Understanding Purlins and Panel Points in Roof Structures

In roof construction, especially with truss systems, purlins are horizontal structural members that span between the main load-bearing elements, such as rafters or truss chords. Their primary function is to support the roof covering (like sheeting or tiles) and transfer the loads from the roof covering and external forces (like wind or snow) down to the main structural frame.

A roof truss is typically composed of members connected at their ends by pin joints. These connection points are called panel points or nodes. Trusses are designed to efficiently carry loads primarily through axial forces (tension or compression) in their members when loads are applied directly at these panel points.

Why Purlins are Placed at Panel Points

The structural behavior of a truss is based on the assumption that loads are applied only at the joints (panel points). When loads are applied directly at the panel points, the forces are transmitted axially along the truss members, resulting in minimal or no bending moments within the members. This is the ideal scenario for a truss, as axial forces are more efficiently carried by slender members compared to bending moments.

If purlins are placed between the panel points on a rafter member of a truss, the load they transfer is applied along the length of that rafter member, away from the joints. This distributed or concentrated load between the joints will cause the rafter member to bend. This bending introduces significant bending moments and shear forces in the rafter, in addition to the axial forces it carries as part of the truss action. Designing a truss member to resist significant bending moments makes it less efficient and potentially requires a larger or heavier section compared to one designed primarily for axial load.

Therefore, placing purlins directly at the panel points of the rafter or top chord ensures that the roof loads are transferred as concentrated loads at the nodes of the truss. This allows the truss to behave as intended – carrying loads through axial forces in its members – and minimizes or eliminates bending moments in the rafter members.

Analyzing the Options

Let's look at the given options in the context of placing purlins at panel points:

• Axial force in rafter: Placing purlins at panel points transfers the load as an axial force component to the rafter as part of the overall truss action. You cannot avoid axial force; it's how the truss supports the load.
• Shear force in rafter: While placing loads between panel points induces shear forces in the rafter member itself (acting like a beam), placing loads *at* panel points minimizes this beam-like shear. However, shear forces are a consequence of bending moment. The primary issue avoided is the bending moment.
• Deflection of rafter: Bending moments cause significant deflection in a beam-like member. By avoiding bending moments, placing purlins at panel points also helps control the deflection of the individual rafter member (between panel points), although the overall truss deflection is due to axial deformations.
• Bending moment in rafter: As explained, applying loads between panel points causes bending moment in the rafter. Placing loads at panel points directly loads the joint, preventing the rafter segment between joints from acting as a beam under load. This is the main reason for placing purlins at panel points.

Therefore, placing purlins at the panel points is done primarily to avoid inducing bending moments in the rafter members, allowing the truss to function more efficiently under axial loads.

Revision Table: Purlins and Rafter Loads

Additional Information: Truss Behavior and Design Loads

Understanding truss behavior is crucial in structural engineering. A key principle is that ideal trusses (with pin joints and loads only at joints) only develop axial forces in their members. Real-world trusses have rigid or semi-rigid joints and may experience loads between joints, which introduces secondary bending moments. However, engineers design trusses aiming for the ideal behavior by placing major loads, like those from purlins, at the panel points.

Design loads for roofs include dead load (weight of roofing materials, purlins, truss itself), live load (people, maintenance), snow load, and wind load. Purlins collect these loads from the roof surface and concentrate them at their support points on the rafters or top chord. By aligning these support points with the truss panel points, the load path is directed efficiently through the truss members as axial forces.

Minimizing bending moments in truss members leads to more economical designs, as sections optimized for axial load are typically lighter and simpler than those designed to resist significant bending in addition to axial load. This is why the practice of placing purlins at panel points is standard in steel and large timber truss construction.