Design Wind Speed

 

Design Wind Speed ( Vz ) –

Vz= Vb k1 k2 k3

Where

Vz=design wind speed at any height z in m/s;

Vb=basic wind speed=39 m/s

k1=probability factor ( risk coeffi.

cient ) ( see 5.3.1

k2=terrain, height and structure size

factor ( see 5.3.2 ); and

k3=topography factor ( see 5.3.3 ).

 

RISK COEF’FICIENTS.FOR DIFFERENT CLASSES OF STRUCTURES IN

DIFFERENT WIND SPEED ZONES

CLASS OF STRUCTURE	MEAN PROBABLE DESIGN LIFE OF STRUCTURE IN YEARS	K1 FACTOR FOR BASIC WIND SPEED (M/S) OF
		33	39	44	47	50	55
All general buildings and structures	50	1.01	1.0	1.0	1.0	1.0	1.0
Temporary sheds, structures such as those used during construction operations ( for example, formwork and falsework ), structures during construction stages and boundary walls	5	0.82	0.76	0.73	0.71	0.70	0.61
Buildings and structures presenting a low degree of hazard to life and property in the event of failure, such as isolated towers in wooded areas, farm buildings other than residential buildings	25	0.94	0.92	0.91	0.90	0.90	0.89
Important buildings and structures such as hospitals communication buildings / towers, power plant structures	100	1.05	1.06	1.07	1.07	1.08	1.08

 

A and B are coefficients having the following values for different basic wind speed zones:

 

Zone	A	B
33 m/s 39 m/s 44 m/s 47 m/s 50 m/s 55 m/s	83.2 84.0 88.0 88.0 88.8 90.8	9.2 14.0 18.0 20.5 22.8 27.3

 

 

 

Basic Wind Speed -  basic wind speed map of India, as applicable to 10 m height above mean ground level for different zones of the country. Basic wind speed is based on peakgust velocity averaged over a short time interval of about 3 seconds and corresponds to mean heights above ground level in an open terrain ( Category 2 ).

 

BASIC WIND SPEED(Vb) AT 10 m HEIGHT FOR SOME IMPORTANT CITIES/TOWNS

 

City/Town	Basic Wind Speed  ( m/s )	City/Town	Basic Wind Speed ( m/s )
Agra Ahmadabad Ajmer Almora Amritsar Asansol Aurangabad Bahraich Bangalore Barauni Bareilly Bhatinda Bhilai Bhopal Bhubaneshwar Bhuj Bikaner Bokaro Bombay Calcutta Calicut Chandigarh Coimbatore Cuttack Darbhanga Darjeeling Dehra Dun Delhi Durgapur Gangtok Gauhati Gaya Gorakhpur Hyderabad Imphal Jabalpur Jaipur Ja mshedpur	47 39 47 47 47 47 39 47 33 47 47 47 39 39 50 50 47 47 44 50 39 47 39 50 55 47 47 47 47 47 50 39 47 44 47 47 47 47	Jhansi jodhpur Kanpur Kohima Kurnool Lakshadweep Lucknow Ludhiana Madras Madurai Mandi Mangalore Moradabad Mysore Nagpur Nainital Nasik Nellore Panjim Patiala Patna Pondicherry Port Blair Pune Raipur Rajkot Ranchi Roorkee R ourkela Simla Srinagar Surat Tiruchchirrappalli Trivandrum Udaipur Vadodara Varanasi Vijaywada Visakhapatnam	47 47 47 44 39 39 47  47 50 39 39 39 47 44 44 47 39 50 39 47 47 50 44 39 39 39 39 39 39 39 39 33 47 39 47 44 47 50 50

Wind speed(m/s)	zone
33	1
39	2
44	3
47	4
50	5

 

 

 K2=1.09  FACTORS TO OBTAIN DESIGN WIND SPEED VARIATION WITH HEIGHT IN DIFFERENT TERRAINS FOR DIFFERENT CLASSES OF BUILDINGS/STRUCTURES

 

HEIGHT	TERRAIN CATEGORY 1			TERRAIN CATEGORY2			TERRAIN CATEGORY 3			TERRAIN CATEGORY4
M	Class			Class			Class			Class
	A	B	C	A	B	C	A	B	C	A	B	C
1	2	3	4	5	6	7	8	9	10	11	12	13
10 15 20 30 50 100 150 200 250 300 350 400 450 500	1.05 1.09 1.12 1.15 1.20 1.26 1.30 1.32 1.34 1.35 1.37 1.38 1.39 1.40	1.03 1.07 1.10 1.13 1.18 1.24 1.28 1.30 1.32 1.34 1.35 1.36 1.37 1.38	0.99 1.03 1.06 1.09 1.14 1.20 1.24 1.26 1.28 1.30 1.31 1.32 1.33 1.34	1 1.05 1.07 1.12 1.17 1.24 1.28 1.30 1.32 1.34 1.36 1.37 1.38 1.39	0.98 1.02 1.05 1.10 1.15 1.22 1.25 1.28 1.31 1.32 1.34 1.35 1.36 1.37	0.93 0.97 1 1.04 1.1 1.17 1.21 1.24 1.26 1.28 1.29 1.30 1.31 1.30	0.91 0.97 1.01 1.06 1.12 1.2 1.24 1.27 1.29 1.31 1.32 1.34 1.35 1.36	0.88 0.94 0.98 1.03 1.09 1.17 1.21 1.24 1.26 1.28 1.30 1.31 1.32 1.33	0.82 0.87 0.91 0.96 1.02 1.10 1.15 1.18 1.20 1.22 1.24 1.25 1.26 1.28	0.80 0.80 0.80 0.97 1.1 1.2 1.24 1.27 1.28 1.30 1.31 1.32 1.33 1.34	0.76 0.760.76 0.93 1.05 1.51 1.2 1.22 1.24 1.26 1.27 1.28 1.29 1.3	0.67 0.67 0.67 0.83 0.95 1.05 1.1 1.13 1.16 1.17 1.191.20 1.21 1.22

 

 

 

 

 

Tojography ( ks Factor ) - The basicwind speed Vb given in Fig. 1 takes account of the general level of site above sea level. This does not allow for local topographic features such as hills, valleys, cliffs, escarpments, or ridges which can significantly affect wind speed in their vicinity. The effect of topography is to accelerate wind near the summits of hills or crests‘of cliffs, escarpments or ridges and decelerate the wind in valleys or near the foot of cli%, steep escarpments, or ridges.

 The effect of topography will be significant at a site when the upwind slope (ϴ) is greater than about 3 degree, and below that, the value of k3 may be taken to be equal to 1.0 The value of k3 is confined in the range of 1.0 to 1.36 for slopes greater than 3degree. A method of evaluating the value of k3 for values greater than 1.0 is given in Appendix C. It may be noted that the value of

K3 varies with height above ground level, at a maximum near the ground, and reducing to 1.0at higher levels.