Wind loads calculation

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  • 1. Wind Loads Calculation (ASCE 7) Henry Kurniadi
  • 2. Structure Loads• Dead Loads: from the weight of the structure itself.• Wind Loads: could be determined based on ASCE 7 (US Standard), Eurocode 1 (EU Code), Wind Code 2004 (Hong Kong Code), GB 50009 (China PR Standard), AS/NZS 1170.2 (Australia/NZ Standard) or other national/international standards/codes.
  • 4. Wind Loads According to ASCE 7• Wind loads are randomly applied dynamic loads.• They are depend on the wind speed, shape, height and topographic location of the structure.• Calculations using US customary units (Metric units also available in ASCE 7-05).
  • 5. Wind Speed Curve in Different Region Sea-side Open area Built-up area Big city (Currently not used)
  • 6. Surface Roughness and Exposure Surface Definitions ExamplesRoughness and Exposure B Urban and suburban areas, wooded areas, or other terrain with numerous closely spaced obstructions having the size of single-family dwellings or larger. C Open terrain with scattered obstructions having heights generally less than 30 ft (9.1 m). This category includes flat open country, grasslands, and all water surfaces in hurricane prone regions. D Flat, unobstructed areas and water surfaces outside hurricane prone regions. This category includes smooth mud flats, salt flats, and unbroken ice.
  • 7. Basic Wind Speed, V• Basic wind speed, V, based on 3-second gusts, 33 ft (10 m) above ground in a Ground Roughness Exposure C (defined in mph or m/s).• Some regions, such as: Taiwan, coastal China, coastal USA and Japan have very high wind speed; others such as: Indonesia, India and inland USA have lower wind speed.
  • 8. Air Flow• The more the air is streamed, the less the reaction force exerted by the structure.• Wind force highly depends on the shape of the structure.
  • 9. Wind Directionality Factor, Kd • Wind Directionality Factor, Kd shall be determined from Table 6-4. • This factor to accommodate the cross-sectional shape of the structure.
  • 10. Importance Factor, I• An importance factor, I, for the building or other structure shall be determined from Table 6-1 based on building and structure categories listed in Table 1-1.• This factor to accommodate the importance of the structure.
  • 11. Velocity Pressure Coefficient Kz• Velocity pressure exponent, Kz, depends on the site relative height to the ground, z.• This means for rooftop structure, z, would be the total height of the component and the building its installed.• This factor to accommodate the absolute height of the structure from ground level.
  • 12. Topographic Factor, Kzt• Local abrupt topography affects wind near the ground.• Wind speed depends on shape of hill, location of building, and height above ground.• The value of Kzt was taken as 1 with assumption flat region environment.• This factor to accommodate the topographic area of the structure location.
  • 13. Force Coefficient, Cf For chimneys, tanks &• Force coefficient, Cf other similar structures: determined based on the shape of the structure.• This factor to accommodate the wind-facing area of the For trussed towers: structure.
  • 14. Velocity Pressure, qz• From Bernoulli’s equation of flow, the wind pressure: (q in psf, V in mph)• The velocity pressure, qz, evaluated at height z shall be calculated by:
  • 15. Gust Effect Factor, G• Factor accounting for: – Gustiness and turbulence – Gust frequency – Gust size – Frequency of structure – Structural damping – Aerodynamic admittance – Gust correlation• Gust effect factor, G, could be calculated by• In general, gust more likely to occur at lower altitude.
  • 16. Wind Loads According to ASCE 7 FWT • Design wind force for each component shall be determined by: d FSP FCT FLe FAC c • Moment because of wind force calculated b FBF as: a M FSF a FBF b FAC c FSP FCT FL d FWT e FSF