The Principle Of Truss

The characteristic of force is that the internal force of the structure is only axial force, but there is no bending moment and shear force. This force characteristic reflects the main factor of the actual structure, and the axial force is called the main internal force of the truss. In actual structures (such as reinforced concrete roof trusses, riveted (bolted) or welded steel truss bridges), due to the non-ideal hinge of the nodes, there are also small bending moments and shear forces at the same time (ideal hinges do not have), and the axial The force also has a small effect (depending on the stiffness of the joint and the ratio of the cross-sectional area of the truss rod to the moment of inertia, generally reduced by 5% to 0.1%), which is called the secondary internal force.

Considering the balance of each node of the truss, the node is subjected to the action of the converging force system, and the projected equilibrium equation of each node is established successively, and all the unknown rod forces can be obtained. This method is called the node method, which is most suitable for simple trusses. When solving, it is advisable to first determine the zero rod according to the composition characteristics, and avoid solving simultaneous equations as much as possible. Sometimes only a few member internal forces are required or for joint trusses and complex trusses, the section method is required when the joint method cannot work. Selective truncation of rods (generally no more than three rods) takes the part of the truss as the balance object. Considering the balance of any part, the required axial force of the rod can be obtained from the balance equation. For some trusses, such as K-trusses, it is more efficient to apply the joint and section methods together. For complex trusses or space trusses with many members, the computer method is the best option.