Study On The Failure Mechanism Under Strong Earthquake Excitation And Isolation Of Space Double-layer Lattice Structure

Since the establishment of the People’s Republic of China(P.R.C.), large span space structure was widely used in China with the economic surge, especially the widely application of space double-layer lattice structure for its safety and economy. However, two typical space double-layer lattice structures, Lushan middle school gymnasium and Lushan gymnasium, located in Lushan county were suffered great damage during the Lushan earthquake which occurred on April 4, 2013. The former was reconstructed and the later was consolidated which aroused much attention on the seismic behavior and design theory of space double-layer lattice structure. In this paper, the main work is as follow:First of all, the main reason for the severe damage of the above two gymnasiums was the strong earthquake motion which was beyond the design fortification intensity based on the seismic damage investigation and the distribution of the strong motion earthquake stations in the Lushan earthquake. On the other hand, the insufficient earthquake resistance ability of the space double-layer lattice structure was the inner reason for the severe damage. The typical failure patterns including bearing failure, member buckling and connection bolts failure after systematical summary and discussion of the earthquake damage in this area.Secondly, the numerical simulation of these two gymnasiums under real motions excitation was conducted. The results show that coupling effects between the lower supporting structure and the upper double-layer roof structure including acceleration amplification effect, filtering effect, the internal force redistribution caused by supports and the inconsistent effect on the displacement input at the support. Therefore, the integrated modeling which could consider the coupling effects is recommended during analysis and design process.Thirdly, the link element can not consider the effect of member dynamic buckling which caused the overvaluation of the seismic capacity of the space double-layer lattice structures during the analysis process. Therefore, two measures, one is to find an analysis method which can consider the member dynamic buckling effect and the other is to invent a new 3-D isolator that can reduce the seismic effect are studied in this paper.The shake table test on the scaled space double-layer lattice structure was carried out. The test model was designed by the current code system and the characteristics of these two space double-layer lattice structures. The results show that the failure of the test model is caused by member buckling under the dynamic amplification effect of the lower supporting frame structure.Actually, the member dynamic buckling is the bending effect of axial compressive force applied on the member. Thus, the modeling method that considers the steel failure constitutive model and curve segmented beam-element considering the dynamic buckling effect has been proposed. In the method, the curve shape of the beam is used to simulate the initial bending of member and the initial eccentricity of joint; the plastic kinematic material model is used to simulate the material failure under axial force and bending moment effect.The compared results between the shaking table test and the six groups of finite element models including the link elements, straight line segmented beam and curve segmented beam show that the minimum analytical error of finite element is the curve segmented beam-element model. Furthermore, the curve segmented beam model including failure constitutive model of steel and accidental eccentric is most accurate after application the six models to simulate these two real structures. Therefore, the curve segmented beam model is recommended when conducting the seismic evaluation of the space double-layer structure under severe earthquake.A new kind of 3-D large displacement isolators are designed and manufactured to reduce the seismic effect of the space double-layer lattice structures. And then the mechanical model and the corresponding mechanics parameters of these isolators are determined based on the test. Furthermore, the principle of virtual work has been used to derive the equations of the mechanical parameters and element parameters of the isolators. The parameters under different spring stiffness and geometry dimensions can be acquired from the equations. At last, the compared results between the numerical simulation and test show that the influence factor of the lateral stiffness is complex. Therefore, the semi empirical and semi theoretical equations of the horizontal stiffness and the component parameters based on the test results are presented in this paper.The 3-D isolator has significantly reduced the dynamic responses of the structure through sacrificing the interlaminar deformation and protected the upper space double-layer lattice structure based on compared test results with or without the application of the isolator. The model using the isolator still kept elastic state and the model without the isolator occurred collapse which demonstrated the obvious isolation effect of the isolator.At last, a simplified 3-D liner spring model of the isolators was proposed to apply this isolator to the actual structure and avoid the complication of nonlinear analysis. The complexity of the mechanical property of the isolated bearing after reaching the limit displacement, especially, the difference between Rayleigh damping model and friction damping was investigated based on the comparison results of the finite element analysis and test. The simplified model of the isolator is acceptable under normal conditions. However, the nonlinear spring model based on test data should be used under extremely motions excitation, especially, in the case of horizontal collision of isolated bearing at the limit state.