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DESIGN WIND LOAD
Huzaif Bin Mohmad
M Tech : CTM
NITTTR Chandigarh
Introduction
Wind is essentially the large scale horizontal moment
of free air. It plays an important role in the design of
tall structures because it exerts load on the building.
High Rise Building- A building having height more
then 15m as per NBC 2005 of india is called High
rise building.
Wind means the motion of air in the atmosphere. The response
of structures to wind depends on the characteristics of the
wind.
DESIGN WIND LOAD
AS PER 875 (III)
Terminology
For the purpose of wind loads, the following definitions shall apply.
Angle of attack — Angle between the direction of wind and a reference axis of the structure.
Breadth — Breadth means horizontal dimension of the building measured normal to the direction of wind.
Depth — Depth means the horizontal dimension of the building measured in the direction of the wind.
NOTE — Breadth and depth are dimensions measured in relation to the direction of the wind, whereas length and width are dimensions related to the plan.
Terminology
Developed height - is the height of upward
penetration of the velocity profile in a new terrain.
Effective frontal area - The projected area of the
structure normal to the direction of the wind.
Element surface area- The area of surface over
which the pressure coefficient is taken to be
constant.
Terminology
Force coefficient- A non-dimensional coefficient such that the total wind force on a body is the product of the force coefficient, the dynamic pressure of the incident design wind speed and the reference area over which the force is required.
Gust- A positive or negative departure of wind speed from its mean value, lasting for not more than say 2 min over a specified interval of time.
Peak gust- Peak gust or peak gust speed is the wind speed associated with the maximum amplitude.
Terminology
Fetch length- is the distance measured along the wind from a boundary at which a change in the type of terrain occurs.
Gradient height- is the height above the mean ground level at which the gradient wind blows as a result of balance among pressure gradient force, coriolis force and centrifugal force.
Pressure coefficient- is the ratio of the difference between the pressure acting at a point on a surface and the static pressure of the incident wind to the design wind pressure.
Terminology
Suction- means pressure less than the atmospheric (static)
pressure and is taken to act away from the surface.
Velocity profile- The variation of the horizontal
component of the atmospheric wind speed at different
heights above the mean ground level is termed as velocity
profile.
Topography-The nature of the earth’s surface as
influenced by the hill and valley configurations.
Design Wind Speed (Vz )
Risk coefficient (k1)
Terrain, height and structure size
factor (k2 )
Design Wind Pressure
The design wind pressure at any height above mean
ground level shall be obtained by the following
relationship between wind pressure and wind velocity:
pz = 0.6 Vz2
where
pz = design wind pressure in N/m2 at height Z, and
Vz = design wind velocity in m/s at height Z.
Force Coefficients
The value of force coefficients apply to a building or structure as a whole, and when multiplied by the effective frontal area, Ae of the building or structure and by design wind pressure, pd give the total wind load on that particular building or structure.
F = Cf Ae pd
where F is the force acting in a direction specified in the respective tables and Cf is the force coefficient for the building.