| Passive flexible barrier is an important rockfall interception structure,usually made of a light structure in which some posts keep in position a steel mesh(diamond wire net or ring-net)able to intercept the moving block and to transfer the stopping forces by steel cables to tendons anchored to the ground.Some of the proposed layouts are also provided with ``energy dissipator'' systems(breakring in this paper)able to dissipate part of the kinetic energy of the block by friction effect or by permanent deformation of their own structure and able to allow large displacements of the structure.The flexible passive network has the advantages of convenient construction and friendly environment,and is suitable for the protection of rockfall hazards in mountain traffic arteries.However,the flexible passive network structure has not yet formed a mature and effective design method.The existing design method in practical engineering is to configure the components by experience first,and then verify by experiment.Due to the lack of design method,a large number of damage phenomena occur in the practical engineering application,such as buckling of supporting steel column,failure of starting of pressure reducing ring,damage of metal mesh,breakage of steel rope,failure of anchorage of steel rope,etc.The uniqueness and complexity of the structure are the main reasons leading to the difficulty in designing and calculating flexible passive networks.The uniqueness of the flexible passive network is reflected in its protection principle: the system reduces the impact force and dissipates the kinetic energy of rockfall through large deformation.The complexity of the structure is embodied in the realization of this protection principle.A large number of flexible components and complex connections are used in the structure.The impact interception process involves the deformation of flexible components,the cooperative motion between components,and the start-up and deformation of energy dissipator.In order to make the structure work effectively,the rational allocation of components and the calculation of structural design are very critical.However,due to the complexity of the structure,the interception analysis of flexible passive nets involves many highly nonlinear problems,such as dynamic impact,large deformation and contact slip.It is difficult,inefficient and costly to carry out refined calculation and analysis(specifically,according to the actual geometry and connection relationship of components,using refined element modeling for numerical calculation and analysis).Therefore,it is very important to establish an equivalent substitution model for efficient calculation and analysis in practical engineering applications.In order to solve the problem of efficient calculation of flexible passive network,the deformation and energy dissipation mechanism of flexible passive network structure,the deformation law of ring network members,the calculation model of ring network and the efficient and fast calculation method of the whole structure are studied by means of component test,full scale impact test and finite element numerical simulation analysis.The main contents and conclusions are as follows:1.A flexible passive network with protection level of 1500 k J was designed,full-scale impact test of the whole structure was carried out.The breakring was arranged outside the span of the structure,the supporting rope passed through the end of the column by setting a transition rope.The process of impact interception deformation and the time history of main cable force were obtained in the test.Through the analysis of the impact interception process and the final tensile state of the pressure reducing ring,the results show that the flexible passive network has a clear two-stage working mechanism of flexible large deformation-energy dissipation large deformation.The starting deformation sequence of the components in the structure was the ring net,the support rope,and the anchor rope.The deformation of ringnet provided the flexible large deformation of the structure,and the deformation of support rope and anchor rope due to the elongation of break ring provided the large deformation of energy dissipation of the structure.2.The impact deformation process and final deformation form of annular mesh in full-scale impact test were analyzed.The regionalized deformation characteristics of ring net and three typical deformation states of net-ring(the basic unit of annular mesh)were obtained.The test and calculation of typical deformation net-ring were carried out,and the net-ring constitutive relation was obtained.The deformation characteristics of ring net under fixed boundary(edge net rings were fixed)and "flexible supporting" boundary(edge net rings could slide)were calculated and analyzed in detail.The relationship between the supporting boundary and the deformation characteristics of the ring network was obtained.Based on the calculation results,the rules of zoning the ring net under the condition of "flexible support" were proposed.Combined with the constitutive relation of net ring and the division rule of regional deformation of ring net,a simplified calculation method of deformation of ring network at rock fall impact point was proposed.3.A "X" shaped 4-truss element connected to the center of the circle was used to simulate a single ring.The initial length of the element was the radius of the ring.The relationship between load and displacement of net ring under typical deformation condition was equivalent to stress strain relationship of truss element.The whole ring net element model was established by connecting truss elements to corresponding nodes.The stress-strain relationship of truss element was given according to the deformation area.The connection mode between the ring net and the supporting rope was simulated by setting the slip-contact mode.In this way,a high efficiency subarea equivalent substitution model for the whole ring net was established.The accuracy of the substitution model was verified by comparing the results with the refined model.The load-displacement relationship and the main performance parameters were obtained by tensile test of the break ring.The equivalent model of a single truss element of the break ring was established by equivalent constitutive relation.Based on the equivalent substitution model of ring net and the truss model of break ring,a high efficiency calculation model of the whole structure was established.The numerical simulation of full-scale impact test of the whole structure was carried out by using the model.The calculated results show the periodic deformation characteristics consistent with the test results.The dynamic response results of the main components were in good agreement with the test results.4.Based on the high efficiency calculation model of the whole structure,the effects of the arrangement of the break ring in support rope,the preset slip length of the support rope and the arrangement of the pressure reducing ring in anchor rope on the structural performance were studied.Through calculation and analysis,it is found that there is a "traction effect" in suppor rope motion.The causes and main influencing factors were also analyzed.The start-up load range of energy dissipation units with protection energy level of 1000-3000 k J was determined by calculation.The starting condition of the anchor rope breakring set was determined.According to the results of calculation and analysis,the design proposal for specific configuration was put forward. |
PDF Full Text Request