The truss in the truss structure refers to the truss beam, which is a kind of beam structure with lattice structure. Truss structure is often used in large-span factory buildings, exhibition halls, stadiums and Bridges and other public buildings. Because of the roof structure that is mostly used in buildings, trusses are often referred to as roof frames.

Main structural features of the span

The stress of each member is mainly unidirectional tension and compression, and the distribution of bending moment and shear force inside the structure can be adapted by reasonable arrangement of the upper and lower chord and the belly bar. Because the tension and compression forces in the horizontal direction achieve self-balance, the whole structure does not produce horizontal thrust on the support. The structure layout is flexible, and the application range is very wide. Compared with the solid belly beam (that is, the beam we generally see), in terms of bending, the tension and compression sections are concentrated at the upper and lower ends, and the internal force arm is increased, so that the same amount of material is used to achieve greater bending strength. In the aspect of shear resistance, the shear force can be transferred to the support step by step by rationally arranging the belly bar. In this way, whether it is flexural or shear resistance, the truss structure can give full play to the strength of the material, so as to be suitable for various spans of building roof structures. The more important significance is that it can transform the complex stress state in the solid belly beam under the action of transverse bending into the simple tension and compression stress state in the truss member, so that we can intuitively understand the distribution and transmission of the force, and facilitate the change and combination of the structure.

Trusses can be classified according to different characteristics.

First, according to the shape of the truss is divided into:

1. Parallel string truss (easy to arrange double-layer structure; Conducive to standardized production, but the rod force distribution is not uniform enough);

2. Folding string truss (such as parabolic truss beam, bending moment diagram of simple supported beam under uniform distribution load, uniform distribution of rod force, economical use of materials, complex structure);

3. Triangular truss (rod force distribution is more uneven, structural layout is difficult, but the slope meets the needs of roof drainage).

Second, by the geometric composition of the truss:

1. A simple truss (composed of a basic hinged triangle successively adding binary bodies);

2. Joint truss (composed of several simple trusses according to the simple composition rules of the geometrically invariant system);

3. Complex trusses (different from other statically determinate trusses of the first two types).

Three, according to the horizontal thrust points:

1. No thrust beam truss (compared with the corresponding solid beam structure, the hollowing rate is large, the upper and lower chord is flexural, the belly rod is mainly shear resistant, the force is reasonable, and the material is economical);

2. Arch truss with thrust (arch ring and arch structure are integrated as a whole, easy to construction, strong crossing ability, saving steel materials).

The force characteristic is that the internal force of the structure is only the 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. Actual structure (such as reinforced concrete roof truss, riveted (bolted) or welded steel truss bridge) due to the joint of the non-ideal hinge and other reasons, there is also a small bending moment and shear force (ideal articulation does not), the axial force also has a small impact (due to the joint rigidity and truss rod cross-sectional area and the size of the inertia moment ratio, generally reduced by 5% to 0.1%), known as the secondary internal force.

Considering the balance of each node of the truss, which is subjected to the action of the confluence force system, the projection balance equation of each node is established step by step, and all the unknown bar forces can be obtained. The null bar should be determined first according to the composition characteristics, and simultaneous equations should be avoided as much as possible. Sometimes only a small number of internal forces are required, or for joint trusses and complex trusses, the section method is required. Selectively truncated members (generally no more than three members) take the local part of the truss as the balance object, and consider any part of the balance, the required axial force of the member can be obtained from the balance equation. For some trusses (such as K-trusses), it is more effective to apply the joint method and the section method together. For complex trusses or space trusses with many members, the best choice should be the computer method.