VARIATION OF WIND

LOAD WITH HEIGHT & ASPECT RATIO

 

What Is Wind & Wind Load ? Wind is one of the nature’s greatest hazards to life on our

planet. The impact of this natural phenomenon is sudden, with little or no warning to make preparations against damages and collapse of buildings/structures.

Wind load is produced due to change in momentum of air current striking the surface of building. A building is less likely to experience the other design loads in its life but it is almost certain that the building is likely to be subjected to the design wind loads.

Types of Wind Flow Wind is caused by airs from high pressure to low pressure. The Wind flow generation is on account of atmospheric pressure differentials itself into various forms, such as,

Gales and monsoonal winds Cyclones/Hurricanes/Typhoons Tornados Thunderstorms Localised storms

Some Definitions…

Angle of Attack : It is the angle between the direction of wind and a reference axis of the structure.

Developed Height :Developed height is the height of upward penetration of the velocity profile in a new terrain.

Effective Frontal Area :It is the projected area of the structure normal to the direction of the wind.

Element of Surface Area :It is the area of surface over which the pressure coefficient is taken to be constant.

Force Coefficient :A non-dimensional coefficient such that the total wind force on a body is the product of the force coefficient.

Ground Roughness: the nature of the earth’s surface as influenced by small scale obstructions such as trees and buildings is called ground roughness.

Gust: a positive or negative departure of wind speed from its mean value, lasting for not more than, say, 2 minutes over a specified interval of time.

Gradient Height: It is the height above the mean ground level at which the gradient wind blows.

Mean Ground Level :The mean ground level is the average horizontal plane of the area enclosed by the boundaries of the structure.

Pressure Coefficient :It 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.

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.

EFFECTS OF WIND 1.General Wind Effects Direct Positive Pressure Aerodynamic Drag Negative Pressure Rocking Effects Harmonic Effects Clean-Off Effect

2.Critical Wind Effects On Buildings Inward Pressure On Exterior Walls Overturn Effect Suction On Exterior Walls Torsional Effect Vortex shedding

Variation of wind speed across India

variation of wind velocity with heightNear the Earth surface the motion is opposed and wind speed reduced by the surface friction .At the surface wind speed reduces to zero and than begin to increase with height .A graph of wind velocity vs height above ground given below.

LITERATURE REVIEW Peter A. Irwin, 2007, proposed that a Bluff body may be defined as

an object whose dimension perpendicular to the wind flow is almost equal to the dimension parallel to the wind flow. Most man-made object s are bluff bodies. Hence the study of bluff bodies is an important aspect of wind analysis.

Kenny C.S. Kwok, Peter A. Hitchcock, and Melissa D. Burton, nowadays all high rise exceeding 1000 m are faced with the challenge of wind vibrations which interfere with the comfort of the occupants. But perception of vibration is totally based on and varies from person to person. Hence no single standard could be set up which defines the maximum limit of human tolerance

About the project

We have started our project taking 9 buildings as example. We have considered the aspect ratios of0.5,1.5 and 3 for buildings.

And in each aspect ratio there are three different types of building having 3 storeys,10storeys and 20 storeys.

The height of each storey is 3 meters. The breadth of the building in the direction of wind is 20 meters. The

Columns of the building are at 5 meters interval.

Working On It

We have done our project by FORCE COEFFICIENT METHOD. The project has been undergone in DURGAPUR, WEST BENGAL. Basic wind speed (Vb) at this region = 47 m/s. Let’s assume all buildings belong to terrain category 1. Taking mean probable design life of structure is 50 years. Also taking the upwind slope of the buildings is below than 3.

Design Wind Speed ( Vz ) Design Wind Speed for any site is depend on : a) Risk level;

b) Terrain roughness, height and size of structure and c) Local topography.

It can be mathematically expressed as follows: Vz = Vb k1 k2 k3

where Vz = design wind speed at any height z in m/sk1 = probability factor ( risk coefficient)k2 = terrain, height and structure size factor and k3 = topography factor

Risk Coefficient (k1 Factor):This factor is taken as 1.0 at 10 m above ground level based

Height and structural size factor (K2): It vary due to variation of height by which the basic wind speed shall be multiplied to obtain the wind speed at different heights, in each terrain category for different sizes of buildings on 50 years mean return period.

Topography (k3 Factor ) :This factor is generally taken as 1.0 Storey shears are also changing due to variation of K2

Design wind speed, Vz =Vb*K1*K2*K3

=47*1*K2*1=47*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.

Wind pressure on the building Pz =0.6*Vz

2

Where Pz =design wind pressure in N/m2 at height z, andVz= design wind velocity in m/s at height z.

Storey Shear The wind force acting on the building is called wind load or storey

shear. It is given by

F = Cf Ae pd WhereCf is the force coefficient for the building .It is depend upon aspect ratio.Ae of the building or structure and by design wind pressurepd is the total wind load on that particular building or structure

Variation of Force Coefficients vs Aspect ratioVariation of Force Coefficients vs Aspect ratio is given below

Example

We consider For the 3 stores building having aspect ratio 0.5A table is given below..

NO. OF STOREY

HEIGHT (m) K2 Vz Cf Aeinter (m2)

Ae end (m2)

Pz(KN\m2) F inter (KN) F end(KN)

STOREY SHEAR (KN)

1 3 1.05 49.35 1.19 15 7.5 1.4613 26.084205 13.0421025 260.84205

2 6 1.05 49.35 1.19 15 7.5 1.4613 26.084205 13.0421025 156.50523

3 9 1.05 49.35 1.19 7.5 3.75 1.4613 13.0421025 6.52105125 52.16841

FOR THE 20 STORES BUILDING HAVING ASPECT RATIO 3

NO. OF STOREY

Ht. (m)

K2 Vz Cf Ae inter (m2)

Ae end (m2)

Pz(kN\m2) F inter(kN) F end(kN) STOREY SHEAR(kN)

1 3 0.99 46.53 1 15 7.5 1.29902454 19.4853681 9.74268405 1810.6663532 6 0.99 46.53 1 15 7.5 1.29902454 19.4853681 9.74268405 1732.724883 9 0.99 46.53 1 15 7.5 1.29902454 19.4853681 9.74268405 1654.7834084 12 1.006 47.282 1 15 7.5 1.341352514 20.12028772 10.06014386 1576.8419355 15 1.03 48.41 1 15 7.5 1.40611686 21.0917529 10.54587645 1496.3607856 18 1.042 48.974 1 15 7.5 1.439071606 21.58607408 10.79303704 1411.9937737 21 1.054 49.538 1 15 7.5 1.472408066 22.086121 11.0430605 1325.6494778 24 1.066 50.102 1 15 7.5 1.506126242 22.59189364 11.29594682 1237.3049939 27 1.078 50.666 1 15 7.5 1.540226134 23.103392 11.551696 1146.93741810 30 1.09 51.23 1 15 7.5 1.57470774 23.6206161 11.81030805 1054.5238511 33 1.0975 51.5825 1 15 7.5 1.596452584 23.94678876 11.97339438 960.041385712 36 1.105 51.935 1 15 7.5 1.618346535 24.27519803 12.13759901 864.254230613 39 1.1125 52.2875 1 15 7.5 1.640389594 24.60584391 12.30292195 767.153438514 42 1.12 52.64 1 15 7.5 1.66258176 24.9387264 12.4693632 668.730062915 45 1.1275 52.9925 1 15 7.5 1.684923034 25.27384551 12.63692275 568.975157316 48 1.135 53.345 1 15 7.5 1.707413415 25.61120123 12.80560061 467.879775317 51 1.1412 53.6364 1 15 7.5 1.726118043 25.89177064 12.94588532 365.434970418 54 1.1448 53.8056 1 15 7.5 1.737025555 26.05538332 13.02769166 261.867887819 57 1.1484 53.9748 1 15 7.5 1.747967421 26.21951132 13.10975566 157.646354520 60 1.152 54.144 1 7.5 3.75 1.758943642 13.19207731 6.596038656 52.76830925

Graph of Storey Shear vs No. of storeyA graph is given of STOREY SHEAR VS NO. OF STOREY GRAPH FOR RA=0.5

 

1 2 30

500

1000

1500

2000

2500

3000

FOR 3 STOREY BUILDINGFOR 10 STOREY BUILDINGFOR 20 STOREY BUILDING

NO. OF STOREY

STO

REY

SHEA

R

STOREY SHEAR VS NO. OF STOREY GRAPH FOR RA=1.5

1 2 30

500

1000

1500

2000

2500

FOR 3 STOREY BUILDINGFOR 10 STOREY BUILDINGFOR 20 STOREY BUILDING

NO.OF STOREY

STO

REY

SHEA

R

STOREY SHEAR VS NO. OF STOREY GRAPH FOR RA=3

 

1 2 30

200

400

600

800

1000

1200

1400

1600

1800

2000

FOR 3 STOREY BUILDINGFOR 10 STOREY BUILDINGFOR 20 STOREY BUILDING

NO. OF STOREY

STO

REY

SHEA

R

GRAPH FOR STOREY SHEAR VS NO. OF STOREY FOR 3 STOREY BUILDING

1 2 30

50

100

150

200

250

300

FOR a/b = 0.5FOR a/b = 1.5FOR a/b = 3

NO. OF STOREY

STO

REY

SHEA

R

GRAPH FOR STOREY SHEAR VS NO. OF STOREY FOR 10 STOREY BUILDING

1 2 3 4 5 6 7 8 9 100

200

400

600

800

1000

1200

FOR a/b = 0.5FOR a/b = 1.5FOR a/b = 3

NO.OF STOREY

STO

REY

SHEA

R

GRAPH FOR STOREY SHEAR VS NO. OF STOREY FOR 20 STOREY BUILDING

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 200

500

1000

1500

2000

2500

3000

FOR a/b =0.5FOR a/b = 1.5FOR a/b =3

NO. OF STOREY

STO

REY

SHEA

R

CONCLUSIONWe can analyse wind load by two method-Static method & Dynamic method. But in this paper we have worked out through static method or force coefficient method. Storey shear at top store is minimum and maximum in bottom. Storey shear vary with top storey to bottom. Wind load depends on height. Storey shear is directly proportional to aspect ratio of the building. For tall buildings difference of storey shear at top is very less with respect to

various aspect ratio. For tall building difference of storey shear at bottom is very high with respect to

various aspect ratio. Wind load is also depends on risk coefficient & topography factor. It depends on terrain category of building. There are four types of terrain

category.

THANK YOU