AS 4055:2021 pdf free.Wind loads for housing.
2.3 Selection of terrain category
The terrain category (TC) 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 may be based on the likely terrain five years after design. Substantial well-established trees may be considered as obstructions for evaluation of terrain category in all wind regions.
NOTE 1 In assessing the terrain category, a reasonable estimate should be made about mull development in the next five years as it is the anticipated development five years after construction that is assessed.
The terrain category for a housing site shall be identified by the notation TC1, TC2, TC2.5 or TC3 and be determined as follows:
(a) Terrain Category 1 (TC1) — Very exposed open terrain with very few or no obstructions, and all water surfaces, e.g. flat, treeless, poorly grassed plains; open ocean, rivers, canals, hays and lakes.
(b) Terrain Category2 (TCZ) — Open terrain including grassland with well-scattered obstructions having heights generally from 1.5 m to 5 m with no more than two obstructions per hectare, e.g. farmland and cleared subdivisions with isolated trees or clumps of trees, equivalent to at least house size, and uncut grass.
(c) Terrain Category2.5(TC2.5) — Terrain with a few trees or isolated obstructions. This category is intermediate between TC2 and TC3 and represents the terrain in developing outer urban areas with scattered houses or clumps of trees equivalent to house size, or large acreage developments with fewer than 10 house-size obstructions per hectare.
(d) Terrain Category 3 (TC3) — Terrain with numerous closely spaced obstructions having heights generally from 3 m to 10 m. The minimum density of obstructions shall be at least the equivalent of 10 house-size obstructions per hectare, e.g. suburban housing, light industrial estates or clumps of trees greater than house size.
In urban situations, roads, rivers, small lakes or canals less than 200 m wide shall be deemed to form part of normal Terrain Category 3” terrain. Parks and other open spaces less than 250 000 m2 in area shall also be deemed 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 500 m from the edge of a development shall be classified as the applicable terrain that adjoins the development, i.e. TC1, TC2, TC2.5 or TC3, as applicable.
NOTE 2 Shielding provisions may still apply to these sites.
NOTE 3 For commentary on terrain categories, see Appendix A.
NOTE 4 For worked examples, see Appendix C.
NOTE 5 Terrain Category 4. as defined in AS/NZS 1170.2, is not applicable to this document.
NOTE 6 Terrain categories are the same as those in AS/NZS 1170.2.
NOTE 7 Vegetation is considered differently for terrain categories in this clause and for shielding classes in C1ai.ie25.
2.4 Selection of topographic class
The topographic class is a measure of the effect of wind on a house on rising ground.
The topographic class for a housing site shall he identified by the notation TO, TI, T2, T3, T4 or T5 and be determined from Table 2.4.
NOTE For a worked example to determine topographic class, see Appendix B.AS 4055 pdf download.