MiTek provides numerous methods, products and details for the construction of timber-framed structures that take into consideration the New Zealand Building code and 3604:2011 requirements for wind design.

MiTek roof truss engineering software has long been established as the most widely used trussed rafter design package in the industry. This provides automatic framing, editing tools, timber optimisation and automatic wind loading based on user selected criteria.

For a structure to be sound and secure the foundation, roof, and walls must be strong and wind resistant. Wind loading analysis is an essential part of the building process. If wind loading analysis is not done correctly the resulting effects could include collapsed windows and doors, ripped off roofing, and more.

Types of wind load forces on buildings:

• Shear Load: Wind pressure that is horizontal and could make a building tilt.
• Lateral Load: A pulling and pushing horizontal pressure that can cause a building to move off its foundation.
• Uplift Load: Pressures from wind flow that result in lifting effects.

Designing for Wind

Make an assessment of wind effects early in the design process. This should include the speed (average and peak) and direction of wind, and how it affects the site at different times of year.

Factors influencing wind speed on a specific site are:

General wind speeds in the region, source from:

• NIWA
• New Zealand Met Service
• The local council should be able to give advice on the wind zone of any property within its boundaries. Some local authorities have this information online. 
• A land information memorandum (LIM) may contain information about the property’s wind zone.
• Level of site exposure determined from on-site observations: A site visit should give some indication of wind speed and predominant or strongest wind direction. Observe vegetation and features on the site.
• Talk to neighbours and observe how neighbouring properties deal with the effects of wind.

Topography: What the wind speed will do:

• Increase as it passes over and around hills
• Slow down as it passes over rougher terrain
• Accelerate over open and flat expanses (eg. large expanses of water such as lakes or ocean during the day). The sun will heat the land, resulting in an increase in temperature relative to the adjacent body of water. As warm air on the land rises, cooler air from over the water will replace the rising air, resulting in afternoon on-shore breezes.

Other buildings and vegetation affect wind speed:

• Lower when a site is surrounded by larger buildings
• Will increase where it flows around or between buildings
• Slowed by vegetation
• Building size: The higher the building, the more exposed it will be to winds, particularly where the building is going to be taller than other buildings and/or vegetation.

MiTek products that aid in high and very high wind environments include:

• Cyclonic Ties CT400 and CT600: Overall length 400mm and 600mm — designed specifically for fixing down rafters or purlins in high wind situations. These are produced in straight pre-twisted lengths which are then folded over timber members on-site, accommodating various width purlins or rafters. Fix with LUMBERLOK Product Nails 30 x 3.15mm diameter.
• Cyclone Ties SSCT400 and SSCT600: Overall length 400mm and 600mm — designed specifically for fixing down rafters or purlins in high wind situations. These are produced in straight pre-twisted lengths which are then folded over timber members on-site, accommodating various width purlins or rafters. Fix with LUMBERLOK Stainless Steel Product Nails 30 x 3.15mm diameter.
• 16kN Truss to Top Plate Fixing: CPC80 achieve 16kN. The required pack comes supplied with the appropriate cleats and screws to penetrate through the timber top plate packer and into the top plate. Compliant with NZS 3604:2011, these conveniently top mounted fixings allow additional face fixing if required.