Structural Loads on Roof Trusses
Learn how to distribute area loads into point loads (nodal forces) for accurate truss analysis and member sizing.
The Principle of Nodal Loading
A structural truss is unique because it is designed to carry loads only at its Nodes (joints). Any load applied directly to the members (like a person walking on a rafter) is transferred to the nearest joints. This allows the internal members to experience only pure Tension or pure Compression, making trusses extremely efficient for long spans.
Types of Truss Loads
- Dead Load (DL): The self-weight of the truss members, purlins, insulation, and roofing sheets. This is a constant force acting vertically downwards ($10$-$25$ kg/m²).
- Live Load (LL): Temporary loads such as maintenance workers, tools, and accumulation of dust. Standards usually specify a minimum of $75$ kg/m² ($0.75$ kN/m²).
- Wind Load (WL): The most critical load for lightweight metal trusses. Wind can create intense "suction" (uplift) that can lift a roof off if not properly anchored.
Load Distribution Formula
Tributary Area Concept
To calculate the load on a specific joint, you must determine the "Tributary Area" that joint supports. For an intermediate node, this is normally half the distance to the adjacent nodes multiplied by the spacing between trusses. End nodes (at the supports) usually carry only half the tributary area of intermediate nodes.
Frequently Asked Questions (FAQ)
What is a 'Purlin'?
Purlins are horizontal beams that span between trusses. They provide the surface to which the roofing sheets are fastened. They play a vital role in transferring the sheet load to the truss and providing lateral stability to prevent the truss chords from buckling.