Steel Purlins

Purlins, also known as purlins and girder, are horizontal roof beams perpendicular to the roof trusses or rafters. They are used to support rafters or roofing materials. Purlins are transverse bending members and are generally designed as single-span simply supported purlins.

Solid purlin

1. Channel Steel Purlins
Channel steel purlins are divided into two types: ordinary channel steel purlins and lightweight channel steel purlins. Ordinary channel steel purlins, due to their thicker profile, do not fully develop their strength and require more steel. While lightweight channel steel purlins offer some improvements over ordinary channel steel purlins, they are still not ideal.
2. Angle Steel Purlins
Angle steel purlins. Ordinary angle steel purlins are easier to source, but they lack rigidity and require more steel. They are only suitable for applications with smaller spans, purlin spacing, and loads.
3. Composite Channel Steel Purlins
Combined channel steel purlins are welded together from two angle steels. They use less steel than ordinary channel steel, but require more welding work. They are suitable for spans ≤ 4m.
4. Composite Z-Shaped Steel Purlins
Combined Z-Shaped Steel Purlins are similar to composite channel steel purlins and are also welded together from two angle steels. When the roof slope i ≥ 1/3, it bears more load than a channel steel section.
5. Hot-rolled Thin-Walled H-Beam and High-Frequency Welded Thin-Walled H-Beam Purlins
Hot-rolled thin-walled H-beam and high-frequency welded thin-walled H-beam are a new type of lightweight steel with advantages such as thin webs, good bending stiffness, relatively close moments of inertia along the two principal axes, and straight flanges for easy connection.

Hollow purlin

Hollow purlins are made of angle steel (upper and lower chords) and tie plates welded together. Their main feature is that they use less steel and can make rational use of small angle steel and thin steel plates. However, due to the dense spacing between tie plates, the workload of assembly and welding is large, so they are less used.

Lattice purlin

Plane truss purlin
Commonly used plane truss purlins can be divided into two categories: one is made of angle steel and round steel; the other is made of cold-bent thin-walled steel.
1. Angle steel and round steel plane truss purlin
The upper chord of the purlin is placed in a V-shape with angle steel. This type of purlin has poor lateral stiffness, but it is easy to obtain materials and has clear force. It is suitable for roof loads or roofs with small purlin spacing. Due to the small purlin spacing, the purlin must be passed through when installing the upper chord support, which makes construction more troublesome. In order to solve this contradiction, the purlin with the upper chord support bay can be changed to a solid-web purlin.
2. Cold-bent thin-walled steel plane truss purlin

Cold-bent thin-walled steel plane truss purlins are divided into two categories.

(1) All the members of the purlin are cold-bent thin-walled steel. It is suitable for roofs with large purlin spacing. It saves steel, has clear force, and has large stiffness both inside and outside the plane.

(2) The main parts of the purlin, the upper chord and the end vertical compression members, are made of cold-bent thin-walled steel, and the remaining members are made of round steel. In order to enhance the stability of the purlin, the end compression web members are preferably made of square tubes. It is mostly used for roofs with a purlin spacing of 1.5m. Compared with the previous flat truss purlin, this type of purlin has basically the same force performance, but is more convenient to obtain and manufacture.

T-shaped truss purlin

The upper chord of the purlin is two small angle steels placed separately, and the web and lower chord are round steel. Its single-piece web system forms a T shape with the upper chord system, so it is called T-shaped truss. Diagonal tie bars and straight braces should be set between the two angle steels of the upper chord to form a truss system to enhance the lateral stiffness of the purlin. To ensure the relative position of the web and top chord planes during loads and to resist partial torque, three to four steel hoops should be installed along the purlin span. The diameter of the round steel hoops should not be less than 10mm. The calculation method for T-truss purlins is the same as for flat truss purlins. The two angles of the top chord are approximately lumped together within the web plane for calculation.

Bracketed Purlins

Bracketed purlins consist of a top chord, two vertical braces, and a bottom chord. They are a statically indeterminate structure. Their main features include reducing the calculated span within the top chord plane, requiring fewer members, and facilitating fabrication. Top chords can be arranged in either vertical or horizontal configurations. The vertical configuration offers greater in-plane stiffness and a more economical top chord cross-section, but suffers from poor lateral stiffness, requiring special attention to prevent torsion during installation. The horizontal configuration offers greater lateral stiffness and facilitates installation, but requires slightly more steel. Bracing is required for both vertical and horizontal configurations.

Space truss purlin

The cross section of the purlin is triangular, and it is composed of three plane trusses ①, ②, and ③ to form a complete space truss system, so it is called space truss type. The characteristics of this purlin are reasonable structure, clear force, large overall rigidity, no need to set braces, and easy installation; but it is more labor-intensive to manufacture and uses a large amount of steel. It is suitable for situations where the span, load and purlin distance are large.

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