Study On Secondary Stress And Numerical Analysis Of Steel Space Grid Roof

Space grid is a sort of preferable roof type for long span buildings. It has many advantages, for example forces are transferred in a 3D manner, save steel material,large rigidity,small building highness,industrialized, beauty sculptect. There are tremendous buildings adopt space grid structure as roof domestic and abroad.The emphasis is put on the theoretical study and numerical analysis about the secondary stress of steel space grid roof. At present, In ordinary steel structure design of Space grid roof, the method that we adopt generally is to calculate according to ideal pin-connected model, thus draw the sectional stress of each chord, and we can choose the type of suitable chord according to these sectional stress. While calculating the internal forces of the Space grid, generally the basical assumptions are adopted as follows:(1) The nodes are pin-connected; (2) The axises of the poles are straight and intersect at the center of node; (3) the action line of the load passes the node of space grid.If the space grid accords with the above-mentioned assumptions totally, its internal force is axial force only, and the stress caused by axial internal forces of the chord is called direct stress. But in the factual structure, these assumptions are not correct absolutely, in practice,the chord of space grid is bending evidently,explanating the chord having secondary stress. Generally speaking, the joint is difficult to be deemed to pure pin-connected, and the tip of the chord has certain rotating restriction more or less. In this kind of space grid structure, besides the axial stress, the production of the bending stress is still unavoidable, and this secondary stress is produced by nodal rigidity. In addition, if the axises of the chords don't intersect in one point, the corresponding extra bending moment will produce because of the influence of the eccentricity, and this kind of moment in the paper is named as secondary moment, which is caused by eccentricity of node. Because of the existence of secondary stress, the sectional stress of the space grid is changed. In base of Code for design of steel structures,advance the criteria and unstability condition inconsideration of secondary stress.The essay build pin-connected, rigid-connected and semi-rigid-connected infinite model,forming the whole stiffness matrix. In the base of M-θcurve, tanφ=M/θ,α=2φ/π,αis named semi-rigid coefficient,ifφ=0,then M/θ=0,α=0, the node is pin-connected;ifφ=π/2,then M/θ=+∞,α=1, the node is rigid-connected;if 0<φ<π/2, then 0