Wind Loads (AS/NZS 1170.2:2011)

How it calculates

The Wind Loads (AS/NZS 1170.2:2011) calculator determines site wind speeds and design wind pressures for rectangular enclosed buildings per AS/NZS 1170.2:2011 (Amdt 5). Wind is evaluated in all eight cardinal and ordinal directions. Terrain, shielding and topography can be set once as a single worst case or separately for each direction.

Regional and site wind speed

The regional wind speed V_R is looked up for the wind region and the chosen return period. The return period is checked against the minimum for the importance level and design working life. The site wind speed in each direction (V_sit,beta) is then:

V_sit,beta = V_R × M_d × (M_z,cat × M_s × M_t)

Where:

• M_d - directional multiplier for each of the eight directions, looked up from the wind region (1.0 in all directions for regions B, C and D)
• M_z,cat - terrain/height multiplier for the terrain category in that direction (TC1 through TC4), linearly interpolated between the tabulated heights
• M_s - shielding multiplier, from the number, height and breadth of upwind shielding buildings
• M_t - topographic multiplier, from the hill-shape multiplier for hills, ridges or escarpments, with a lee multiplier you enter for New Zealand sites

Each building face uses its design wind speed V_des,theta: the highest site wind speed within 45° either side of that face. V_h, the maximum site wind speed over all eight directions, sets the equivalent AS4055 wind class.

Design wind pressures

External pressure coefficients (C_pe) are applied to each building surface - windward wall, leeward wall, side walls, roof zones - per AS/NZS 1170.2:2011 Cl. 5. Internal pressure coefficients (C_pi) follow the building's permeability. For an impermeable building they depend on which walls are permeable. For a permeable building they depend on the dominant opening ratio: the largest opening area on one surface divided by the total opening area on all other surfaces.

Each external and internal pressure is calculated from the design wind speed of that face:

p = 0.5 × ρ_air × (V_des,theta)² × C_p × C_dyn

Here C_p is C_pe or C_pi, and C_dyn is the dynamic shape factor you enter. The load table then combines external and internal pressures for each surface, using the combination factors K_c,e and K_c,i and any other K factors you enter.

Outputs

The calculator reports V_R, V_h, the equivalent AS4055 wind class, and a load table for each surface you list. Each row gives the maximum and minimum net pressure across all wind directions. Serviceability wind speeds and pressures are computed separately from the serviceability return period. The pressure coefficient and pressure tables cite their AS/NZS 1170.2:2011 clauses, and the load table can be linked to any Calcs.com design calculator.

Assumptions and scope

The calculator assumes a rectangular enclosed building per Cl. 5, flat suburban terrain by default. Lee wind effects and special K factors are entered manually where required. A positive wind pressure acts toward the surface. Note: the Climate Change Multiplier introduced in the 2021 edition is not present in this 2011 calculator.