Wind Load

The wind load imposed on a roof structure is taken to apply at right angles to the roof cladding over a nominated area. The design wind load is affected by the design of the building and is modified using factors called pressure coefficients. Wind design load is measured in kilopascal (kPa); 1 kPa equals 1 kN/m². 

Wind Uplift Load

Wind-Uplift-Load-2

 

Under wind loads, inwards loads are resisted by the purlin or girts, whilst outward loads are only resisted by the fastener head or clip. Therefore, outwards load is the more critical figure.

Engineers calculate both the serviceability load and the ultimate load. They compare these values with the maximum failure loads of the products and systems they are considering.

Load/span data for standard corrugate, and low rib 5 and 6 ribbed trapezoidals is at the end of this section.

Refer to manufacturer's load/span tables for all other profiles, which should give the maximum recommended load for continuous spans when tested as described in Testing Procedure.

Sheltered by Trees, "Urban Terrain"

Scattered obstructions of a similar height or lower within 500 m from a building will considerably lessen wind speeds and lower design wind pressures.

Surrounded by flat ground, "Open Terrain"

Unsheltered Building

Structures in open land such as flat pasture and playing fields, or by water, will be subjected to higher design wind pressures.

Topographical Influences

Topgraphical Influence

Terrain also has a big effect. Structures near the crest of a rise or on flat land near a steep face will have increased design wind pressure.

Wind Design Load is affected by building design factors such as building height, shape, proportions, orientation, and roof pitch. Permeability can also be a big factor; buildings with large openings on one side but completely closed on the other three sides will suffer high internal wind pressures. These internal pressures must be added to the suction load on the outside of the roof when calculating wind design load.

Local territorial authorities are usually able to give wind speed figures for a specific address in their area. All other factors, including topographical influences, internal, and local pressure factors must be considered by a suitably qualified professional to calculate the design wind load on a structure.