GUIDE TO WIND CLASSIFICATION
An accurate wind classification is essential input to determining the installation standard to ensure that the performance of a tiled roof is not compromised by environmental conditions.
It is normal practice that the advice of a suitably qualified building engineer be sort to accurately determine the Wind Classification for a site.
The information provided in this guide has been created to assist the designer or the main contractor in their responsibility in determining the appropriate wind classification (or design wind velocity (m/s)) for a site.
Also note that the process used in the guide is an approximate method for estimating wind classifications for residential structures only. For full analysis refer to Australian/NZ Standards;
• AS/NZS 1170.2:2011 and by its Amendment 2 (2012);Definition of Terrain category
• AS1170.2-2002 – Structural design actions, Part 2:Wind actions; This Standard is generally applicable to determining the wind loads on any structure.
• AS 4055-2002: Wind loads for housing”. In NZ, the relevant standard is NZS 4203.
•AS 4055-2002:房屋的风荷载。在新西兰，相关标准是NZS 4203。
It is important to categorize each building on a case-by-case basis. Each site should be assessed individually for its wind classification. Each building must be assessed for compliance with geometry and for evaluation of pressures.
Therefore, Monier Roofing does not accept liability for any loss or damage suffered as a result of any errors in the interpretation or application of this design guide. Monier Roofing recommends that the advice of a suitably qualified building engineer is sort in assessing the wind classification of a particular site.
What is wind classification
The wind classification relates to gust wind speeds (m/s) that affecting a given site. The determination of a wind classification is critical in ensuring that a building is designed and constructed to the level appropriate to withstand the wind forces it will be subjected to. The wind classifications defined in the standards are summarised in the Table 1 below:
TABLE 1 – WIND CLASSICATION CONVERSION TABLE
Determining a wind speed classification
The selection of wind speed classification for a house depends on the conditions at the site of the house. The classification shall be determined from Table 2 on Page 8 using the following site conditions:
STEP 1 The region where the site is located as defined in Figure 1 (Region A B C or D as given in AS/NZS 1170.2).
第一步，定义所在区域(AS /NZS 1170.2中给出的A、B、C或D区域),如图1。
STEP 2 The terrain that surrounds or is likely to surround the site within the next 5 years, as defined in Figure 2 on Page 4
STEP 3 The topography of the site, as defined in Figure 3 on Page 5
STEP 4 The shielding that a structure is subjected to, as defined in Figure 4 on Page 6 FS PS or NS
Step 1. Geographic wind speed region
Please note: Specific design wind speed recommendations are available for some regions via government recommended websites. Refer to the links below for specific regionally information;
> SOUTH AUSTRALIA http://tinyurl.com/mns36yv
Step 2. Selection of terrain category
Terrain categories are used for determining a structure’s exposure to wind as a result of terrain that surrounds it.
The terrain category for a housing site is a measure of the lowest effective surface roughness from any radial direction within a distance of 500 m of the proposed housing site.
It shall be based on the likely terrain five years after design.
The terrain category for a housing site shall be identified by the notation TC1, TC1.5, TC2, TC2.5 or TC3 and shall be determined as follows:
Well established trees may be considered as obstructions for evaluation of terrain
In selecting a terrain category due regard to the permanence of the obstructions should be made.
In urban situations, roads, rivers, small lakes or canals less than 200 m wide shall be considered to form part of normal ‘Terrain Category 3’ terrain. Parks and other open spaces less than 250 000 m2 in area shall also be considered to form part of normal ‘Terrain Category 3’ terrain provided they are not within 500 m of each other,or not within 500 m of open country.
Housing sites less than 200 m from the boundaries of open areas larger than 250 000 m2, e.g. golf courses, that are completely surrounded by urban terrain,shall be considered to have the terrain category applicable to the open area itself Shielding provisions may still apply to these sites.
Housing sites less than 500 m from the edge of a development shall be classified as the applicable terrain that adjoins the development, i.e. TC1, TC1.5, TC2, TC2.5 or TC3, as applicable.
Where the water body is large (i.e. the wind has been blowing over the water for more than 10 km), then wind has the chance to develop long wavelength waves that will shoal on reaching the shore irrespective of the wind. Hence:
(a) Water bodies less than 200 m wide in the direction that the wind is blowing to affect a site do not affect the terrain category.在受风影响地点的方向上，不足200米宽的水体不影响地形类别。
(b) Water bodies greater than 200 m wide, but less than 10 km wide in the direction that the wind is blowing to affect a site are deemed to be Terrain Category 1.在风向影响某一地点的方向上，宽逾200米但宽不足10公里的水体被视为第一类地形。
(c) Only water bodies greater than 10 km wide in the direction that the wind is blowing to affect a site are deemed to be Terrain Category 1.5.只有在受风影响的地点的方向上超过10公里宽的水体才被视为1.5类地形。
Terrain Category 2.5 addresses acreage subdivisions where the house density is less than 10 per hectare. Where trees are large and robust (similar size to houses) then wooded areas with:
(i) Fewer than 2 large trees per hectare can be categorized as TC2.每公顷少于2棵大树可列为2级树木。
(ii) Between 2 and 10 large trees per hectare can be categorized as TC2.5.每公顷2到10棵大树可以被归类为TC2.5。
(iii) More than 10 large trees per hectare can be categorized as TC3.每公顷超过10棵大树可列为TC3。
In some cases, the 500 m radius circle may contain combinations of smooth features. For example, a small lake with diameter less than 200 m may be adjoined by a park also with width less than 200 m. However, the combine width of smooth features is greater than 200 m and the lake should be considered as part of the park, giving limiting terrain category for the building at the centre of the 500 m radius circle as Terrain Category 2.
Step 3. Selection of topography
The topographic classification determines the effect of wind on a structure due to its location on a hill, ridge or escarpment relative to the height and slope of a hill, ridge or escarpment.
A hill, ridge or escarpment is a natural feature that rises in height by more than 3° from the surrounding area. The slope of a hill, ridge or escarpment is measured by measuring the slope throughout the top half of the hill, ridge or escarpment at its steepest point. The second consideration is to determine whether the structure is located in the bottom, middle or top third of the hill, ridge or escarpment in terms of height.
With the slope and the position of the structure known, the adjacent diagrams may be used to determine the topographic classification i.e T0, T1,T2, and T3.
根据已知的坡度和结构的位置，可以用相邻的图来确定地形分类i。e T0 T1 T2 T3。
In assessing the wind pressures on any structure, the prevailing wind direction must be assessed and if unknown, wind pressures from all wind directions should be assessed.
In areas where the maximum slope does not exceed 1 in 20 (approximately 3°) the topographic classification shall be T0.
在最大坡度不超过1 / 20(约3°)的地区，地形分类为T0。
Step 4. Selection of shielding
Shielding classification is required as wind speeds on a house is influenced by obstructions around the house that are of similar size. Shielding classification should be considered based on the likely shielding five years hence.
In Geographic Wind Speed Regions A and B trees or groups of trees with similar face area to houses may be considered as shielding elements. In Geographic Wind Speed Regions C and D trees and vegetation shall not be considered as shielding elements.
This is because the long duration of the wind events in tropical cyclones can denude the trees and reduce their effectiveness as obstructions.
The shielding class for a housing site shall be identified by the notation FS, PS or NS, and shall be determined as follows:
(a) Full shielding FS
The application of full shielding shall be appropriate for typical suburban development greater than or equal to 10 houses, or similar size obstructions per hectare.
Full shielding shall apply where at least two rows of houses or similar size permanent obstructions surround the house on all sides. In Regions A and B, permanent heavily wooded areas within 100 m of site provide full shielding.
The effects of roads or other open areas with a distance measured in any direction of less than 100 m shall be ignored. However, the first two rows of houses abutting permanent open areas with a least dimension greater than 100 m, such as parklands, large expanses of water and airfields, shall be considered to have either partial shielding or no shielding.
(b) Partial shielding PS
Partial shielding shall apply to intermediate situations where there are at least 2.5 houses or sheds per hectare, such as acreage type suburban development or wooded parkland.
The second row of houses abutting open parkland, open water or airfields may be classified as having partial shielding.
(c) No shielding NS
No shielding shall apply where there are no permanent obstructions or where there are less than 2.5 obstructions per hectare, such as the row of houses or single houses abutting open parklands, open water or airfields.
Recent amendments of AS/NZS 1170.2 provide that where buildings are situated on steeply rising land, the roof of a building being designed will not be shielded by the roof of a similar-sized building lower on the slope. Higher topographic classes are associated with steeper slopes, for which full shielding is not appropriate.
Calculating Wind Classification
Referring to TABLE 2 – WIND CLASSICATION FROM WIND REGION AND SITE CONDITIONS on Page 8, and following these 4 steps below, the wind classification for your site is able to be determined.
From FIGURE 1 (Page 3): Determine your Geographic wind speed region, either A B C or D从图1(第3页):确定您的地理风速区域，A B C或D
From FIGURE 2 (Page 4): Determine the most appropriate terrain category (TC) for your site从图2(第4页):为您的站点确定最合适的地形类别(TC)
From FIGURE 3 (page 5): Determine the Topographic Class most relevant for your structure by calculating the slope of site at its maximum point and the location of the structure in terms if bottom,middle or top 1/3 of slopes height.
From FIGURE 4 (Page 6) : Determine the Shielding most relevant to site from fully shielded FS partially shielded PS or not shielded NS from the prevailing wind direction.