COP:structure; wind-load

3.7 Wind Load 

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

AS/NZS 1170.2 and NZS 3604 contain the basic wind speed regions for New Zealand and the modifying factors that govern the design wind load. The predominant wind speed for New Zealand is 45 m/s. The exceptions are either side of Cook Strait and areas in the lee of mountainous areas.

3.7A Sheltered Building

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

 

3.7B Unsheltered Building

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

3.7C Topographical Influences

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.

3.7.1 Local Pressure Factor (Kl) 

The local pressure factor (Kl) is an important design consideration required by the Loadings Code. The peripheral areas of roof and wall surfaces are subjected to greater uplift loads than the main body of the roof. Designers need to include local pressure factors in the calculation of wind loads on the cladding.

When determining fixing requirements, the engineer must prepare a roof map showing purlin spans and local pressure factors for each section of the cladding. See 13.6.3A Pull-Over.

When designing to NZS 1170 the local pressure factors are:

  • 1.5—applied to the edges of all buildings at a dimension equal to 0.2 or 20% of the width or height of the building whichever is the least.
  • 2.0—applied to the edges of all buildings at a dimension equal to 0.1 or 10% of the width or height whichever is the least.
  • 3.0—applied to roof pitches less than 10°, at the corners where the dimensions in (a) intersect. It also applies to corners of walls where the building height is greater than the building width.

 

 

 

 

 

 

 

 

 

3.7.2 Conversion of Wind Speed To Pressure 

The basic formula for converting a wind speed to wind load is: 0.6 x velocity²= wind load.  However, to get a true design wind speed is a lot more complex; various factors have to be applied including roof self-weight, internal pressure and local pressure coefficients. 

The most influential of these factors is generally the local pressure factor, but internal pressure can also have a profound influence—particularly on unlined structures. Even houses built to NZS3604 have internal pressure factors incorporated into their design load calculations.

3.7.3 Roof Weight 

The self-weight of light-weight profiled sheet cladding should be included in the calculation of net wind load, but is a minor factor.