A truss is a geometrically invariant structure composed of triangular frames composed of straight rods. The joints between members are called nodes (or nodes). According to the axis of the truss members and the distribution of the external force, the truss can be divided into plane truss and space truss. Spatial structures such as roof trusses or bridges are composed of a series of parallel plane trusses. If they mainly bear plane loads, they can be simplified as plane trusses for calculation.

The truss in the truss structure refers to the truss beam, which is a latticed beam structure. Truss structures are often used in public buildings such as large-span workshops, exhibition halls, gymnasiums, and bridges. Trusses are also commonly referred to as roof trusses as they are mostly used in roof structures of buildings.

Main structural features of the mid-span

The stress of each member is mainly unidirectional tension and compression. Through the reasonable arrangement of the upper and lower chords and web members, the bending moment and shear force distribution inside the structure can be adapted. Since the internal forces of tension and pressure in the horizontal direction achieve self-balance, the whole structure does not generate horizontal thrust on the support. The structure is flexible and the application range is very wide. Compared with truss beams and solid web beams (that is, beams we generally see), in terms of bending resistance, due to the concentrated arrangement of the tension and compression sections at the upper and lower ends, the internal force arm is increased, so that with the same amount of material, Greater flexural strength is achieved. In terms of shear resistance, the shear force can be gradually transferred to the support by arranging the web rods reasonably. In this way, whether it is bending or shearing, the truss structure can give full play to the strength of the material, so it is suitable for building roof structures of various spans. The more important significance is that it transforms the complex stress state inside the solid web beam under the action of transverse bending into the simple tensile and compressive stress state in the truss member, so that we can intuitively understand the distribution and transmission of force, which is convenient for structural Variations and combinations.

Trusses can be classified by different characteristics.

1. According to the shape of the truss, it is divided into:

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

2. Chord truss (such as a parabolic truss beam, the bending moment diagram of a simply supported beam with the same shape and uniform load, the rod force is evenly distributed, the material is economical, and the structure is more complicated);

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

2. According to the geometric composition of the truss, it is divided into:

1. Simple truss (consisting of a basic hinged triangle followed by two elements);

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

3. Complex truss (different from other statically indeterminate trusses of the first two).

3. According to the horizontal thrust received:

1. Beam truss without thrust (compared with the corresponding solid beam structure, the hollowing rate is large, the upper and lower chords are resistant to bending, the web members are mainly shear resistant, the force is reasonable, and the material is economical);

2. Arch truss with thrust (the arch ring and the upper structure of the arch are integrated into one, which is easy to construct, has strong spanning ability, and saves steel materials).

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 ＆#8203;＆#8203;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.

Comparison of various beam trusses

Beam-type trusses can be regarded as evolved from beams. The internal forces of beams with the same span and common beam-type trusses under the same uniform load are compared as follows. The shape of the truss has a great influence on the internal force distribution of the members. The internal force of the parallel chord chord decreases from the mid-span to both ends, while the internal force of the triangular truss chord increases from the mid-span to the two ends. This is because the truss relies on the internal force of the upper and lower chords to form the section bending moment, and the internal force of the chord can be expressed as:

F=±M°/r

In the formula, M° is the section bending moment at the corresponding truss node position of the simply supported beam with the same span, and r is the force arm of the chord internal force to the distance center. Under the uniform load, the bending moment of the simply supported beam is distributed according to the parabolic law, and reaches the maximum ＆#118alue in the mid-span. Because the moment arm of the parallel truss chord is constant, the internal force decreases from the mid-span to the two ends; the moment arm of the triangular truss chord decreases linearly from the mid-span to both ends, faster than M° decreases according to the parabolic law. speed, so the internal force of the chord increases from the mid-span to the two ends. When the upper chord node of the truss is located on a parabola, the moment arm of the lower chord and the horizontal component force of each upper chord to the centroid changes according to the parabolic law like M°, so the internal force of each lower chord and the horizontal component of each upper chord are the same equal, so that the internal forces of each upper string are also nearly equal.

The internal force of the vertical rod of the parallel chord truss and the vertical component of the inclined rod are equal to the shear force at the corresponding position of the simply supported beam, so it increases from the middle to the two ends; the upper chord of the parabolic truss conforms to the reasonable arch axis, and acts on the upper chord node at this time The vertical force of the truss is completely balanced by the axial force of the upper chord, so the internal force of the web member is zero; the internal force of the web member of the triangular truss increases from the middle to the two ends.

1. A truss bridge is a form of bridge.

2. Truss bridges are generally more common in railways and highways; they are divided into two types: upper chord and lower chord.

3. The truss is composed of upper chord, lower chord and web rod; the form of web rod is divided into inclined web rod and straight web rod; because the rod itself is relatively long and thin, although the connection between the rods may be "fixed", However, the actual rod end bending moment is generally very small, so the design analysis can be simplified as "hinged". When simplifying the calculation, the rods are all "two-force rods", which are subjected to pressure or tension.

4. Since the span of the bridge is large, and the rigidity of the single truss "out-of-plane" is relatively weak, therefore, the "out-of-plane" needs to be supported. When designing a bridge, the "out-of-plane" is generally designed in the form of a truss, so that the bridge forms a whole with good stiffness in both directions.

5. Some bridge decks are set on the top chord, so the force is mainly transmitted through the top chord; some bridge decks are set on the bottom chord. Due to the requirement of out-of-plane stiffness, the top chords still need to be connected to reduce the calculated length of the top chord outside the plane.

6. The chord of the truss is relatively large in the mid-span part, and gradually decreases in the direction of the support; while the force of the web member is mainly in the attachment of the support, and the force of the web member in the mid-span part is relatively small, even some The theoretical "zero pole".