BRANZ

BRANZ Helps Custom Design Our Sheds for Issues with Wind

BRANZ is a multi-faceted, science-led organisation, BRANZ uses independent research, systems knowledge and its broad networks to identify practical solutions that improve New Zealand’s building system performance.

Designing Sheds  for Wind

Wind direction, speed and frequency will influence the building design including bracing requirements, roof and wall cladding selection, weathertightness detailing, building entry locations, window size and placement and provision of shelter for outdoor spaces.

Site Analysis

Understanding all the features of a site, using and protecting the best, and minimising the impact of the worst.

On this page:

• designing for wind
• wind regions and zones
• specific design
• gathering information about local wind conditions.

Regional and Site Features

General wind speeds in the region sourced from:

• NIWA
• Metservice

Level of site exposure: Determined from on-site observation

Wind speed & Terrain:

• Wind increases as it passes over or between hills
• Wind slows down as it passes over rougher terrain (drag effect)
• Wind accelerates over open and flat expanses of land or water

Around a large expanse of water nearby, e.g. sea or a lake

During the day, solar gain will heat the land mass, resulting in an increase in temperature relative to an adjacent large body of water. As air warmed by the land rises, cooler air from over the water will replace the rising air, resulting in the generation of afternoon, on-shore breezes

Wind speed with adjacent buildings / vegetation

• is lower when a site is surrounded by taller buildings
• will increase where it funnels around or between buildings
• is slowed by trees and vegetation

Wind speed & Building Height
The higher the building, the more exposed it will be to higher winds, particularly where the building is taller than adjacent buildings or vegetation.

Prevailing wind directions
The prevailing wind direction must be considered in relation to the design of a building, in particular, for locations of doors and opening windows, and provision of shelter for outdoor areas. Other aspects of wind to consider include:

• the direction of the strongest wind
• the direction of the coldest wind
• humid/dry winds
• wind that comes off the sea (salt spray issues)
• the wind direction that brings most of the rain.

Wind regions and zones

NZS 3604 Timber framed buildings requires that buildings are designed to withstand the winds that they are likely to be subjected to. Bracing requirements must be calculated separately for both wind and earthquake loadings; the bracing must be designed for the greater of the two loads. The amount of bracing required to resist the lateral wind loads on the building under NZS 3604 is based on the design wind speed.

NZS 3604 divides New Zealand into two wind regions (A and W) and several lee zone areas – these are areas where the landforms create localised wind acceleration resulting in higher wind speeds than the rest of the region.

It also classifies wind zones into categories, set out in the following table, according to maximum ultimate limit state speeds.

Wind zone classifications (from NZS 3604 Timber framed buildings

Table 5.1 in NZS 3604 provides steps to determine wind zone.
From Table 5.1, determination of the wind zone for a particular site requires the following steps:

• Determine the wind region.
• Determine whether in a lee zone.
• Determine ground roughness.
• Determine site exposure.
• Determine topographic class.

In practice, there are several different ways to establish the wind zone for a specific site. They are, with the most precise method at the top:

• Calculations made specifically for a site following AS/NZS 1170.2:2011 Structural design actions – Part 2: Wind actions
• Calculations made specifically for a site following NZS 3604:2011 Timber-framed buildings
• Council wind maps (if based on AS/NZS 1170.2:2011 or NZS 3604:2011)
• BRANZ Maps. BRANZ Maps is an experimental tool that can assist in determining a wind zone – it is not yet sufficiently precise to give a definitive wind zone for a specific site.

Specific design

The wind zones defined in NZS 3604 are based on a number of simplifying assumptions, such as wind direction, general topography and shelter.

In some situations, the wind zone classifications may under- or over-estimate the site wind speed.

In such cases, employing an engineer to assess the site conditions may be a more cost-effective design option. It will be required where the wind speed is over 55 m/sec.

Gathering information about local wind conditions

A site visit may 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.

Indicators of high wind speeds include:

• a general lack of developed planting
• stunted tree and shrub growth
• windbreak fences on adjacent properties
• wind shaping of existing planting.

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.

The National Institute of Water and Atmospheric Research (NIWA) provides climate station data for its 30 climate stations throughout New Zealand, which includes wind speeds, number of gale days per month, and wind roses (diagrammatic representation of historical wind patterns).

Level of Importance

Clause A3 of the Building Code defines the significance of a building by its importance level (IL), which is related to the consequences of failure.
There are five levels of importance, considered by the importance of the building to society:

• Level 1: Structures presenting a low degree of hazard to life or property, such as walkways, outbuildings, fences and walls.
• Level 2: Normal structures and structures not covered by other categories, such as timber-framed houses, car parking buildings or office buildings.
• Level 3: Structures that may contain crowds, have contents of high value to the community or pose a risk to large numbers of people in close proximity, such as conference centres, stadiums and airport terminals.
• Level 4: Buildings that must be operational immediately after an earthquake or other disastrous event, such as emergency shelters and hospital operating theatres, triage centres and other critical post-disaster infrastructure.
• Level 5: Structures whose failure poses a catastrophic risk to a large area or a large number of people, such as dams, nuclear facilities or biological containment centres.

The required level of seismic performance increases with each level of importance.

In general, important structures, such as hospitals, communications centres and those that provide occupation for many people, are designed for a greater level of earthquake shaking than ordinary commercial structures.

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