The Structural Engineer's Corner

Eng. Onorio Francesco Salvatore

Coefficient of exposure for wind action – Difference between ISO 4354 and AS/NZS 1170.2

Written By: Lexatus - Oct• 01•13


The Australian Standards adopt in many parts recognized International Standards. For what regards the Wind Action the reference is the ISO 4354 “Wind action on structures”.

One of the differences between the AS/NZS 1170.2 and the ISO 4354 is the exposure factor Cexp. The Australian Standard decided to not adopt that factor because it has to be considered as dependent on wind direction and cannot be expressed as a single factor in a directional standard. Another difference regards how to apply the factors: directly to wind speed in the Australian Standard and to dynamic pressure in the ISO 4354. The coefficients are applied to the site wind speed, Vsit,b, for 8 cardinal directions (compass points) incorporating multipliers for directionality, terrain/height, shielding and topography. The square of the product of these multipliers can be regarded as a direction-dependent exposure multiplier as used in ISO 4354:

Cexp ≈ [Md Mz,cat Ms Mt]2


Md = wind direction multiplier;

Mz,cat = terrain/height multiplier;

Ms = shielding multiplier;

Mt = topographic multiplier.

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Eng. Onorio Francesco Salvatore

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