yield point concentrically braced frames under strong earthquakes more is not easy to damage.In fact,under frequently occurred earthquake,the two structures are still in elastic state,the braces did not enter the yield stage,little energy consumption.But under rarely occurred earthquake,the peak of top displacements and base shear of low yield point concentrically braced frames are less than ordinary concentrically braced frames,which also illustrates that low yield point concentrically braced frames have better seismic performance again.(4)In order to be more intuitive to compare the energy dissipation performance of ordinary concentrically braced frames(BBF)with the energy dissipation performance of low yield point concentrically braced frames(LBBF),two 10 layer three span structures are simulated in ANSYS and cycle loading is applied in top of the structure.The final results can be found that the area of the hysteresis loop of low yield point concentrically braced frames are more than ordinary concentrically braced frames,which suggests that low yield point concentrically braced frames have better energy dissipation performance.In addition,the energy dissipation coefficient of low yield point concentrically braced frames are more than ordinary concentrically braced frames,which also illustrates the hysteresis curve of the low yield point concentrically braced frames are more full.