2007 SLOVAK UNIVERSITY OF TECHNOLOGY28

1 INTRODUCTION

Certain aspects necessary to determine wind action on a structure are dependent on the location and the availability and quality of the meteorological data and the type of terrain. Wind actions are determined from the basic values of wind velocity, and the response of structures should be calculated from the peak velocity pressure qp . qp depends on the wind climate, the terrain’s roughness and orography and the reference height and is equal to the mean velocity pressure plus any contribution from short-term pressure fluctuations.

2 WIND VELOCITY AND TURBULENCE

Wind velocity and velocity pressure are composed of a mean and a fluctuating component. The wind velocity at height z is:

(1)

The mean wind velocity depends on the roughness factor cr (z) and the orography factor c0 (z)

(2)

for (3)

for (4)

where vb is the basic wind velocity, zo is the length of the roughness, zmin is the minimum height, zmax is 200 m.The expression (3) is valid when the upstream distance with a uniform degree of terrain roughness is long enough to stabilise the profile.Terrain factor kr depends on the roughness length zo:

O. HUBOVÁ

THE EFFECT OF WIND ON A STRUCTURE

KEY WORDS

• Wind pressure and forces• Mean wind velocity• Turbulence intensity• Terrain category

ABSTRACT

Wind action is represented by a simplified set of pressures or forces, whose effects are equivalent to the extreme effects of turbulent wind. In accordance with implementing Eurocodes into our National Standards, it is necessary to take into account differences between the peak velocity pressure at the reference height according to STN, EN and DIN.

Oľga HUBOVÁ, Assoc. Prof., Ing., PhD. Department of Structural MechanicsFaculty of Civil EngineeringSlovak University of TechnologyRadlinského 11, 813 68 Bratislava, Slovakiae-mail: olga.hubova@stuba.skResearch field: Static and dynamic wind action on structures

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29THE EFFECT OF WIND ON A STRUCTURE

(5)

where: z0,II = 0,05 m (terrain category II), zo is the roughness length (see Tab.1)

The turbulent component of wind velocity has a standard deviation σv = kr . vb . kl, and according to the EN determination of turbulence intensity, is:

(6)

An illustration of turbulence intensity is in Fig.1.

3 PEAK VELOCITY PRESSURE

The recommended rule for determining peak velocity pressure according to EN 1991-1-4 is:

(7)

where ρ = 1,25 kg/m3 is air density

Velocity pressure according to DIN 4131: q(z) = q0 + 0,003 . z (8)

where: z is the height above the terrain,q0 is the wind pressure (depends on the weather zone)

The wind pressure for different wind areas is in Fig. 3.

Fig. 1 Turbulence intensity as a function of height and terrain category

Fig. 2 Peak velocity pressure as a function of height and terrain category

Fig. 3 Velocity pressure as a function of height and wetter zone

Tab. 1 Terrain categories and terrain parametersTerrain category z0 [m] zmin [m] kr

0 – sea or coastal area 0.003 1 0.156I – lakes or flat area with negligible vegetation, without any obstacles

0.01 1 0.1697

II – area with low vegetation and isolated obstacles (trees, buildings)

0.05 2 0.19

III – area’s regular cover of vegetation or buildings (villages, suburban terrain, permanent forest)

0.3 5 0.215

IV – area in which 15% is covered with buildings; their height exceeds 15 m

1.0 10 0.234

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30 THE EFFECT OF WIND ON A STRUCTURE

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The velocity pressure according to STN 75 0035:

(9)

where: wo is the wind pressure given in Tab. 2 (depends on the wind area )

is the height factor (depends on the terrain category – Tab.3)

Wind pressure for different wind areas is in Figs. 4 and 5.A numerical analysis of a steel stack due to along-wind turbulence according to the EN has been compared with procedures according to STN 73 0035, see Fig. 6 and Hubová, O. (2006). Cantilevered steel stack 25 m height and 1.3 m diameter, equivalent mass per unit length m = 225 kg/m, logarithmic decrement of damping δ= 0,012:

Tab. 2 Wind pressure and velocity due to wind area Wind area III IV V VIWind pressure w0 [kN.m-2] 0.45 0.55 0.7 0.85Wind velocity v [m.s-1] 26.83 29.66 33.46 36.87

Tab. 3 Height factor

Terrain category

A – open terrain - lakes or flat area and obstacles (trees, buildings) less than 10 m. B – area’s regular cover of vegetation or buildings (cities, permanent forest) their height exceeds 10 m

Fig. 4 Velocity pressure as a function of height, terrain and wind area

Fig. 5 Velocity pressure as a function of height, terrain and wind area

Fig.6 Wind forces for the whole stack due to different codes

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4 CONCLUSION

The paper shows that a detailed analysis of peak velocity pressures according to different codes gives us different wind loads. The fluctuating part of pressure, i.e., its dynamic nature, depends on the atmospheric turbulence. The turbulence intensity in Fig.1 gives us bigger values near the ground. The result of these pressures gives

rise to a system of forces (see Fig.6). The differences between EN and STN are more than 40.7% near the cantilevered part.

ACKNOWLEDGEMENTS

The presented results were achieved under the sponsorship of the VEGA Grant Agency of the Slovak Republic (Registration No.1/2142/05).

REFERENCES

• EN 1991-1-4 (2005) European Standard EN 1991-1-4 Eurocode 1: Actions on structures-Part 1-4: General actions - Wind actions, 2005

• Background document of ENV 1991-2-4 (1996) Background document of ENV 1991-2-4: Wind Action, Draft: Feb.1996

• Koloušek, V. - Pirner, M. - Fisher, O. – Náprstek, J. (1983) Wind Effects on Civil Engineering Structures, Academia Praha 1983

• STN 73 0035 Zaťaženie stavebných konštrukcií (Load on Building Structures), Slovak Standard STN 73 0035

• Petersen, Ch. (1996) Dynamik der Baukonstruktionen, Braunschweig/Wiesbaden, 1996, Friedr. Vieweg & Sohn Verlag-gesellschaft mbH, ISBN 3-528-08123-6

• Koloušek V. et col. (1967) Stavebné konštrukcie namáhané dynamickými účinkami (Dynamic Effects on the Building Structures), Slovenské Vydavateľstvo Technickej Literatúry Bratislava 1967 (in Slovak).

• Hubová, O. - Prekop, Ľ. (2000) Wind excited response of slender line structures. Proceedings of Int. Conference on Dynamics of Civil Engineering and Transport Structures and Wind Engineering, Vyhne, Sept., 2000 pp. 41-44

• Hubová, O. (2006) Dynamické účinky vetra s využitím noriem. (Dynamic Wind Effects usingStandards), Proceedings of Int. Conference DYNA 2006, Brno 11.-12.5.2006, ISBN 80-214-3164-4 pp.107-115.

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