What Is a Scissor Truss?

The truss in the truss structure refers to the truss beam, which is a kind of latticed beam structure. Truss structures are often used in large-span public buildings such as factory buildings, exhibition halls, stadiums and bridges. Due to the roof structure most commonly used in buildings, trusses are often referred to as roof trusses.

Truss structure

Main structural characteristics

The stress of each member is mainly one-way tension and compression. Through reasonable arrangement of upper and lower chords and webs, it can adapt to the bending moment and shear force distribution inside the structure. Because the horizontal tension and internal pressure achieve self-balance, the entire structure does not generate horizontal thrust to the support. The structural arrangement is flexible and the application range is very wide. Compared with truss beams and solid web beams (that is, the beams we generally see), in terms of bending resistance, because the tension and compression sections are concentrated at the upper and lower ends, the internal force arm is increased, so that the same amount of material, Achieved greater flexural strength. In terms of shear resistance, by properly arranging the webs, the shear force can be gradually transferred to the support. In this way, whether it is bending or shearing, the truss structure can make full use of the material strength, which is suitable for building roof structures of various spans. The more important significance is that it converts the complex stress state inside the solid web beam under the transverse bending into a simple tension and compression stress state in the truss member, so that we can intuitively understand the distribution and transmission of force, which is convenient for the structure. Variations and combinations.

Historical evolution of trusses

The ideal hinge system of an iso-straight bar, which is only affected by node loads, is called a truss structure. It is abstracted and simplified from some engineering structures where the shafts and axes intersect. Trusses were the first to be practical in the construction of wooden bridges and roof trusses. Ancient Romans built tres across the Danube with trusses

Trusses can be classified according to different characteristics.

1. According to the shape of the truss:

1. Parallel chord trusses (easy to lay out double-layer structure; conducive to standardized production, but the rod force distribution is not uniform enough);

2. Folding chord trusses (such as parabolic truss beams, with the same bending moment diagram of simply supported beams under uniformly distributed loads, uniform rod force distribution, economical use of materials, and complex structure)

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

Second, according to the geometric composition of the truss:

1. Simple truss (consisting of a basic hinged triangle in order to increase the binary body);

2. Joint truss (pressed by several simple trusses

The force characteristic is that the internal force of the structure is only the axial force, and there is no bending moment and shear force. This stress characteristic reflects the main factors of the actual structure. 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 non-ideal hinges at the nodes, etc., there are also small bending moments and shear forces (there is no ideal hinge). Force also has a small effect (due to node stiffness and truss bar

Comparison of various beam trusses

Beam trusses can be regarded as evolved from beams. The internal forces of beams of the same span and common beam 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 string truss 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. The internal force of the chord can be expressed as:

F = ± M ° / r

Where M ° is the section bending moment of the corresponding truss joint position of the simply supported beam with the same span, and r is the force arm of the chord internal force to the centroid. Under uniformly distributed loads, the bending moments of simply supported beams are distributed according to a parabolic law, reaching a maximum in the span. Because the force arm of a parallel truss chord is constant, the internal force decreases from mid-span to both ends; the force arm of a triangular truss chord decreases linearly from mid-span to both ends, decreasing faster than M ° according to a parabolic law. Speed, so the internal force of the chord increases from mid-span to both ends. When the upper chord node of the truss is located on a parabola, the lower chord and the horizontal chord's horizontal force on the center of gravity's moment arm are changed according to the parabolic law like M ° . Therefore, the internal force of each lower chord and the horizontal sub-force of each upper chord Equal, so that the internal forces of the windings are almost equal.

The internal force of the vertical rod of the parallel string truss and the vertical component of the inclined beam 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 vertical force of is completely balanced by the axial force of the upper chord, so the internal force of the web is zero; the internal force of the web of the triangular truss increases from the middle to the ends.

1. Truss bridge is a form of bridge.
2. Truss bridges are more common in railways and highways; they are divided into upper and lower chord forces.
3. The truss is composed of upper chord, lower chord, and web. The form of web is divided into oblique web and straight web. Because the bars themselves are relatively long, although the connection between the bars may be "fixed", However, the actual bending moment at the rod end is usually very small, so the design analysis can be simplified to "hinge". When simplifying the calculations, the rods are all two-force rods that can withstand pressure or tension.
4. Because the bridge spans are large, and the rigidity of the single-frame truss is "out-of-plane", the support is needed for "out-of-plane". When designing a bridge, the "out-of-plane" design is generally designed as a truss, so that the bridge forms a whole with good stiffness in both directions.
5. Some bridge decks are set on the upper chord, so the force is mainly transmitted through the upper chord. Some bridge decks are set on the lower chord. Due to the out-of-plane stiffness requirements, the upper chords still need to be connected to reduce the calculated length of the upper chord.
6. The chord of the truss is relatively stressed in the middle of the span and gradually decreases toward the support; the load of the web is mainly the largest in the support attachment, and the force of the web is relatively small in the mid-span. The theoretical "zero shot". ^[1]