What Is a Girder Truss?

Héng (truss): A structure in which rods are hinged to each other at both ends. A truss consists of a straight bar, generally a flat or spatial structure with triangular elements. The truss members mainly bear axial tension or pressure, so that the strength of the material can be fully utilized. When the span is large, it can save materials and reduce the weight and Increase stiffness.

The word "héng" is pronounced "héng". Because the word "truss" is used less frequently, it is mistakenly pronounced as "háng" (line), hence the name "line frame". Definition of truss: A support beam structure formed by welding, riveting or bolting of rods is called a "truss".

The advantage of a truss is that the members mainly bear tensile or compressive forces, which can fully play the role of materials, save materials, and reduce the weight of the structure. Commonly used steel trusses, reinforced concrete trusses, prestressed concrete trusses, wood trusses, steel and wood composite trusses, steel and concrete composite trusses. ^[1]

Chinese name: truss
Foreign name: truss

Features: Lattice load-bearing structure
Application: Bridges, transmission line towers, crane frames, etc.
Applied discipline: Water conservancy technology; engineering structures, building materials, etc.

Truss theory

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

Plane truss

The axis of the members that make up the truss and the external forces are on the same plane (Figure 1). A plane truss can be thought of as adding members to a basic triangular frame. For every two rods added, a new node must be formed in order to maintain the geometry of the structure. This type of truss that can maintain geometrical rigidity is called a truss (or no redundant rod) truss. If you only add members without adding nodes, you cannot maintain the geometric robustness of the truss. This kind of truss is called a redundant (or redundant) truss.

There are two methods for analyzing the force of statically determinate plane trusses:

Section method

Nodal method

Maxwell-Cremona

Space truss

The axes of the members that make up the truss and the external forces to which they are subjected are not on the same plane. In engineering, some space trusses cannot be simplified as plane trusses, such as grid structures. Towers, cranes, etc. The nodes of the space truss are smooth ball hinge points. The axis of the rods passes through the ball hinge center of the joint point and can rotate around any axis of the ball hinge center. Each node has three degrees of freedom in space. The relationship between the number of nodes and members is W = 3 j - n , W > 0 is a geometrically variable truss, and W = 0 is a geometrically invariant space without unnecessary constraints. The space truss is the same as the plane truss, and the internal force on all members in the truss can be obtained by the partial cutting method and the nodal method. The partial cutting rule is to find the internal force of each rod by using six equilibrium conditions of arbitrary force system in space. The nodal method is to intercept the nodes as insulators, and use the three equilibrium conditions of the spatial convergence force system that each node receives to find the internal force of each rod. ^[2]

Classification of truss structures

various types From the perspective of mechanics, when the truss shape is similar to the bending moment diagram of the simply supported beam, the axial force of the upper and lower chords is evenly distributed, the axial force of the web member is small, and the material is most economical. From the perspective of materials and manufacturing, the wooden truss is made into a triangle The steel truss adopts trapezoidal or parallel chord shape, and the reinforced concrete and prestressed concrete truss are polygonal or trapezoidal.

The ratio of the height of the truss to the span. Generally, the three-dimensional truss is 1/12 ~ 1/16, the three-dimensional arch is 1/20 ~ 1/30, and the tensioned three-dimensional arch is 1/30 ~ 1/50. And specifications. The truss has a wide range of uses. When selecting the truss type, the truss' use, materials, support methods, and construction conditions should be considered. The principle of selecting the best form is to meet the requirements for use, and strive to manufacture and install materials and materials. The amount of labor is minimal.

Triangle truss

truss Under the nodal load evenly distributed along the span of the triangular truss, the axial force of the upper and lower chords is the largest at the end points and gradually decreases toward the mid-span; the axial force of the webs is opposite. Triangular trusses are mostly used in roof trusses of tile roofs due to the large difference in internal force of chords and insufficient material consumption.

Trapezoidal truss

Compared with the trapezoidal truss and the triangular truss, the force of the rod is improved, and it can more easily meet the technical requirements of some industrial plants when used in roof trusses. If the upper and lower chords of the trapezoidal truss are parallel, it is a parallel chord truss. The force of the member is slightly worse than that of the trapezoid, but the type of the web is greatly reduced. It is mostly used in bridges and trestle bridges.

Polygon truss

Polygon truss

truss Shaped truss. The upper chord node is located on the quadratic parabola. If the upper chord is arched, the bending moment caused by internode loads can be reduced, but the manufacturing is more complicated. Under the uniform load, the bending moment pattern of the truss is similar to that of the simply supported beam, so the upper and lower chord axial forces are evenly distributed, the web element axial force is small, and the material is the most economical. It is a truss form commonly used in engineering.

Open web truss

The hollow web truss basically adopts the shape of a polygonal truss, without oblique webs, and only connected by vertical webs and upper and lower chords. The axial force distribution of the member is similar to that of the polygon truss, but the bending moment value at the rod end changes greatly under asymmetric load. The advantage is that there are fewer members at the intersection of nodes, which is convenient for construction and manufacturing.

Truss bridge

Truss (13 photos)

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".

Truss product classification

Aluminum truss 1. Fixed truss:

The strongest type of truss has high reusability. The only disadvantage is the higher transportation cost. The product is divided into square tube and round tube.

2. Folding truss:

The biggest advantage is the low transportation cost and less reusability. The product is divided into square tube and round tube.

3. Butterfly truss:

The most artistic type of truss, the shape is strange and beautiful.

4. Ball joint truss:

It is also called ball joint frame, with beautiful shape and good ruggedness. It is also the most expensive type of truss.

Truss process characteristics

Wooden truss 1. Seiko welding: large machines, flow operations, many years of technicians, professional welding, quality assurance.
2, paint process: automotive quality, professional paint room, Seiko paint process.
3, standard processing: large machine positioning, accurate hole location.
4. Leading technology: Has its own rental installation company, front-line research and development, always leading

Truss design requirements

Sufficient strength-no fracture or plastic deformation occurs; sufficient rigidity-no excessive elastic deformation occurs; sufficient stability-no collapse due to a sudden change in equilibrium form; good dynamic characteristics-seismic and wind resistance.

Design requirements for truss: There must be rods that meet the requirements; there must be good connections, including rivets, pins and welds. These involve the type of truss, the size and material of the members, but the first is a static analysis.

Choice of truss form

truss Some reference values such as: cost per square meter, maximum component weight, maximum span, structural form, eave height, etc. These can provide empirical data when determining whether the plant is heavy steel or light steel. National regulations and technical documents do not say that heavy steel, many buildings are light and heavy steel. In order to distinguish the light steel structure of the house, it may be more appropriate to call the general steel structure ordinary steel. Because the scope of ordinary steel structures is very wide, it can contain a variety of steel structures, regardless of the load size, and even includes many contents of light steel structures. The technical regulations for light house steel structures only specify some more specific for their "light" characteristics. Content, and the scope is limited to a single-layer portal frame. It can be seen that the distinction between light steel and heavy steel is not in the weight of the structure itself, but in the weight of the supporting material it bears, and the concept of structural design is still consistent.

Calculation of truss structure

Steel truss For trusses used for building construction, generally only static calculations are performed; for dynamic loads such as wind, seismic forces, moving vehicles, and running machinery, the equivalent static loads multiplied by the dynamic coefficient are calculated; particularly significant dynamic loads Load trusses, such as long-span bridges and aircraft wings, require dynamic analysis based on dynamic loads (see Loads).

Planar trusses are generally calculated based on ideal hinged trusses, that is, assuming that loads are applied to the truss nodes (if the load is applied to the joints, it can be converted to a node load by simply supported beams), and all members of the truss are in the same plane Inside, the center of gravity axis of the rod is on a straight line, and the nodes are hinge points that can rotate freely. In the ideal statically determinate truss, the axial force of the member can be taken as an unknown quantity, and the axial tensile force or pressure of the member under a known load can be obtained according to the static numerical method or the graphical method (see Static Analysis of Rod Structures) ).

The truss joints used in engineering are generally nodes with a certain rigidity rather than ideal hinged joints. The bending stress and axial stress of the members due to the influence of the rigidity of the joints are called secondary stresses. The calculation of the secondary stress needs to consider the axial deformation of the member, which can be solved by the statically indeterminate structure method or the finite element method.

Wooden truss The space truss is composed of several plane trusses, and the load can be decomposed into the same plane as the truss to calculate the component force according to the plane truss or the space articulated rod system using the finite element method.

According to the materials used in the truss members and the calculated internal forces, selecting an appropriate section should ensure the overall stiffness and stability of the truss, as well as the strength and local stability of each member, to meet the requirements for use.

The overall stiffness of the truss is ensured by controlling the maximum vertical deflection of the truss to not exceed the allowable deflection; the out-of-plane stiffness of the plane truss is poor and must be ensured by the support system. The support system includes upper chord support, lower chord support, vertical support and truss to form a space stabilization system.

Examples of truss buildings

Wenzhou Science and Technology Museum The steel structure in the Wenzhou Science and Technology Museum project is divided into six units: 1. The sign tower is a steel structure tower with a height of 62.016 meters. The tower body is a pipe frame structure and adopts a spatial steel pipe structure system. 2. The roof of the middle gallery adopts a spatial steel tube structure system. Directly intersecting welded joints are used between the steel tubes. The main truss is a flat truss. 3. The roof of the North Exhibition Hall adopts a spatial steel pipe structure system. Directly intersecting welding nodes are used between the steel pipes. The main truss is a flat truss. 4. The square quadrangular pyramid grid in the South Hall's bolt ball joint. The roof is made of double laminated color steel plate. 5. Spherical grid with a diameter of 33.5 meters. 6. The steel folding canopy is H-type simple steel structure.