管桁架结构设计

 Tube truss is a lattice structure composed of round rods connected with each other at the end. Compared with the traditional steel joist with open section (H-beam and I-beam), the material of the cross-section of tube joist structure is more uniformly distributed around the center and axis, so that the cross-section has good compressive and bending and torsion bearing capacity as well as large stiffness at the same time. This kind of steel structure doesn't need node plate, simple structure, easy to make and install, good structural stability and large roof rigidity. Space triangular steel tube truss shows the force mechanism of shear resistance of web bar and bending resistance of chord bar when it is subjected to vertical uniform load. The main influencing factor of axial force of chord bar is the height of section, while the main influencing factor of axial force of vertical diagonal web bar is the inclination angle between vertical web bar and vertical straight line. Horizontal web bars are subjected to smaller forces under vertical loads, but if they are subjected to significant torsional moments, consideration must be given to appropriately enlarging their cross-sectional dimensions.

General Calculation Principle. Pipe truss structure should be calculated for internal force and displacement under gravity load and wind load, and should be calculated for displacement and internal force under earthquake, temperature change, bearing settlement and construction and installation load according to specific conditions. The internal force and displacement can be calculated according to the elasticity theory, using the finite element method of space rod system. For non-seismic design, the effect of the role and the combination of the role should be calculated according to the current national standard "Building Structural Load Code". In the design of rod section and node, the design value of internal force shall be determined according to the effect of basic combination of roles. For seismic design, the effect of seismic combination shall be calculated according to the current national standard "Code for Seismic Design of Buildings". In displacement checking, the deflection shall be determined according to the effect of standard combination of action (not multiplied by load sub-coefficient). When analyzing pipe truss, the nodes can also be assumed to be articulated when the ratio of internodal length of rods to section height (or diameter) is less than 12 (main pipe) and 24 (branch pipe). External loads can be in accordance with the principle of static equivalence of the node within the area under the jurisdiction of the node load concentration on the node. When there is a local load on the rod, the effect of local bending stress should be considered separately. Structural analysis, should consider the upper space grid structure in the lower supporting structure of the mutual influence. In addition, reasonable boundary constraints should be determined according to the structural form, the position, number and structure of the support nodes and the rigidity of the supporting structure. Boundary constraints of the bearing nodes, should be used according to the actual structure without side shift or one side can be shifted side of the articulated bearing or elastic bearing.

      Static force calculation. Pipe truss structure should be designed after displacement and internal force calculation, if the cross section of rod needs to be adjusted, it should be re-designed so as to meet the design requirements. After designing, the rods should not be replaced. If the rods have to be replaced due to the difficulty of material preparation, the replacement should be carried out according to the principle of cross-section and stiffness equivalence, and the replaced rods should not be the main stress rods of the structure and the quantity should not be too much (usually not more than 5% of all the rods), or else it should be recalibrated. To analyze the internal force of pipe truss structure due to temperature change, the solid end reaction force of rods caused by temperature difference can be reversed as equivalent load on the nodes at both ends of rods, and then analyzed by finite element method.

      Seismic calculation. Under the single-dimensional seismic action, the revitalization decomposition reaction spectrum method can be used when carrying out the calculation of the effect under the action of multiple earthquakes, and the time-range analysis should be used to supplement the calculation for the large-span structures with complex or important body shapes. When adopting the time-course analysis method, not less than two groups of actual strong earthquake records and one group of artificially simulated acceleration time-course curves shall be selected according to the category of the building site and the design seismic grouping, and the average seismic effect coefficient curve shall be consistent with the seismic effect coefficient curve adopted in the revitalization decomposition reaction spectrum method in the statistical significance. The peak acceleration curve shall be adjusted according to the maximum value of the acceleration time-range curve of the multiple earthquakes corresponding to the seismic protection intensity, and a sufficiently long duration of ground shaking shall be selected. When the seismic action analysis of pipe truss structure is carried out by vibration decomposition reaction spectrum method, it is recommended to take at least the first 25-30 vibration shapes, and more vibration shapes are needed to combine the effects for the body shape which is especially complicated or important. In seismic analysis, the influence of supporting system on its force should be considered. At this time, the truss structure and the supporting system can be considered at the same time and calculated according to the overall analysis model; the supporting system can also be simplified as the elastic bearing of the pipe truss structure and calculated according to the elastic bearing model. When analyzing the effect of seismic action of structure, for the pipe truss structure with peripheral landing, the damping ratio can be taken as 0.02, and for the pipe truss structure supported by concrete structure, the damping ratio can be taken as 0.03. For the pipe truss structure with complicated body shape or large span, it is appropriate to analyze the effect of multi-dimensional seismic action. For multi-dimensional seismic effect calculation, multi-dimensional random vibration analysis method, multi-dimensional response spectrum method or time course analysis method can be adopted.