What Is a Steel Truss?

Steel trusses refer to roof truss beams, bridges, and hydraulic gates of truss industry and civil buildings made of steel. Common steel trusses are used as the main load-bearing members. Various types of towers, such as mast towers, television towers, and transmission line towers, are commonly used in space steel trusses consisting of three-sided, four-sided, or multi-sided planar trusses.

Steel truss

The most commonly used is a flat truss, which is essentially a lattice beam under lateral loads. Compared to steel trusses and solid web beams,

Steel trusses are often classified according to mechanical sketches, shapes and structural characteristics.

Divided into simple supported and continuous according to the simple diagram of mechanics; statically deterministic and hyperstatically determinate, plane and spatial. Simple supported steel trusses are the most widely used.

According to the shape, it can be divided into triangle, trapezoid, parallel chord and polygon. Triangular steel trusses are often used for roof trusses with steep slopes (Figure

Steel truss connection method

Steel trusses can be welded, ordinary bolted, high strength bolted or riveted. Welding is the most widely used; ordinary bolted joints are often used for detachable structures, transmission towers and support systems; high-strength bolted joints are often used for site connection of heavy steel trusses; riveting is used for heavy steel trusses subjected to large dynamic loads. Replaced by high-strength bolted connections.

High span ratio of steel truss

The height of the steel truss is determined by economic, stiffness, use and transportation requirements. Increasing the height can reduce the cross section and deflection of the chord, but increase the amount of web and the height of the building. The height-span ratio of steel trusses is usually 1/5 1/12; steel trusses with high steel strength and rigid requirements should adopt relatively high values. The height of the triangular steel roof truss is usually determined by the roof slope height; generally, when the roof slope is 1/2 to 1/3, the height-span ratio is corresponding to 1/4 to 1/6.

Steel truss web system

The web system of steel truss is usually in the form of herringbone or monoclinic. Herringbone webs have fewer webs and nodes and are widely used; in order to reduce the internode size of a chord under load or a chord under pressure, some vertical bars are usually added. Single inclined web members are usually arranged to make longer inclined rods stretched and shorter vertical rods compressed, sometimes used for steel trusses with large spans. If you need to further reduce the length of the chord and web, you can use the split web system. When the height of the steel truss is large and the internodes are small, the K-type or diamond web system can be used. In supporting trusses and towers, a cross-type web member system that can better withstand changing loads is often used. Crossed inclined rods are usually designed as tie rods. The inclination of the inclined web to the chord is usually in the range of 30 ° to 60 °.

Force characteristics of steel truss

The section centroid axis of each member of the steel truss should meet at one point at the joint. The calculation of the internal force is generally based on the hinged truss. When the truss is only subjected to the joint load, all members are only subject to axial tensile or compressive forces; if the load is also carried in the section of the member, the member will be bent at the same time. Steel truss members are generally thin, and local bending moments should be avoided or reduced as much as possible when arranging the joints. For steel trusses with a large ratio of section height to length of the members, the secondary stress of the members due to joint stiffness should be considered when necessary.

Steel truss support system

In order to ensure the rigidity and stability of the flat steel truss outside the truss plane, to reduce the calculated length of the chord outside the truss plane, and to support possible lateral loads, the steel truss should be arranged laterally (Figure 2). The supports can be generally divided into horizontal supports (upper and lower chord planes, horizontal and vertical), vertical supports (both ends and middle of the truss), and tie rods. The pair of steel trusses can be arranged with horizontal horizontal supports along the lower chord and upper chord planes respectively, and vertical supports can be arranged at the ends of the steel truss and in the middle of the vertical rod planes at appropriate distances. There are many steel trusses in the roof structure. The horizontal and vertical supports of the upper and lower chords can be set only at the two ends and between two adjacent trusses at a certain distance. The remaining trusses are only provided with tie bars at the appropriate distance between the upper and lower chords. When there is a heavy crane or if necessary, a longitudinal horizontal support can also be added between the lower chord end sections of the truss. In four-sided or multi-sided towers, transverse partitions should be set at certain heights to ensure the stiffness of the tower and the geometric invariance of the cross section.

Section design of steel truss members

The section form of the steel truss member is selected according to the conditions of saving steel, convenient connection and simple manufacturing, and pay attention to make the slenderness ratio (the ratio between the calculated length of the member and the radius of section rotation) of the members as close as possible. Steel truss tie rods shall meet the requirements of strength and allowable slenderness ratio; compression rods shall meet the requirements of strength, stability and allowable slenderness ratio.

When calculating the strength and stability of the rod, the internal force is considered according to the axial force; when the rod is subjected to both the axial force and the bending moment, the joint effect should be considered according to the eccentric force. When calculating the stability and slenderness ratio of the members, the two directions of the truss plane and the out-of-plane direction, or the unfavorable direction with a relatively large slenderness, should be considered. The allowable slenderness ratio of the members is determined according to the conditions under which the members are compressed or stretched, subjected to static or dynamic loads.

Steel truss arching

For steel trusses with a relatively large span, in order to offset all or part of the deflection under the load and load, it is usually required to pre-arch when manufacturing. The arch (f) of the roof truss is generally 1/500 of the span.