Wind Actions According To EC1

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This presentation is intended for stage-2 BEng/MEng Civil and Structural Engineering Students. The main purpose is to present how characterize wind loading on simple building structures according to Eurocode 1

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Wind Actions According To EC1

  1. 1. Wind actions according to Eurocode I Dr Alessandro Palmeri <A.Palmeri@lboro.ac.uk>
  2. 2. Learning Goals  When we have completed this unit (2 lectures + 1 tutorial), you should be able to: ◦ Identify the key parameters influencing wind loads on structures ◦ Apply Eurocode 1 to evaluate wind loads on a simple civil engineering structure Structural Design @ Lboro 2
  3. 3. Introduction  Wind is flowing air  Structures in the wind are subjected to forces which vary with time and space 3
  4. 4. Introduction  Flexible structures(e.g. tall buildings and long- Akashi-Kaikyo Bridge, Kobe span bridges) are [central span of 1,991 m] particularly sensitive to dynamic interaction phenomena ◦ Theory of aeroelasticity must be embedded in the structural design Burj Khalifa, Dubai [828 m tall] 4
  5. 5. Introduction  The lesson from the Tacoma Narrows Bridge… ◦ Probably the most spectacular collapse in the whole history of structural engineering (7th Nov 1940) 5
  6. 6. Introduction  For ordinary buildings, … with conventional structural systems, … with “simple” shape and “small” extension, … designers are allowed to use equivalent static forces to represent the actual effects of the turbulent wind  This is what you have to do for the Structural Design coursework Structural Design @ Lboro 6
  7. 7. Introduction  You will need to become familiar with two documents: ◦ BS EN 1991-1-4:2005  Eurocode 1: Actions on structures, Part 1-4: General actions, Wind actions ◦ NA to BS EN 1991-1- 4:2005  Corresponding UK National Annex ◦ Both documents can be downloaded by using your account in the MyAthens website 7
  8. 8. vb,0= Fundamental value of the basic wind velocity  Is the 10-minute mean wind velocity ... at 10 m above the ground ... of a terrain with low vegetation, ... having an annual probability of exceedance p= 0.02 (i.e. a return period R= 1/p= 50 years)  This can be evaluated as: = vb,map ⋅ calt vb,0 Structural Design @ Lboro 8
  9. 9. “MAP” VALUES OF THE vb,map = 22 m/s BASIC WIND VELOCITY [m/s] NA to BS EN 1991-1-4:2005, Figure NA.1 Structural Design @ Lboro 9
  10. 10. calt= Altitude factor  It depends on: ◦ Altitude of the site above the mean sea level, A [m] ◦ Reference height of the structure, zs [m] (why?) 1 + 0.001 ⋅ A , zs ≤ 10 m  calt =  0.2  10  A 50 m 1 + 0.001 ⋅ A ⋅  z  , zs > 10 m   s  1.20 A 100 m A150 m 1.15  calt 1.10 1.05 1.00 0 10 20 30 40 50 zs m 10
  11. 11. REFERENCE HEIGHT FOR DETERMINING THE STRUCTURAL FACTOR BS EN 1991-1-4:2005, Figure 6.1 Depending on the terrain category, zmin can vary from 1 to 10 m Structural Design @ Lboro 11
  12. 12. vb= Basic wind velocity  For particular design situations, is it possible (not recommended) to reduce the fundamental value of the basic wind velocity, vb,0, taking into account: ◦ Directional factor, cdir ◦ Seasonal factor, cseason vb = cdir ⋅ cseason ⋅ vb,0 ≤ vb,0 Structural Design @ Lboro 12
  13. 13. cprob= Probability factor  When required, this multiplier allows adjusting the annual risk of being exceeded:  1 − K ⋅ ln ( − ln (1 − p ) )  n cprob =   1 − K ⋅ ln ( − ln (1 − 0.02 ) )    where R= 1/p is the return period, while K and n are two parameters defining the probabilistic distribution of extreme winds ◦ Suggested values: K= 0.2 and n= 0.5 Structural Design @ Lboro 13
  14. 14. PROBABILITY FACTOR 1.20 1.15 1.10  1.05 cprob 1.00 0.95 0.90 0.85 0.80 10 20 50 100 200 500 1000 R years Structural Design @ Lboro 14
  15. 15. vm(z)= Mean wind velocity  The wind velocity is not uniformly distributed along the height of the structure  The shape of the vertical profile of mean wind velocity, vm(z), is determined by two functions of the height z: ◦ Roughness factor, cr(z) ◦ Orography factor, co(z) vm ( z ) = cr ( z ) ⋅ co ( z ) ⋅ vb Structural Design @ Lboro 15
  16. 16. cr(z)= Roughness factor  EN suggests a logarithmic profile which depends on four parameters, namely terrain factor kr, roughness length z0, minimum height zmin, and maximum height zmax ln ( zmin z0 ) , 0 ≤ z ≤ zmin  cr ( z= kr ⋅  ) ln ( z z0 ) , zmin < z ≤ zmax  ◦ In turn, the first three quantities (kr, z0 and zmin) depends on the terrain category of the site, while the fourth quantity takes the value zmax= 200 m Structural Design @ Lboro 16
  17. 17. cr(z−hdis)= Roughness factor  NA to EN provides a log-log design chart with: ◦ Distance upwind to shoreline [km] in the horizontal axis ◦ Effective height z−hdis [m] in the vertical axis  hdis being the so-called displacement height, which takes into account the reduction in the wind velocity due to the presence of closely spaced buildings and other obstructions Structural Design @ Lboro 17
  18. 18. DISTANCE UPWIND TO SHORELINE Google Earth, downloadable from: http://earth.google.co.uk/ Structural Design @ Lboro 18
  19. 19. DISPLACEMENT HEIGHT BS EN 1991-1-4:2005, Figure A.5 min {0.6 h,0.8 have } , 0 ≤ x ≤ 2 have  = min {0.6 h,1.2 have − 0.2 x} , 2 have < x < 6 have hdis 0 , x ≥ 6 h  ave Structural Design @ Lboro 19
  20. 20. cr(z−hdis)= Roughness factor Roughness factors at 11 and 14.5 m of height, assuming that 80 km is the distance upwind to shoreline and 8 m is the displacement height Values of cr are read by interpolating 14.5-8.0 0.93 between the contour of the 11.0 – 8.0 chart given as 0.80 Figure NA.3 in the NA to EN 80 Structural Design @ Lboro 20
  21. 21. cr,T(z−hdis)= Roughness correction factor for sites in Town terrain Roughness correction factors at 11 and 14.5 m of height, assuming that 1 km is the distance inside town terrain and 8 m is the displacement height 14.5-8.0 0.76 11.0 – 8.0 0.67 80 Structural Design @ Lboro 21
  22. 22. ALTITUDE TO BE Lu= actual length of the upwind slope CONSIDERED FOR Ld= actual length of the downwind slope SIGNIFICANT OROGRAPHY Le= effective length of the upwind slope NA to BS EN 1991-1-4:2005, Figure NA.2 … To be considered only if the structure falls within one of the shadowed areas below, otherwise co(z)= 1 Structural Design @ Lboro 22
  23. 23. Iv(z)= Turbulence intensity  Wind velocity has a turbulence component which must be considered in the structural analysis  EN defines the turbulence intensity, Iv(z), at a given height z, as the dimensionless coefficient of variation of the wind velocity, i.e. standard deviation divided by mean value: σ v ( z) I v ( z) = vm ( z ) Structural Design @ Lboro 23
  24. 24. Iv(z)= Turbulence intensity  The general expression suggested by NA to EN can be posed in the form: I v ( z − hdis )flat = I v ( z) ⋅ kI,T ( z − hdis ) co ( z ) ◦ where co(z) is the orography coefficient, equal to 1 when orography is not important ◦ and kI,T(z−hdis) is the turbulence correction factor for sites in town terrain, equal to 1 in country terrains Structural Design @ Lboro 24
  25. 25. Iv(z−hdis)flat= Turbulence intensity in flat terrain Turbulence intensity at 11 and 14.5 m of height, assuming that 80 km is the distance upwind to shoreline and 8 m is the displacement height 14.5-8.0 0.18 11.0 – 8.0 0.20 80 Structural Design @ Lboro 25
  26. 26. kI,T(z−hdis)= Turbulence correction factor for sites in town terrain Turbulence correction factors at 11 and 14.5 m of height, assuming that 1 km is the distance inside town terrain and 8 m is the displacement This quantity is height analogous to the roughness 14.5-8.0 1.64 correction coefficient for 11.0 – 8.0 the turbulent 1.80 component of the wind 80 velocity Structural Design @ Lboro 26
  27. 27. qp(z)= Peak velocity pressure  Once mean wind velocity, vm(z), and turbulence intensity, Iv(z), are known at a given height z of interest, the corresponding peak velocity pressure can be evaluated as: 1 qp ( z ) = (1 + 3 ⋅ I v ( z ) ) ⋅ ρ ⋅ vm ( z ) 2 2 2 where ρ= 1.226 kg/m3 (recommended value in the NA to EN) is the air density, while in town terrain both terms vm(z) and Iv(z) should include the pertinent correction factors Structural Design @ Lboro 27
  28. 28. Wind pressures  External (cpe) and internal (cpi) pressure coefficients are required to get the actual pressures on external (we(ze)) and internal (wi(zi)) surfaces from the peak velocity pressure  we ( ze ) cpe ⋅ qp ( ze )  =   wi ( zi= cpi ⋅ qp ( zi )  ) ze and zi being the pertinent reference heights Structural Design @ Lboro 28
  29. 29. PRESSURE ON DIFFERENT SURFACES BS EN 1991-1-4:2005, Figure 5.1 Structural Design @ Lboro 29
  30. 30. Wind forces  External and internal pressures allow computing the wind forces on external (Fw,e) and internal (Fw,i) surfaces  Fw,e = cs ⋅ cd ⋅ ∑ we ( ze ) ⋅ Aref  surfaces  = ∑ wi ( zi ) ⋅ Aref  Fw,i  surfaces where cs and cd are size factor and dynamic factor, respectively, while Aref is the reference area for each surface considered in the summation Structural Design @ Lboro 30
  31. 31. Wind forces  For external surfaces parallel to the wind the friction force Ffr is given by cfr ⋅ ∑ Ffr = qp ( z ) ⋅ Afr surfaces where cfr is the friction coefficient, while Afr is the reference area for each surface considered in the summation Structural Design @ Lboro 31
  32. 32. External pressure coefficients for building structures  External pressure coefficients depends on the size of the loaded area A= Aref, along with the position (zone) in the building  EN provides two limiting values which can be used for small elements with an area of 1 m2 or less, cpe,1, and for large elements with an area of 10 m2 or more, cpe,10  For intermediate situations, i.e. 1<A<10 m2, a logarithmic interpolation is suggested Structural Design @ Lboro 32
  33. 33. EXTERNAL PRESSURE COEFFICIENT BS EN 1991-1-4:2005, Figure 7.2 cpe =cpe,1 − (cpe,1 − cpe,10 ) ⋅ log10 ( A) Why cpe,10 MUST be always < cpe,1 ? Structural Design @ Lboro 33
  34. 34. EXTERNAL PRESSURE For the external pressure COEFFICIENT acting on vertical walls of BS EN 1991-1-4:2005, Table 7.1 rectangular plan buildings, EN gives a table with five zones, i.e. three side zones (A, B and C), windward (D) and leeward (E) windward leeward side zones Structural Design @ Lboro 34
  35. 35. EXTERNAL PRESSURE COEFFICIENT BS EN 1991-1-4:2005, Figure 7.5 35
  36. 36. PRESSURE PROFILES WINDWARD WALLS BS EN 1991-1-4:2005, Figure 7.4 The eternal pressure should assumed to be uniform over each single horizontal strip considered in the analysis The shape of the pressure profile changes with the width/height ratio 36

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