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.