Understanding the Difference Between Trusses and Purlins in Construction
Trusses and purlins are both crucial structural elements in building and construction, each serving distinct functions and contributing to the stability, strength, and efficiency of roof and bridge designs. Understanding their differences is essential for architects, engineers, and builders. This article will delve into the definitions, functions, structures, and types of trusses and purlins, providing a comprehensive guide that will aid in their proper usage.
What is a Truss?
Definition: A truss is a framework made of interconnected triangular units, typically constructed from wood or metal. It is designed to support significant loads and maintain structural integrity over long distances.
Function: Trusses distribute weight and provide stability, allowing for larger spans without the need for internal supports. They are commonly used in roofs, bridges, and towers to ensure they can bear substantial loads and remain stable.
Structure: The triangular units in trusses help to evenly distribute and resist bending, making them highly effective in load-bearing applications. Examples of truss designs include the Pratt, Warren, and Howe trusses, each suitable for different applications.
What is a Purlin?
Definition: A purlin is a horizontal structural member that supports the roofing material and transfers loads from the roof to the rafters or walls. It is an essential component in roof framing.
Function: Purlins provide additional support for the roof covering and help distribute the load evenly. They work in concert with trusses or rafters to maintain the roof's structure and stability.
Structure: Purlins are usually made of steel or wood and can come in various shapes, such as C and Z sections. They are placed perpendicular to the rafters to provide maximum support.
Types of Trusses and Purlins
Trusses: Pratt Truss: Features vertical members that are inclined inward and outward, with top chords that are inclined outward. Warren Truss: Composed of equilateral triangles, providing equal strength in all directions. Howe Truss: Similar to the Pratt truss, but with the diagonal members running upward from the bottom chord.
Purlins: Eave Purlin: Located near the eaves of the roof, providing support for the end of the roofing material. Ridge Purlin: Located at the highest point of the roof, typically used in traditional construction. Common Purlin: Used in timber roof structures, placed between the rafters to support additional roof weight.
Practical Applications and Examples
In Steel Framed Buildings: The trusses support the purlins, which, in turn, support the cladding. The cladding is usually a form of corrugated steel sheet.
In Traditional Timber Roofs: Common purlins are supported by struts and the common rafters are supported by the purlins. Timbers battens are laid across the rafters, and tiles or slates are fixed to the battens. Modern house roofs are commonly trussed rafters, also known as Gang-nail trusses, with the battens fixed to the rafters. There are no purlins in this case.
Conclusion
In summary, while trusses are large structural frames designed to span distances and support significant loads, purlins are smaller horizontal members that provide support for roof coverings. Both trusses and purlins play vital roles in construction, with trusses acting as the backbone of a roof and purlins serving as supportive muscles that hold the roof covering in place. Proper understanding and application of these structural elements are crucial for ensuring strength, stability, and durability in construction projects.