Research On Forming Technology Of Large-span Annular Cable Structures

Large span annular cable structures have distinguishing features as self-equilibrium, besides regular ones of tension structures. So the requirements for substructures are low. Because of the large opening in the center of the structure, it has great adaptability for large structures as stadiums. Plenty of this kind of structures have been built in foreign, while the application has just started in large venues in China. Relevant researches are quite few domestically, especially in the area of shape-forming technique. Shape-forming is the most important point during construction, just as regular tension structures. Thus the in-depth research here is focused on shape-forming of the structures.The process analog simulation is an essential point during the construction of large span annular cable structures. By introducing the nonlinear finite element method, the general software-ANSYS is used to perform the simulation of shape-forming process. And in this process, the fundamental principal, implement steps and elements choosing of the simulation are presented in detail. Later analyzing works is done according to this.It’s priority to choose a proper plan for construction. Large span annular cable structures are classified by different topologies, and different ways on shape-forming are researched based on that. Different kinds of ways are put forward to build prestressing forces by tensioning radial cables reasonably, after combining the cable force distribution, construction efficiency, economy and convenience. Specific ways are as follows:1) For structures with one inner ring of cables and two outer rings, there are two suitable ways. First one, the top radial cables are integrally lifted in position, and then the lower radial cables are also hoisted to their places. The second one, top cables are lifted to a certain place at first, then stretching the lower cables in place, at last the upper ones are put in their places.2) For structures with two inner rings and an outer ring, there are also two methods as above.3) For those with both one ring inner and outer, the method can be simpler, in which several cable points are tensioned at the same time synchronously. Or we can hoist the track cables first and stable cables later. 4) Those with rigid cables of inner rings can also be hoisted as above, except that the initial configuration should be controlled specially because of the rigidness, to prevent the plastic deformation of rigid cables due to uneven structure deformation.5) Stretching ring cables needs to confirm the initial shape of cables. Lifting the brace rods leads to larger deformation and needs a rather long adjustable distance for brace rods, which can be difficult. The methods of structure forming that put forward in the paper are proven reasonable and reliable in engineering practice, and can be used in this kind of projects in the future.Forming process control of structure tensioning is very important. Structure with two inner tension rings and one outer pressed ring is analyzed combining practices. According to the analysis, following conclusions can be made.1) When towing the top radial cable, the displacement of ring cables has a linear relationship with the length of towing cables while the cable forces change little at first. So this process can be controlled by the cables’length in the beginning.In the end, cable forces can be the controlling factor as they change rapidly.2) As to tow lower radial cables, there are three steps. In initial phase, with the changing of towing cables, the forces change quite little while the displacements have risen. The process can be controlled by the length of towing cables. At the middle stage of the tensioning, the forces of towing cables don’t change much, but the displacements do as the cable length changing. The structure can be controlled by the vertical displacement. At last, when the tensioning comes to an end, cable forces can be the controlling factor as the structure rigidity and cable forces both increasing rapidly.It is vital to build prestressing forces on cable structures properly. The error caused by cable length is analyzed in detail here, applying both normal and uniform distributions. The results show that the cable length error has few effects on the cable forces, because of the little rigidity of the cables of large span annular cable structures. Cables with fixed lengths can be used for this kind of structures, especially when the stiffness of the surrounding is very small or the boundary conditions are controlled well.As the configuration of the forming process changes a lot, it’s also a long distance towing process with a long period, so the effect of cable break is simulated mainly on towing process. Analysis shows that:Right after cable break, the forces, acceleration and displacements of the structures all change suddenly and make a big difference on the structure. Those changes can only tend to stability after a long time. Then safety problems will follow up. Large vibration can probably result in safety accidents with the outer beam ring as temporary platform. Thus, the safety of long-distance towing is vital, it should be guaranteed with the safety of towing cables.