Wind LoadingWind Loading

Akshay KaushalAkshay KaushalRam KakkarRam Kakkar

Wind Loads- Key PointsWind Loads- Key Points

In reality dynamic…In reality dynamic… … … but for most structures converted to but for most structures converted to

constant pressure (and shear)constant pressure (and shear) Normally only horizontal load is presentNormally only horizontal load is present

The Effects of Wind on a StructureThe Effects of Wind on a Structure

PositiveInternal

pressure

Winddirection

OpeningOn upstreamside

Suction on roof

PositiveNegative

The Effects of Wind on StructureThe Effects of Wind on Structure

NegativeInternal

pressure

Winddirection

OpeningOn downstreamside

Suction on roof

Positive Negative

The Effects of Wind on StructureThe Effects of Wind on Structure

Winddirection

Frictionalforces onface

Suction out of plane(similar to roof suction)

Wind SpeedsWind Speeds Wind speed usually consists of two partsWind speed usually consists of two parts

– Mean wind speedMean wind speed– Fluctuating partFluctuating part

Obtain a mean wind speed fromObtain a mean wind speed from– Basic value for location adjusted for…Basic value for location adjusted for…

AltitudeAltitude Terrain Roughness (e.g. sea, forest, city)Terrain Roughness (e.g. sea, forest, city) Season (for short-term structures only)Season (for short-term structures only) Direction (can often be ignored)Direction (can often be ignored) Possibly other factors (cliffs, buildings etc.)Possibly other factors (cliffs, buildings etc.)

Obtain degree of fluctuation fromObtain degree of fluctuation from– Terrain (e.g. sea, forest, city)Terrain (e.g. sea, forest, city)

Velocity PressureVelocity Pressure Design wind speed is calculated from;Design wind speed is calculated from;

VVzz = V = Vbb . k1 . k2 . K3 . k1 . k2 . K3

VVbb = basic wind speed (fig. 1 map) = basic wind speed (fig. 1 map) k1 = risk coefficient (clause 5.3.1)k1 = risk coefficient (clause 5.3.1) k2 = terrain height & structure size factor (clause 5.3.2)k2 = terrain height & structure size factor (clause 5.3.2) k3 = topography factor (clause 5.3.3)k3 = topography factor (clause 5.3.3)

Design Wind speed is converted to “design wind Design Wind speed is converted to “design wind pressure” through expression of the form; pressure” through expression of the form;

ppz z =0.6 =0.6 VVzz22 (clause 5.4) (clause 5.4)

Wind forces can be calculated as;Wind forces can be calculated as;

F = (CF = (Cpepe - C - Cpipi) . A . p) . A . pz z

where,Cwhere,Cpepe = Coeff. For external pressure (clause 6.2.2) = Coeff. For external pressure (clause 6.2.2)CCpipi = Coeff. For internal pressure (clause 6.2.3) = Coeff. For internal pressure (clause 6.2.3)A = surface area of structural elementA = surface area of structural element

Steps Steps Obtain base wind speed at locationObtain base wind speed at location Calculate design wind speed at siteCalculate design wind speed at site Calculate design wind pressureCalculate design wind pressure Identify pressure coefficientsIdentify pressure coefficients Calculate forces on structureCalculate forces on structure

Example 1Example 1 Structure in LudhianaStructure in Ludhiana Very boring!!Very boring!! Calculate force on upstream wallCalculate force on upstream wall Opening in “downstream” faceOpening in “downstream” face

Wind direction

10m

10m

5m

3m

5m

Basic wind velocity, VBasic wind velocity, Vbb= 47ms= 47ms -1-1 (from map (Fig. 1) (from map (Fig. 1)

It is ‘Class A’ structure in ‘Terrain Category 3’ with It is ‘Class A’ structure in ‘Terrain Category 3’ with design life of 100 years. design life of 100 years. – Risk coefficient, k1=1.07 (Table 1)Risk coefficient, k1=1.07 (Table 1)– Terrain height factor, k2 =0.91 (Table 2)Terrain height factor, k2 =0.91 (Table 2)– Topography factor, k3 = 1.0 (clause 5.3.3)Topography factor, k3 = 1.0 (clause 5.3.3)

Design wind speed, VDesign wind speed, Vzz = 47 x 1.07 x 0.91 x 1.0 = 47 x 1.07 x 0.91 x 1.0 = 45.76 ms= 45.76 ms -1-1

Design wind pressure, pDesign wind pressure, pzz = 0.6 x 45.76 x 45.76 = 0.6 x 45.76 x 45.76 = 1256.39 N/m= 1256.39 N/m22

Building height ratio, h/w = 0.5Building height ratio, h/w = 0.5Building plan rario, l/w = 1.0Building plan rario, l/w = 1.0CCpepe = +0.7 = +0.7

Area of wall, A = 10 x 5 = 50 mArea of wall, A = 10 x 5 = 50 m22

As opening area is larger than 20% of wall area,As opening area is larger than 20% of wall area,CCpipi = -0.7 = -0.7

Hence, Wind Force (F) = [0.7-(-0.7)] x 50 x 1256.39Hence, Wind Force (F) = [0.7-(-0.7)] x 50 x 1256.39 = 87.95 KN= 87.95 KN

Example 2Example 2 Chimney (As per IS 4998 Part-1)Chimney (As per IS 4998 Part-1)

Drag force, FDrag force, Fzz = p = pzz . C . Cdd. d. dzz

Where, pWhere, pzz = design wind pressure (IS 875 Part-3) = design wind pressure (IS 875 Part-3) CCdd = drag coefficient for chimney taken as 0.8 = drag coefficient for chimney taken as 0.8

(clause A-4.1)(clause A-4.1) ddzz = dia of chimney at z m. height from top of = dia of chimney at z m. height from top of

foundation foundation

Example 3Example 3

Towers (As per IS 875 Part-3)Towers (As per IS 875 Part-3)

Design wind load, F = CDesign wind load, F = C ff . p . pzz . A . Aee Where, CWhere, C ff = force coefficient (Table 30 or 31) = force coefficient (Table 30 or 31)

AAee = effective area against wind = effective area against wind