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Wind Loads on Structures: Analysis and Design Considerations

Understand wind loads on structures, their effects, and analysis methods according to building codes. Explore factors like wind velocity, structure shape, and dynamic excitation for accurate load assessment.

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Wind Loads on Structures: Analysis and Design Considerations

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  1. Chapter 3 – Loads and Load effects Dr. -Ing. Adil Z. Eng. Asmerom

  2. Loads on Structures • Classification of loads • Area of application: Concentrated, Distributed (UDL) • Direction: Vertical (Gravity), Horizontal (Lateral) • Response: Static, Dynamic • Variation with time: Permanent (Dead), Variable (Live)

  3. Loads on Structures • Classification of loads in Building Codes • Permanent (Dead) • Variable (Live) • Environmental Loads • Wind • Earthquake (Seismic) • Snow • Rain • Earth pressure

  4. Wind Load EBCS -1, 1995

  5. Wind load • Wind is air in motion • Structure deflects or stops the wind, converting the wind’s kinetic energy into potential energy of pressure • The wind loads that act on a structure result from movement of the air against the obstructing surfaces. • Wind effects induce forces, vibrations, and in some cases instabilities in the overall structure as well as its non-structural components.

  6. Wind load • Wind velocity increases with the power of the structural height

  7. Wind load

  8. Wind load • These wind effects depend on: • the wind speed, • density of the air, • shape of the structure • location and geometry of the structure, and • vibrational characteristics of the system. • Wind Forces According to EBSC-1, 1995 wind pressure in this section is valid for rigid surface only and neglects their resonant vibration

  9. Wind Load Two method of analysis is provided • the static procedure Only used for structures whose structural properties do not make them susceptible to dynamic excitation (Cd ≤ 1.2) • A detailed Dynamic Procedure must be used for those structures which are likely to be susceptible to dynamic excitation (Cd > 1.2) • In order to determine Dynamic coefficient Cd , Charts and figures can be used (EBCS 1-1995 fig 3.7 to 3.13)

  10. Wind Load

  11. Wind load (the simple procedure) Wind Pressure: The external and internal wind pressures are given as: We = qref Ce (ze )cpe Wi = qref Ce (zi )cpi • Where: We and Wiare the external and internal pressures; Ce(ze ) and Ce(zi ) are the external and internal exposure coefficients; Cpeand Cpi are the external and internal pressure coefficients. • The design wind pressure that is used to establish the wind load on a structure is directly related to reference velocity pressure (qref) and is given by:

  12. Wind load • Where: ρis the density of air and Vrefis the reference wind velocity to be taken as 22m/s. • The air density is a function of altitude and depends on the temperature and pressure to be expected in the region during storms. A temperature of 200C has been selected as appropriate for Ethiopia and the variation of mean atmospheric pressure with altitude is given in Table 2.3.(NS)

  13. Table 2.1 Values of air density ρ

  14. Wind load • Exposure Coefficient : takes into the account the effects of terrain, topography and elevation. Where: KT - the terrain factor Cr(z) - the roughness coefficient Ct(z) - the topography coefficient *** Or use table 3.5 from EBCS-1 1995 for Ce(ze ) & Ce(zi ) (See next slide)

  15. Exposure coefficient Ce Table 3.5 from EBCS-1 1995

  16. Wind load • Terrain Category: The terrain category attempts to take into account the effect of the land coverage, and is given below. The terrain type is classified into 4 groups as follows: • Category I: Lakes with at least 5 km fetch upwind and smooth flat country without obstacles. • Category II: Farmland with boundary hedges, occasional small farm structure, houses or trees • Category III: Suburban or industrial areas and permanent forests. • Category IV: Urban areas in which at least 15% of the surface is covered with buildings and their average height.

  17. Wind load • Cr(z) - the roughness coefficient • Attempts to take into account the effect of the land coverage • Where kT terrain factor zo roughness length zmin minimum height • For ground height above 200 m specialist advice is recommended.

  18. Wind load Terrain factor (KT) can be taken from table or calculated as follows: Z0= minimum height defined Z0,II= minimum height of category II (0.05)

  19. Wind load • Ct(z ) - the topography coefficient • It accounts for the increase of mean wind speed over isolated hills and escarpments Ct(z ) =1 for φ<0.05 Ct(z ) =1+2S φ for 0.05≤ φ <0.3 Ct(z ) =1+0.6 S for φ >0.3 Where: S the orographic location factor, φ the upwind slope H/Lu in the wind direction Le the effective length of the upwind slope, Lu the actual length of the upwind slope in the wind direction Ld the actual length of the downwind slope in the wind direction H the effective height of the feature X the horizontal distance of the site from the top of the crest z the vertical distance from the ground level of the site

  20. Wind load

  21. Wind load

  22. Wind Load • Pressure Coefficient: The shape factor takes into account the effect of shape of structure on the pressure distribution. • The external pressure coefficients Cpe for buildings and individual parts of building depend on the size of the loaded area A. They are given for loaded area A of 1m2 and 10m2 in the relevant tables for the appropriate building configuration as cpe,1 and cpe,10, respectively. For areas between 1m2 and 10m2, values are obtained by linear interpolation. That is:

  23. Wind Load (Ext. Pressure Coeff.) • Cpe = Cpe,1 for A≤1m2 • Cpe = Cpe,1 +( Cpe,10 – Cpe,1)log10A for 1m2<A<10m2 • Cpe = Cpe,10 for A≥10m2

  24. Wind Load (on building face) • Values of external pressure coefficients for different cases are given in Table A.1 to Table A.5 of EBCS-1, 1995.

  25. Wind Load (on building face) • It accounts for the variation in dynamic pressure in different zones of the structure due to • Its geometry • Area and • proximity to other structures

  26. Wind Load (on building face)

  27. Wind Load (on building face) • Reference height and wind pressure profile

  28. Wind Load (on building face) • Reference height and wind pressure profile

  29. Wind Load (on building face) EXTERNAL PRESSURE COEFFICIENTS FOR VERTICAL WALL.

  30. Wind Load (on flat roofs) • Flat roofs are defined as having a slope (α) of –5°< α < 5° • The roof should be divided into zones as shown in Figure below.

  31. Wind Load (on flat roofs)

  32. Wind Load (on flat roofs) • External pressure coefficients for flat roof

  33. Wind Load (on Monopitch roofs) • The roof, including protruding parts, should be divided into zones as shown in Figure below and NS • The reference height Ze should be taken equal to h.

  34. Wind Load (on Monopitch roofs)

  35. Wind Load (on Monopitch roofs) • External pressure coefficients for Monopitch roof ϴ =0º and ϴ =180º

  36. Wind Load (on Monopitch roofs) • External pressure coefficients for Monopitch roof ϴ =90º

  37. Wind Load (on Duo pitch roofs) • The roof, including protruding parts, should be divided into zones as shown in Figure below and NS • The reference height Ze should be taken equal to h.

  38. Wind Load (on Duo pitch roofs)

  39. Wind Load (on Duo pitch roofs) • External pressure coefficients for Duopitchroof ϴ =0º

  40. Wind Load (on Duo pitch roofs) • External pressure coefficients for Monopitch roof ϴ =90º

  41. Wind Load (on Hipped roofs) • The roof, including protruding parts, should be divided into zones as shown in Figure below and NS • The reference height Ze should be taken equal to h.

  42. Wind Load (on Hipped pitch roofs)

  43. Wind Load (on Hipped pitch roofs) • External pressure coefficients for Monopitch roof ϴ =0º and ϴ =0º

  44. Wind Load Internal pressure Coefficient Cpi • Internal and external pressures shall be considered to act at the same time. The worst combination of external and internal pressures shall be considered for every combination of possible openings and other leakage paths. • The internal pressure coefficient Cpi for building w/o internal partition is a function of opening ratio m defined as

  45. For closed buildings with internal partitions and opening windows the extreme values : Cpi= 0.8 and Cpi= -0.5

  46. Wind load • Net pressure: the difference of the pressures (external and internal) on each surface due account of their signs.

  47. Local effects of wind pressure • Wind around a corner Images from FEMA Multi Hazard Seminar

  48. Local effects of wind pressure Images from FEMA Multi Hazard Seminar

  49. Local effects of wind pressure • Uplift on roof Images from FEMA Multi Hazard Seminar

  50. Local effects of wind pressure Images from FEMA Multi Hazard Seminar

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