Wind Loads Calculation (ASCE 7)

Henry Kurniadi

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.

WIND LOADS

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).

Wind Speed Curve in Different Region

Sea-side Open area Built-up area Big city (Currently not used)

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.

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.

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.

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.

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.

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.

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.

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:

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:

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.

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