A purlin is a horizontal beam or bar used for structural support in buildings, most frequently on a roof, in architecture, structural engineering, or construction. Historically, purlin also referred to purline, purloyne, purling, or perling. Rafts or the building’s walls serve as the purlins’ supports. Even though they occasionally take the place of closely spaced rafters in wood frame structures, they are most frequently utilized in metal buildings.
The weight of a roof’s roof deck is supported by its purlins. The roof deck is the wood panel, plyboard, or metal sheeting forms the roof’s surface. When made of wood, it is frequently covered with a weatherproofing layer and occasionally with an insulating one.
There are various purlin types. They are categorized based on the substance they are formed of and their shape. Purlins serve multiple functions, such as providing structural support for floors or walls. Purlins are essential to the roof’s structure because there would be no frame for the roof’s sheeting to rest on without them.
- Wood Purlin
Utilizing wood purlins with fiber cement sheeting is a beautiful idea. The wood purlin and sheeting work well together to keep the chamber below ventilated and safe for storing anything you need to stay there, including grain, cattle, or other organic things.
But because they are composed of wood, purlins can decay. In addition, the main issue with wood is that it is dry when it is put up—the optimal time to dry it before installation is thus. Additionally, moisture can drastically increase weight, causing sagging.
- Steel Purlin
Steel purlins can easily replace wood purlins. They are thin, precise, straight, and dimensionally stable. When the temperature drastically changes, they expand and contract properly.
Cold-formed steel that has been thin enough to allow for screw penetration is typically used to make steel purlin. Rolling or pressing thin steel sheets into the required shape produces cold-formed steel. In addition to being less expensive for the maker, it is also simpler to work with than hot-rolled steel. Despite being more robust than hot-formed steel, cold-formed steel is more likely to break under pressure than to bend.
Like many other standard lightweight steel structural building components, purlins are made of hot-dipped galvanized steel with a coating. In the majority of exposed interior locations, this offers good protection. Avoid contact with or runoff from substances that are incompatible with zinc.
They also paint the area around the purlins to protect it. Combined (synergistically), zinc and paint give corrosion protection roughly twice as effective as either would alone.
- Purlin and Girt
A significant part of many pre-engineered metal buildings is secondary framing. This kind of framing, sometimes known as “secondary structural,” runs between primary framing components to create a structure within a structure, similar to cross beams in wooden construction.
Secondary framing is used to distribute loads from the building’s surfaces to the primary structure and base. Additional longitudinal support the secondary frame provides can aid earthquake and wind resistance. Additionally, it can offer lateral bracing for compression flanges that are a part of the primary framework, boosting the capacity of the entire frame.
Girts and purlins, which are secondary frame elements, function as follows
Girts offer extra wall support by supporting vertical loads in conjunction with columns and wall panels, increasing strength and stability. They also aid in supporting and attaching wall cladding.
Purlins offer additional roof support by forming a horizontal “diaphragm” that bears the weight of the roof deck of your building, regardless of the material you choose for the roof itself. They also contribute to the overall rigidity of your roof structure. Purlins allow for greater spans and the addition of mid-span support, allowing you to design a broader building.
Another type of additional framing is eave struts: These are essentially a combination of the two and are also referred to as eave girts or eave purlins. They are used where sidewalls and roofs overlap, with a top flange supporting the roof and a “web” supporting the walls.
The two secondary framing configurations are CEE and ZED. They are formed into a web with two flanges on a bending press. They occur in various sizes; purlins, for example, might be over 30 feet long.
Cold-formed steel is the most common material for girts, purlins, and eave struts. It has some structural stability difficulties that must be considered when choosing your metal building framing alternatives and overall design, but it is more economical and simpler to work with. The compression flange, web, or connections, in particular, may bend or shift from their original positions due to local or distortional buckling or lateral displacement.
Under the right circumstances, problems might arise even in situations of relatively low stress or even significant stress. Considering metal construction, you shouldn’t view these engineering challenges as drawbacks. In addition to girts, purlins, and eave struts, other stiffeners can also provide additional stability or support.
The dimensions, primary framing system, intended purpose of the building, and other criteria will all affect how many and what size secondary framing parts your project may need. Your metal building firm can thoroughly explain the nuances and direct you in the proper direction.
- Purlin Roof
Anyone in the construction sector is familiar with roof purlins. Purlins are subjected to ambient loads, dead loads (such as the own weight of sheeting materials and accessories), and living loads during their design life (e.g., wind and snow load). A purlin should, therefore, be robust enough to sustain the loads it will experience over its design life and should not sag noticeably, which would cause the roof sheeting to appear undulating and unattractive. The design of steel purlins employing cold-formed sections will be the main topic of this essay.
- Purlin Span
The purlin’s span is the distance between the centers of the cleat bolts at either end. Each span type knows that employing distinct parts (such as internal and end span) is not a proper method and represents an entirely purlin-run system.
- A Single Purlin Span: The span that is only held up by attaching the purlin’s web to a cleat or other complex structure is known as a single purlin span. Under these circumstances, bridging does not affect outward capacities, but the number of rows of bridging affects inward abilities.
- A Double Purlin Span: A double purlin span is supported at both ends and in the middle. One purlin may run the entire length, or two purlins may be lapped together over the center support to create continuity. Bridging in double spans affects both internal and outward capacities.
- A Continuous Purlin Span: Supported at each end with a sequence of evenly spaced intermediate supports, continuous purlin spans are constructed. These tables are for spans where a lap of purlins covers each support with a lap length of 15% of the span. Tables are provided for spans of five or more. The illustrated bridging is necessary for outward weights on equal continuous spans at the end. If inward capacity, the prescribed minimum bridging, and practical spacing requirements allow, one fewer row of bridging can be employed for interior spans. All spans must bridge the distance specified in the table for inward weights.
- Purlin Laps
15% of the span is usually suggested as the lap length. Each purlin should have 7.5% of each neighboring span added rather than 7.5% of that purlin’s span where the span lengths are uneven (such as lowered end spans).
Less than 10% of a lap length (or 5% on any side of support) may not offer complete structural continuity and may also have local failures that are not taken into account by this method. As a result, they must be regarded as outside the purview of this handbook.
- Purlin Cleats
Most circumstances, including lapped Z purlins, call for single cleats. Double cleats play when successive purlins (often unlapped) are butted together. Double cleats may also be employed in applications with a high reaction load to reduce bolt strains. The complete details, in this case, would require further consideration.
- Purlin Spacing
Purlin sections automatically assume the pitch of the roof they are supporting. Purlin spacing typically requires careful planning, as it should match the nodal pattern of the supporting trusses. To avoid applying additional bending and shearing pressures to the truss members, purlins should be positioned at the nodes of trusses rather than directly on the members. Furthermore, since we typically assume pinned connections, such secondary stresses cannot be identified if the manual analysis is used to evaluate a truss loaded in this way.
They often offer advantages like lightweight, high strength and stiffness, ease of fabrication and installation, ease of shipping and transportation, etc., compared to bulkier hot-rolled shapes. The purlin connection may be butted or sleeved depending on the chosen building method.
Single spans with staggered sleeves and butts, single/double spans with staggered sleeves, double span butt joint systems, and single-span butt joint systems are the different layout options. The length of the sections that are widely available on the market, the necessity to prevent wasting offcuts, the weight and span of the roof, how the rafters are arranged, and other factors can all influence the choice of arrangement to be used. The roof designer must therefore make a complete plan. However, because of their ease of use and the tradition of using shorter roof spans, single and double-span butt joint systems are the most common in Nigeria. They are less effective structurally than sleeve connections, though.
- Purlin Bridging
When installing roof and wall sheeting, bridging prevents purlins from rotating. Due to this, it is advised to limit bridging (or bridging to cleat) spacing to a maximum of 20 x purlin depth and a maximum of 4000. Failure to do so may result in fastenings that are out of alignment, putting additional strain on the fasteners and roof sheeting. During construction, excessive purlin rotation may present a safety risk. As a result, it is recommended that each purlin span use at least one row of bridging. Extreme spam/bridging configurations are displayed to the left or above the red line. However, the stated values are valid for structural reasons in cases where sheeting is successfully put outside this suggestion.
- Purlin Installation
Under metal roofs, purlins are laid horizontally. A felt underlayment or vapor barrier is placed on top of them after they are installed on top of the roof rafters. 2 by 4-foot purlins are installed similarly to metal roofing. They offer the roof additional stability and a nailing surface for the drip edge and end panels.
- Purlin Laps
As seen below, purlin laps must be fastened in the upper web hole and the bottom flange hole at both ends of the lap. Complete structural continuity is not provided by bolting simply in the web of lapped purlins, and excessive loads may be exerted on roofing screws that pierce both purlins inside a lapped zone.
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