Girts and purlins are components of the secondary frame of a metal building. A girt is a horizontal structural member in a framed wall that provides lateral support to the wall panel to resist wind loads. Purlins perform the same service for the roof panels.
Girts and purlins may also be called sheeting rails. These secondary frame members are used in commercial, residential and industrial steel buildings. Another secondary frame member, the eave strut, is also known as an eave girt or eave purlin. The eave strut is the last girt at the top of the wall or the first purlin at the connection between the roof and the wall.
Using Purlins and Girts
Both girts and purlins are available in two shapes: Z- purlins or girts and C-purlins or girts. Each appears like the letter of the alphabet after which it is named when looked at from its end.
If a load is to be suspended from a purlin, the load must be included in the load calculations, and the load should be connected to the purlin web by hangars or something similar. Never connect a load to the purlin's lip. Also, never suspend a load from a girt in the roof assembly.
The Z-shaped members are asymmetric sections which have uneven flange widths allowing them to be lapped for structural continuity. Z-sections also provide flexural continuity between spans. Using lapped Z-sections saves on material costs and, when lapped, increases the strength and rigidity to compensate for the extra material required to produce the member.
C-shaped members are also called channel sections when they have flange stiffeners and U-sections when they do not. C-sections are mono-symmetrical and cannot be lapped. However, the freestanding and stable shape allows for easy packaging and transport. The C-section is adaptable to the box and I- configurations and is preferred for stability in single spans.
- Girts work in conjunction with columns and wall panels to support the vertical load and improve strength and stability.
- They also attach and support the wall cladding.
- Purlins provide additional roof support, creating a horizontal diaphragm that supports the weight of the roof and deck.
- They run parallel to the building eave and are supported by rafters or walls.
- Purlins add rigidity to the roof and mid-span support to allow longer spans for a wider building.
Girts and purlins are fabricated from cold-formed steel and can reach up to 30 feet or more in length. The number and size needed depends on the building dimensions, the primary framing system, how the building will be used and the engineering design. Stiffeners can be added to prevent local or distortional buckling or lateral displacement if needed.
When fastened to cleats, both Z and C-sections must have the top flange pointing up the slope to minimize rotation.
The advantage to lapping is the dual thickness of material placed over the interior supports where the bending moment and shear are most crucial. Lapping is done by rotating one Z-purlin 180 degrees and fitting it to another.
Lap length is the distance between the bolt-hole centers and the end of the laps and is expressed as a percentage of the span. The sections are bolted at the outer holes closest to the sheeting and the inner flange at both ends of the lap.
- Z-sections of the same size but different thicknesses can be combined for a lower-cost span configuration.
- Lapping increases load capacity and rigidity.
- Where purlin spans are unequal between supports, each purlin will require around 7% of each adjacent span added rather than 7% of that purlin’s span. The amount is specified in the drawings.
At the end bays where the capacities for strength and deflection are limited for continuous lapping, the limitation can be overcome in one of three ways.
- Extra span is bridging where one additional row of bridging is used only in the end bays of equal continuous spans to increase the outward strength. No change is made in deflection.
- Reduced end spans where the end bays are reduced in length to improve the capacity of the ends. This requires slightly longer internal spans to maintain the same building length. The more bays there are, the shorter the internal spans will be. Improvement is seen in both strength and deflection.
- Increase end thicknesses where each end bay has Z-purlins of increased thickness to improve both strength and deflection.
If Z-purlins are designed with down-turned lips, they cannot be lapped.
Stabilizing Purlins and Girts
Purlins and girts have two types of lateral instability - deflection and rotation. Both can be reduced by various techniques including bridging and attaching sheeting with screw fasteners to provide a lateral brace to the flange to which it is attached.
Bridging can prevent lateral deflection of the free flange and rotation of the section. Bridging elements include ridge bridging, bridging struts, girt hangers, fascia bridging or a girt foot.
Spacing and Spans
Purlin and girt spacing are dependent on the member and roof sheeting capacity. On large or medium sized buildings you can reduce costs by reducing the purlin or girt spacing at the building’s edges and ends to provide stability for higher wind pressures. Additional intermediate runs of purlins can be positioned around the areas of the highest load.
Wherever there is penetration or additional loading as in the case of an AC unit, you should closely space the purlins or girts to increase strength and decrease deflection.
Purlins and girts can be installed in spans. A span is the length between the centers of the cleat bolts. Each span type represents a complete purlin system. Mixing components from an internal span and an end span is not a valid procedure.
- Single span is a span supported by bolting the purlin web to a cleat or other rigid structure. Bridging does not influence inward capacity and outward capacity varies according to the number of rows it bridges.
- Double span is where spans are supported at each end and the center. There may be a single purlin over the entire length, or two lapped together over the central support. Both inward and outward capacities are influenced by bridging.
- Continuous span refers to spans supported at each end and a series of equally spaced intermediate supports.
- Reduced end continuous span is used with equal spans, but the end spans are reduced in length by 20%. There is a significant impact on the outward capacity. This design becomes less costly as the number of spans increases.
Girts and purlins are essentially the same type of secondary frame member. Girts support the wall panels while purlins support the roof cladding. Two shapes are available. C-purlins are appropriate for single spans and I and box configurations while Z-purlins can be lapped and support double and continuous spans.
Girts and purlins provide additional protection against wind loads, especially at the corners and penetrations. Both are critical components of your steel building system.