wind loads calculation
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Wind Loads Calculation (ASCE 7)
Henry Kurniadi
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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.
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WIND LOADS
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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).
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Wind Speed Curve in Different Region
Sea-side Open area Built-up area Big city (Currently not used)
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Surface Roughness and ExposureSurface
Roughness and Exposure
Definitions Examples
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 obstructionshaving heights generally less than 30 ft
(9.1 m). This categoryincludes flat open country, grasslands,
and all water surfaces inhurricane prone regions.
D Flat, unobstructed areas and water surfaces
outside hurricane prone regions. This category includes
smooth mud flats, salt flats, and unbroken ice.
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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.
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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.
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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.
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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.
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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.
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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.
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Force Coefficient, Cf
• Force coefficient, Cf determined based on the shape of the structure.
• This factor to accommodate the wind-facing area of the structure.
For chimneys, tanks & other similar structures:
For trussed towers:
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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:
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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.
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Wind Loads According to ASCE 7
• Design wind force for each component shall be determined by:
• Moment because of wind force calculated as:
eFdFFFcFbFaFM WTLCTSPACBFSF
FWT
FSP FCTFL
FSF
b
d
e
FBF
a
cFAC