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Wind zone SIA in Switzerland, with basic wind pressure

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SIA Wind zones and loads map in Switzerland

Wind zones map in Switzerland

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In Switzerland, wind regions vary from zone 0.9 (lowest) to zone 3.3 (highest).

These zones indicate the variation of the reference value of the dynamic pressure in kN/m².

Reference value of the dynamic pressure in kN/m², by zone according to SN 261:2020 §6.2 (+ Annex E)
Wind zones Jura General Alps
Reference value of the dynamic pressure in kN/m² 2.4 0.9 1.1 1.3 1.1-2.7 1.4-3.3 3.3

Lisa.blue extract for a wind zone 1.1

Wind

The building is located in wind zone 1.1.
Wind standard
The standard applicable for wind load calculation on this building is SIA 261:2020 §6 (08/2020) and its annex Annexe E (08/2020).
Reference value of the dynamic pressure
The reference value of the dynamic pressure, qp0, corresponds to the peak speed (wind gusts of a few seconds) , at 10 m above ground level in terrain of 'open country' (category III) type.
The fundamental values of the dynamic pressure are specified by the Annex E.
This one provides a country map divided into climatic zones.
For the zone 1.1, the specified value of the pressure is: qp0 = 1.1 kN/m².
Fundamental values of the peek wind velocity
The fundamental value of the peek wind velocity, vp0, is deduced from the reference value of the dynamic pressure and from the density of the air.
The density of the air for a building located at an altitude of 1605 m..
(Swiss Federal Office of Energy, 1999: Planning of Wind Power Plants - Guide for Switzerland).
The relation between the dynamic pressure qp0 and the peek wind velocity vp0 is :

The basic wind velocity vp0 is therefore :

Peek wind velocity vp on the building site during a period of 50 years
Probability coefficient of exceeding
The probability p of exceedance is considered on the useful life of the project, itself based on the use of the project.
For our building project, the use is dwelling house, the recommended duration of use, necessary for determining the return period, is therefore 50 years.
The 10 minutes mean wind velocity having the probability p for an annual exceedence is determined by multiplying the basic wind velocity vp by the probability factor of severe wind cprob:
(Not referenced by SIA standards - Application of EN 1991-1-4 Equation 4.2)
Directional factor of wind
'wind direction' means the direction from which the wind comes.
The SIA standard does not mention a wind direction coefficient to reduce the resulting pressures. We therefore assume that cdir = 1.
Season factor
The SIA standard does not mention a season factor coefficient to reduce the resulting pressures. We therefore assume that cseason = 1.
The peek wind velocity is therefore
(Not referenced by SIA standards - Application of EN 1991-1-4 Equation 4.)
Peak velocity pressure qp
For each surfaces of the building, wind effects are observed in 4 nominal angular sectors of 90°:
  • the highest basic wind velocity vp contained in each of its sectors is retained
  • it is then adjusted taking account of the impact of terrain roughness on the variability of the mean wind velocity.
  • the peak velocity pressure qp is finally calculated with taking account of the density of air and the rapid fluctuations in velocity.
Graph of the peak velocity pressure for each direction of the wind
Maximum equivalent peak wind velocity (at the ridge Lvl +7.5 m)
  • 174 km/h to control the vibrations and the deformations of the structure at serviceability limit states (SLS).
  • 213 km/h to control the resistance of the structure at ultimate limit state (ULS).
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