IS 802 RECOMMENDATIONS ON WIND LOAD ON

TRANSMISSION LINE TOWERS

(IS802 (Partl/Sec 1):1995)

Indian StandardUSE OF STRUCTURAL STEEL IN

OVERHEADTRANSMISSION LINE TOWERS -

CODE OF PRACTICEPART 1 MATERIALS, LOADS AND

PERMISSIBLE STRESSES

Section 1 Materials and Loads

Basic Wind Speed, Vb

• Ref: IS 875 (3) – WIND MAP

Meteorological Reference Wind Speed, VR

Design Wind Speed, Vd

• Risk coefficient K1 depends on the basic wind speed Vb (or wind zone) and the reliability level

• Reliability level 1 shall be adopted for EHV transmission lines up to 400 kV class.

• Reliability level 2 shall be adopted for EHV transmission lines above 400 kV class. Triple and quadruple circuit towers up to 400 kV lines shall be designed corresponding to the reliability level 2.

• Reliability level 3 shall be adopted for tall river crossing towers and special towers

Terrain Roughness Coefficient, K2

Design Wind Pressure, Pd

• The design wind pressure on towers, conductors and insulators shall be obtained by the following relationship :

Wind Load on Tower• the tower is divided into different panels

having a height ‘h’. (normally be taken between the intersections of the legs and bracings).

• For a lattice tower of square cross section, the resultant wind load Fwt in Newtons, for wind normal to the longitudinal face of tower, on a panel height ‘h’ applied at the centre of gravity of this panel is:

Pd = design wind pressure, in N/m2

Cdt = drag coefficient for panel under consideration against which the wind is blowing.

Ae = total net surface area of the legs, bracings, cross arms and secondary members of the panel projected normal to the face in m2

GT = gust response factor,

Wind Load on Conductor and Ground wire

WIND LOAD ON ROAD BRIDGES

(IRC 6 Section II – 1966)

• The lateral wind force against any exposed moving live load shall be considered as acting at 1.5 m above the roadway and shall be assumed to have the following values

While calculating the wind force on live load, the clear distance between the trailers of a train of vehicles shall not be omitted.

WIND LOAD ON COOLING TOWERS

IS : 11504 - 1985

Indian StandardCRITERIA FOR STRUCTURAL

DESIGN OFREINFORCED CONCRETE

NATURALDRAUGHT COOLING TOWERS

Wind Pressure• The basic wind pressure shall, in general, conform

to IS : 875-1964* excepting in places where local conditions warrant special investigations

• The wind pressure coefficient distribution on the shell should preferably be derived from wind tunnel tests of a model of the proposed tower shell shape

• the wind pressure distribution for cooling towers not more than 100 m in height and not more than 120 m in base diameter built singly or in groups spaced at clear distance of not less than 0.5 times the base diameter of the largest cooling tower in the group can be computed as follows

• The wind pressure distribution on the outside of the shell is assumed to be symmetrical about the centre line in the direction of wind.

• For practical design these values may be increased by 10 percent to take into account geometrical imperfections

• The wind pressure coefficient distribution around the shell is defined by the following equation

Pressure FluctuationsThe steady pressure coefficients discussed

earlier are for uniform pressure distribution in laboratory conditions and further allowances should be made in assessing the wind loading for

• load intensification due to natural turbulence in the incident wind, and

• load intensification due to turbulence induced in the incident wind by adjacent cooling towers in a group or of the structures of significant dimension in the vicinity

WIND LOAD ON RC CHIMNEYS

IS 4998 ( Part 1) : 1992

Indian StandardCRITERIA FOR DESIGN OF

REINFORCEDCONCRETE CHIMNEYS

PART 1 ASSESSMENT OF LOADS

ESTIMATION OF WIND LOADSTWO METHODS• The first is a simplified method and is likely

to yield slightly conservative results as far as across wind loads are concerned.

• The second method is based on random response method.The wind loads shall be estimated by

both the methods and the loading which yields higher moments shall be

considered for design of chimneys.

SIMPLIFIED METHOD1. Along-Wind Load or Drag ForceThe along-wind load or drag force per unitheight of the chimney at any level shall becalculated from the equation

• The design wind pressure ( pz ), for the along wind response, shall be obtained in accordance with IS 875 ( Part 3 ) ; 1987, taking the appropriate factor depending upon the class of the structure as defined in that standard.

• The chimney shall be divided into ten or more sections along its height and the load at any section shall be calculated by suitably averaging the loads above and below it.

The moments are calculated from the sectional forces treating the chimney as a free standing structure

RANDOM RESPONSE METHODAlong-Wind Load on a Chimney• along-wind response of a chimney shall

also be calculated by the Gust Factor method

• The use of the Gust Factor method requires a knowledge of Hourly Mean Wind Speed ( HMW ).

• Hourly mean wind speed at any height (z), shall be obtained as per IS 875 ( Part 3 ) : 1987

IS 6533 (Part 2) : 1989

Indian StandardCODE OF PRACTICE FOR DESIGN ANDCONSTRUCTION OF STEEL CHIMNEY

PART 2 STRUCTURAL ASPECT

WIND LOAD ON STEEL CHIMNEYS

• The wind loads shall be calculated in accordance with the provisions contained in IS 875 (Part 3) : 1987

• Wind force on ladders and other fixtures fixed to a chimney shall be determined and added to the force on the chimney

Calculation of Static Wind Load• Static wind pressure, q, acting normal to

the surface of chimney shall be taken as specified in IS 875 (Part 3) : 1987 for the appropriate wind zone, terrain and topography

• the chimney height shall be divided into a number of convenient zones such that the number of zones shall not be less than three and the zone height shall not exceed 10 m

• Static wind force acting at the midpoint of Kth zone ( K varying from 1 to r ) shall be calculated from the formula

Calculation of Dynamic Wind Loads• Dynamic effect of wind is influenced by a

number of factors, such as, mass and its disposition along chimney height, period and mode of natural oscillation, Logarithmic decrement of dampening, pulsation of velocity thrust, etc.

• Values of dynamic components of wind load should be determined for each mode of oscillation of the chimney as a system of inertia forces acting at the centre of the zone being considered.

WIND LOAD ON RAILWAY BRIDGESBRIDGE RULES

(IN SI UNITS) RULES SPECIFYING THE LOADS FOR

DESIGN OF SUPER-STRUCTURE AND SUB-STRUCTURE OF BRIDGES AND FOR ASSESSMENT

OF THE STRENGTH OF EXISTING BRIDGES

• For purposes of design where no meteorological records are available, the Map as given in IS: 875 (Part 3) in conjunction with the Table therein, may be used for determining the basic wind pressures.

• The wind pressure specified above shall apply to all loaded or unloaded bridges provided that a bridge shall not be considered to be carrying any live load when the wind pressure at deck level exceeds the following limits:

• The wind load shall be computed from the appropriate basic wind pressure and the exposed area

exposed area• For unloaded spans net exposed area

shall be considered as one and half times the projected area of the span, except for plate girders for which the area of the leeward girder shall be multiplied by the factors based on its depth and the distance from the windward girder

• For loaded spans the net exposed area shall be computed as the sum of (i) and (ii)(i) as detailed above(ii) The projected area of the moving load

• In the case of railway bridges: the area of the moving load shall be taken as from 600mm above rail level to the top of the highest stock for which the bridge is designed

• In the case of foot bridges: the height of the moving load is to be taken as 2m throughout the length of the span