New Zealand’s Wind Climate
New Zealand is one of the windiest countries in the world, and wind loading is a significant design consideration for buildings throughout the country — but particularly in Wellington (among the windiest cities in the world), exposed coastal locations, elevated sites, and the Cook Strait region. Designing and building structures that resist wind loading adequately is a fundamental requirement, and understanding the concepts behind wind design helps builders understand why specific structural requirements are specified.
Wind loading is governed by NZS 1170.2 (Wind Actions) and the Building Code. The wind design speed for a building depends on its location, its exposure (terrain roughness, topography), and its importance level (a hospital has higher design requirements than a standard house). Buildings in high-exposure locations face design wind speeds that are dramatically higher than those in sheltered suburban environments.
Cladding and Window Wind Resistance
External cladding and windows must resist the wind pressure that acts on them — both positive pressure (wind pushing on the windward face) and negative pressure (suction on the leeward face and on roof surfaces). Cladding fixing patterns, window frame specifications, and glazing thickness are all determined by the wind loading the element must resist. Installing cladding with insufficient fixings, or specifying windows for sheltered conditions in an exposed coastal location, creates a serious risk of failure in high wind events.
The Structural System
The building’s structural system must transfer wind loads from the cladding through the framing and bracing to the foundation. NZS 3604 addresses wind loading for light timber frame buildings through the bracing system, with different bracing requirements for different wind zones. For buildings outside the scope of NZS 3604 — high wind zones, unusual geometry, large spans — specific structural engineering is required. Understanding the wind zone classification of a site and its implications for bracing requirements is a basic competency for residential builders.