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Eurocode 1 - directional factor cdir

What is the directional factor for ?

High velocities of wind are observed more frequently in some sectors directions; the directional factor allows a reduction when the wind comes from a direction where the probability of occurrence of severe winds is lesser.

How do the direction factor values apply ?

« Wind direction » means the direction from which the wind comes. By convention, this direction is identified by the angle it forms with the North, increasing from 0° to 360° clockwise.

For the calculation of wind actions, only a few wind directions are considered; for example the normal directions to the facades in the case of buildings.
  • In the case of buildings, the pressure coefficients given in the Section 7 of EN 1991-1-4 take account of this simplification of the procedure; their values correspond to the most unfavorable direction of a so-called « nominal » angular sector of ±45° around the conventionally considered direction.
    Just as the standard requires, in 4.3.2 (3), to keep the roughness of the most unfavorable terrain in the nominal angular sector, it is advisable to use the most unfavorable directional factor of the directions contained in this sector.
  • In other cases, in particular when the construction has been the subject of a specific study, in a wind tunnel or by digital simulation, exploring all the wind directions in 20° or 30° steps, the most unfavorable directional factor of the directions contained in the sector of ±15° around the direction considered should be used.

What are the directional factor values ?

Belgium

In Belgium, you can use the values of cdir defined by the table below.

Wind direction 0°
(North)
22.5° 37.75° 45° 56.25° 90°
(East)
120° 150° 180°
(South)
270°
(West)
Directional factor cdir 1.0 1.0 0.95 0.90 0.85 0.85 0.90 0.95 1.0 1.0

France

In Metropolitan France, you can use the values of cdir defined by the map below, provided that the nominal angular sector including the wind directions associated with the pressure or force coefficients used in the calculations, is entirely contained by the interval.

Zone 1 2 3
Nominal angular sector included in the interval (North at 0°) 10° - 150° 70° - 150° 50° - 250°
Directional factor cdir 0.7 0.7 0.85
Zoning map of the directional factor of Metropolitan France (2011-2021) Zoning map of the directional factor of Metropolitan France (2011-2021)

The rotary nature of tropical cyclones which affect certain Overseas Departments does not allow any reduction for the effect of management in these departments.

Italy

In Italy, the recommended value is 1.0 (no reduction is allowed).