Steel Pipe | H & Box Section | Angle Iron Trusses Design

Steel Truss Structure Brief Introduction

AGICO offers fabrication services of customized steel trusses, which are common prefab steel members for roof, girder, and column fabrication and are applied in large public buildings such as stadiums, stations, and airports.

In recent years, they have also been widely adopted for the station buildings of rapidly developing high-speed railways and intercity rail transit systems. These steel truss structures can be fabricated into long spans and diverse shapes and designs.

Different Truss Types According to Steel Material

A prefab steel truss structure generally consists of chords, web members, and gusset plates. Depending on the steel materials used for the steel truss members, there are basically 4 types: steel pipe trusses, H-section steel trusses, box-section trusses, angle steel / iron trusses. Different types of truss structures have their own characteristics, and corresponding considerations and requirements must be taken into account during detailed design.

Different Prefabricated Steel Truss Types & Designs

Steel Pipe Truss Design Tips

The structural members of a steel pipe truss are round, square, and rectangular steel pipes. The connections between members are mostly intersecting (directly welded) joints.

The main factors to consider during the detailed design of pipe trusses are as follows:

1. The truss members are mostly subjected to axial forces. Eccentricity should be avoided, and the centerlines of the members should lie in the same plane. When multiple members are connected at a joint, their centerlines should intersect at a single point.
2. The external dimensions of the main member (chord) shall be no smaller than those of the branch member (web), and the wall thickness of the main member shall be no less than that of the branch member. When members intersect, the larger pipe shall intersect the smaller pipe, and branch members shall not be inserted into the main member.
3. When members intersect, the angle between them shall not be less than 30 degrees.
4. The weld connecting a branch member to the main member shall be continuous around the entire circumference and shall transition smoothly.
5. When multiple pipes intersect, the intersection sequence must be carefully arranged to ensure that every member can be continuously welded around its full circumference.
6. When a truss is fabricated in sections, the splice points of the upper and lower chords must be staggered. A backing tube (internal sleeve) should generally be provided at the splice point to ensure the quality of the butt weld. Appropriate measures must be taken to prevent the backing tube from hindering the installation of the truss.

H‑Section Steel Truss Design Tips

A prefab H‑section steel truss can have chords and web members both made of H‑section steel, also known as H beams, or chords made of H‑section steel with web members made of round, square, or rectangular steel tubes.

During the detailed design of an H‑section steel truss, the following factors should be noted:

1. For the upper and lower chords of the H‑section steel truss, the splice length of the flange plates of welded H‑section steel shall be no less than twice the plate width; the splice width of the web plate shall be no less than 300 mm, and its splice length no less than 600 mm (follow the design if special requirements are specified).
2. For field splices of the H‑section steel truss, the butt welds of the upper and lower chords shall not be at the same location; they shall be staggered vertically by more than 200 mm.
3. If the cross‑section of the upper or lower chord of the H‑section steel truss needs to be changed, the change point should not be located at a joint (the structural design drawings show the change at the joint). Meanwhile, the cross‑section change point can be combined with the field splice location to reduce the number of butt welds on the chords.
4. During detailed design of an H‑section steel truss, care should be taken to control the overall external dimensions. The fabrication should be carried out using full‑scale lofting to avoid misalignment during field installation.

Box‑Section Truss Design Tips

A prefab box‑section truss can have chords and web members, both of box section, or chords of box section with web members of round, square, or rectangular steel tubes. The connections between web members and chords, and between web members themselves, are mostly intersecting (directly welded) joints.

When both chords and web members are of box section, the following additional factors should be noted:

1. The butt welds of the members must be of good quality, using connection details that facilitate field welding. Typically, a top cover plate or a side-opening cover plate is adopted.
2. The assembly sequence of the box plates and the location of the assembly welds must take into account the installation of internal stiffeners inside the box, ensuring that stiffeners not accessible for electroslag welding can achieve 4‑sided fillet welding through the assembly process.
3. When field‑connecting a box‑section web member to a box‑section chord, it is common practice to provide a stub (short stub member) at the panel point of the chord to simplify field welds and ensure weld quality.

Angle Steel / Iron Truss Design Tips

Angle steel or angle iron trusses are mostly prefabricated and used in roof structural systems, commonly found in industrial plant buildings and power transmission towers. They often adopt a T‑section composed of double angles.

During detailed design, the following aspects should be noted:

1. The truss shall be aligned with the centroidal axes of the members. The centerlines of connecting structural bolts shall be placed as close as possible to the centroidal axes of the members. All axes at a joint shall intersect at one point, and eccentricity shall be avoided where possible.
2. When the same chord uses two different cross‑section materials, the cross‑section change shall occur at a panel point. The gravity line of the member shall coincide with the centroidal line of the truss. If the top fibers are required to be at the same elevation, steel plates may be used for leveling.
3. Except for the support panel points, other gusset plate connections should use plates of the same thickness. Support gusset plates shall be at least 2 mm thicker than other gusset plates.
4. The shape of gusset plates is generally rectangular or trapezoidal, with two sides parallel to each other as a usual requirement. Gusset plates generally extend 10–15 mm beyond the edges of the angle steel.