AS 4055:2021 – Wind loads for housing
AS 4055:2021 – Wind loads for housing.
Section 5 Racking forces
Racking forces are lateral (horizontal) forces transferred to the foundations through bracing provided for each storey of the house and the subfloor.
The racking forces occur in walls parallel to the wind direction and are calculated from the horizontal component of wind blowing on the external envelope of the house and resisted by bracing walls.
Racking forces shall be calculated as follows:
(a) Determine the wind class as given in Section 2.
(b) Determine area of elevation of the house as given in Clause 5.2.
(c) Determine the wind pressure, as given in Table 5.2(A), for buildings presenting a flat vertical surface to the wind.
(d) Determine the wind pressure, as given in Tables 5,2(B) to 5.2(M), using the width (shorter dimension) of the building and roof pitch of the building being designed.
Pressures are given for single storey and upper storey of two storeys for both long and short sides of the building, and for lower storey of two storeys or subiloor for both long and short sides of the building.
(e) Calculate racking force, in kN. as follows:
Total racking force Area of elevation (m2) x Lateral wind pressure (kPa).
The racking force shall be calculated for both directions (long and short sides) of the building. The total racking force for each storey or level of the building shall be determined as the sum of the forces on each of the areas facing the direction In which racking forces are evaluated. Racking forces shall be calculated to address the most adverse loading situation.
NOTE 1 For intermediate values bctwen those given in Tables 5.2(A) to 5.2(M). use linear interpolation.
NOTE 2 For the explanation of Tables S.2A) to 5.2(M), sec Appendix A.
NOTE 3 For worked examples. see Appendix B.
5.2 Area of elevation
Area afelevation appropriate for calculation of racking forces shall he as shown in Figures 5.2(A) to S.2C).
The wind direction used shall be that resulting in the greatest load for the length and width of the building, respectively. As wind can blow from any direction, the elevation used shall be that for the worst direction.
NOTE 1 In the case of a single-storey house with a gable at one end and a hip at the other, the gable end lacing the wind will result In a greater amount of load at right ingles to the width of the house than the hip end facing the wind.
NOTE 2 For complex building shapes, buildings that are composed of a combination of storeys or rectangles (i.e. L, H or Ii shapes), the shapes may be considered Individually, and forces added together later, or the total area as a whole can be calculated.
Irrespective of which method is used, racking forces shall be calculated for the most adverse situation.
If a veranda or similar structure Is present and is to be enclosed, it shall be included in the area of elevation calculations.
Other combinations of smooth features may be less apparent. For example, a freeway reserve of width 150 m may be adjacent to a creek and reserve of width 100 m. Here the total width of smooth features is greater than 200 m and so can be categorized as a region ofTC2.
Where the smooth features do not adjoin they do not have to be combined. For example, a freeway reserve of width 150 m and a creek reserve of width 100 m separated by two rows of houses can be treated as two separate smooth features, each with a width of less than 200 m. The two separated features do not affect the terrain category.
The following satellite images show different terrain categories consistent with the definitions and the guidance in this appendix.
In Fieures A.3.3(A) to A.3.3(E) the red lines differentiate regions of different terrain categories. Figure A33(A) provides an example of the assessment of the terrain category of an individual site shown by the intersection of the blue lines. IL will be the terrain with the lowest TC number wIthin 500 m of the site. In this case, a yellow circle of radius 500 m has been drawn around the site. Although it contains mostly TC3 areas, some TC2 does encroach on the circle. TC2 includes parks, parking, public open spaces and vacant land. The vacant land has been undeveloped for more than 20 years and there are no plans to develop it in the next five years. Although TC2 is less than 200 000 m2, It is in contact with a large area of the more open TCI so can be regarded as a very large area of open terrain. This house would be classified as TC2. Different houses in this figure may have different classifications.