Truss: A structure in which members are hinged at both ends. Truss is a plane or spatial structure of triangular elements composed of straight bars. Truss members mainly bear axial tension or pressure, so that when the span is large, the strength of materials can be fully utilized, materials can be saved, self-weight can be reduced, and stiffness can be increased. The word "truss" is pronounced "héng". Because the word "truss" is rarely used, it is mispronounced as "háng" (line). Therefore, "Hangjia" got its name. Definition of truss: The supporting beam structure composed of members connected by welding, riveting or bolts is called "truss". The advantage of truss is that the members mainly bear tension or pressure, which can give full play to the role of materials, save materials and reduce the weight of the structure. Commonly used are steel truss, reinforced concrete truss, prestressed concrete truss, wood truss, steel-wood composite truss and steel-concrete composite truss.

Structural classification-triangular truss: under the action of node loads evenly distributed along the span, the axial force of the upper and lower chords is the largest at the end points, and gradually decreases towards the midspan; The axial force of the web is opposite. Triangular trusses are mostly used for roof trusses with tile roofs because of the great difference in internal forces of chords and unreasonable material consumption. Trapezoidal truss: Compared with triangular truss, the stress of members is improved, and it is easier to meet the technological requirements of some industrial workshops when used as roof truss. If the upper and lower chords of a trapezoidal truss are parallel, it is a parallel chord truss, and the stress of its members is slightly worse than that of a trapezoidal truss, but the types of web members are greatly reduced, which are mostly used in bridges and trestles. Polygonal truss is also called folding truss. The upper chord node is located on the quadratic parabola. If the upper chord is arched, the bending moment caused by the load between nodes can be reduced, but the manufacture is more complicated. Under the action of uniform load, the shape of truss is similar to the moment distribution law of simply supported beam, so the upper and lower chord axial forces are evenly distributed, the web axial forces are small and the materials are the least, so it is a common truss form in engineering. Open-web truss: basically polygonal truss shape, with straight lines between upper chord nodes, no inclined web members, only vertical web members connected with upper and lower chord members. The axial force distribution of members is similar to that of polygonal truss, but the bending moment at the end of members changes greatly under asymmetric load. The advantages are that there are few intersecting members at the nodes, and the construction and manufacture are convenient.