Timber Pole Sheds

Buildings using Small End Diameter (SED) timber poles paired with light-gauge steel rafters offer a straightforward, lightweight, and cost-effective structural solution for various low-rise buildings in New Zealand. This method leverages the natural durability and accessibility of treated timber poles alongside the strength, uniformity, and lightweight nature of cold-formed steel sections.

The fundamental structural system is made up of treated SED timber poles that support cold-formed steel (CFS) rafters. These timber poles serve as the main vertical supports, channeling roof and lateral loads to the foundations or directly into the ground when a suitable pole foundation system is used.

Applications

SED timber pole and cold-formed steel rafter buildings are ideal for straightforward, lightweight, and low-risk constructions. Common uses include:

• Storage Buildings – Designed for storing equipment, materials, agricultural products, and general goods where a simple and cost-effective structure is needed.

• Farm and Agricultural Buildings – Suitable for sheds, machinery storage, livestock-related structures, and other agricultural purposes, depending on specific building needs.

• Workshops – Offers an economical structural solution for small workshops and utility buildings where large open areas and lightweight construction are advantageous.

• Shelters and Open-Sided Buildings – The pole and rafter design is ideal for covered shelters, carports, and open-sided structures where traditional wall framing may not be required.

• Importance Level 1 (IL1) Buildings – These structures are often chosen for Importance Level 1 applications, generally covering buildings where failure poses a low risk to human life or the community.

• Simple, Low-Risk Structures – The system is especially effective for buildings with a straightforward layout, relatively low structural loads, and a simple load path.

  
  

Advantages of SED Timber Poles and Cold-Formed Steel Rafters

Lightweight Construction – CFS rafters offer excellent structural capacity with a relatively low weight, reducing the load on the timber poles and foundations.

Cost-Effective Structural System – SED timber poles serve as a cost-efficient alternative to traditional timber framing or heavier steel columns, especially for simple low-rise structures.

Simple Connections – CFS rafters can be attached to timber poles using straightforward plates, bolts, screws, or proprietary connection systems, minimizing fabrication complexity and easing installation.

Faster Construction – The lightweight components and simple connection details facilitate handling and assembly, potentially decreasing construction time and labor requirements on-site.

Bird-Proof Roof Detailing – Incorporating bird-proof purlins or closed/protected purlin arrangements into the roof framing helps prevent birds from entering the roof space, making the system ideal for agricultural, storage, and open-sided buildings.

Typical Construction Methodology

• Primary Frames: Cantilevered vertical round poles serve as the main support columns and lateral resisting elements, supporting pitched CFS rafters.

• Secondary Framing: Cold-formed steel wall girts and roof purlins offer structural restraint to the primary rafters and act as a substrate for external metal cladding.

• Connection Detailing: The connection designs between the timber pole and CFS rafters are simplified, as the primary lateral resistance is achieved through pole bending instead of moment-resisting frame joints.

• Foundation: Timber poles are usually embedded in bored cylindrical concrete footings. This foundation system is designed to safely transfer vertical and lateral forces into the ground and provide sufficient resistance to overturning.

• Wind Loading: Wind is a crucial design consideration for lightweight and open-sided structures. Roof uplift and lateral wind forces are transmitted through the roof framing, connections, and timber poles to the foundations, necessitating a continuous load path throughout the structure.

• Timber Specifications: Poles are specified according to NZS 3605 and treated to H5 (hazard class 5) as per NZS 3640.

• Longitudinal Stability: Roof and wall bracing, or diaphragm action from the metal cladding, contributes to stability along the building's length and transfers lateral loads to the foundations.

Conclusion

The use of SED timber poles alongside cold-formed steel rafters provides a versatile solution for building design, especially when prioritizing open spaces, efficient material utilization, and straightforward construction. Its appropriateness is influenced by factors like site conditions, building layout, exposure, and structural requirements, highlighting the importance of early engineering involvement to achieve an optimal balance between performance and constructability.

AB Consulting Engineers have extensive experience in designing and delivering a variety of pole structures. We collaborate closely with pole shed suppliers and contractors to ensure the best possible outcome for our clients. Whether you are planning your next storage structure or a hobby room, please do not hesitate to contact us.