Chapter 4Wind Load

NBC 2016 (SP 7:2016) Volume 1

2016 · PDF page 435

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NATIONAL BUILDING CODE OF INDIA 2016

4.1 General

4.1.1 This clause gives wind forces and their effects (static and dynamic) that should be taken into account when designing buildings, structures and components thereof.

4.1.2 Wind speeds vary randomly both in time and space and hence assessment of wind loads and response predictions are very important in the design of several buildings and structures. A large majority of structures met with in practice do not however, suffer wind induced oscillations and generally do not require to be examined for the dynamic effects of wind. For such normal, short and heavy structures, estimation of loads using static wind analysis has proved to be satisfactory. The details of this method involving important wind characteristics such as the basic wind speeds, terrain categories, modification factors, wind pressure and force coefficients, etc, are given in 4.4 and 4.5.

4.1.3 Nevertheless, there are various types of structures or their components such as some tall buildings which require investigation of wind induced oscillations. The influence of dynamic velocity fluctuations on the along wind loads (drag loads) for these structures shall be determined using Gust Factor Method, included in 4.8. A method for calculation of across wind response of tall buildings is included in 4.8.3.

4.1.4 This Section also applies to buildings or other structures during erection/construction and the same shall be considered carefully during various stages of erection/construction. In locations where the strongest winds and icing may occur simultaneously, loads on structural members, cables and ropes shall be calculated by assuming an ice covering based on climatic and local experience.

4.1.5 Wind is air in motion relative to the surface of the earth. The primary cause of wind is traced to earth's rotation and differences in terrestrial radiation. The radiation effects are primarily responsible for convection either upwards or downwards. The wind generally blows horizontal to the ground at high wind speeds. Since vertical components of atmospheric motion are relatively small, the term 'wind' denotes almost exclusively the horizontal wind; vertical winds are always identified as such. The wind speeds are assessed with the aid of anemometers or anemographs which are installed at meteorological observatories at heights generally varying from 10 to 30 m above ground.

4.1.6 Very strong winds (more than 80 km/h) are generally associated with cyclonic storms, thunderstorms, dust storms or vigorous monsoons. A feature of the cyclonic storms over the Indian area is that they rapidly weaken after crossing the coasts and move as depressions/lows inland. The influence of a severe storm after striking the coast does not; in general exceed about 60 km, though sometimes, it may extend even up to 120 km. Very short duration hurricanes of very high wind speeds called Kal Baisaki or Norwesters occur fairly frequently during summer months over North East India.

4.1.7 The wind speeds recorded at any locality are extremely variable and in addition to steady wind at any time, there are effects of gusts which may last for a few seconds. These gusts cause increase in air pressure but their effect on stability of the building may not be so important; often, gusts affect only part of the building and the increased local pressures may be more than balanced by a momentary reduction in the pressure elsewhere. Because of the inertia of the building, short period gusts may not cause any appreciable increase in stress in main components of the building although the walls, roof sheeting and individual cladding units (glass

panels) and their support-ing members such as purlins, sheeting rails and glazing bars may be more seriously affected. Gusts can also be extremely important for design of structures with high slenderness ratios.

4.1.8 The liability of a building to high wind pressures depends not only upon the geographical location and proximity of other obstructions to air flow but also upon the characteristics of the structure itself.

4.1.9 The effect of wind on the structure as a whole is determined by the combined action of external and internal pressures acting upon it. In all cases, the calculated wind loads act normal to the surface to which they apply.

4.1.10 The stability calculations as a whole shall be done considering the combined effect, as well as separate effects of imposed loads and wind loads on vertical surfaces, roofs and other part of the building above general roof level.

4.1.11 Buildings shall also be designed with due attention to the effects of wind on the comfort of people inside and outside the buildings.

NOTES

1 This Section does not apply to buildings or structures with unconventional shapes, unusual locations, and abnormal environmental conditions that have not been covered in this Code. Special investigations are necessary in such cases to establish wind loads and their effects. Wind tunnel studies may also be required in such situations.

2 In the case of tall structures with unsymmetrical geometry, the designs may have to be checked for torsional effects due to wind pressure.

3 The provisions given in good practice [6-1(4)] may be suitably referred by the design engineer/planner for reference.

4.2 Notations

The notations to be followed unless otherwise specified in relevant clauses under wind loads are given in Annex B.

4.3Terminology

4.3.1 For the purpose of wind loads, the definitions given below shall apply.

4.3.1.1 Angle of attack — An angle between the direction of wind and a reference axis of the structure.

4.3.1.2 Breadth — It means horizontal dimension of the building measured normal to the direction of wind.

4.3.1.3 Depth — It 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 wind, whereas length and width are dimensions related to the plan.

4.3.1.4 Developed height — It is the height of upward penetration of the velocity profile in a new

PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS

terrain. At large fetch lengths, such penetration reaches the gradient height, above which the wind speed may be taken to be constant. At lesser fetch lengths, a velocity profile of a smaller height but similar to that of the fully developed profile of that terrain category has to be taken, with the additional provision that the velocity at the top of this shorter profile equal to that of the un-penetrated earlier velocity profile at that height.

4.3.1.5 Effective frontal area — The projected area of the structure normal to the direction of wind.

4.3.1.6 Element of surface area — The area of surface over which the pressure coefficient is taken to be constant.

4.3.1.7 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.

NOTE — When the force is in the direction of the incident wind, the non-dimensional coefficient will be called as ‘drag coefficient’. When the force is perpendicular to the direction of incident wind, the non-dimensional coefficient will be called as ‘lift coefficient’.

4.3.1.8 Ground roughness — The nature of the earth's surface as influenced by small scale obstructions such as trees and buildings (as distinct from topography) is called ground roughness.

4.3.1.9 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.

4.3.1.10 Peak gust — A peak gust or peak gust speed is the wind speed associated with the maximum amplitude.

4.3.1.11 Fetch length — It is the distance measured along the wind from a boundary at which a change in the type of terrain occurs. When the changes in terrain types are encountered (such as, the boundary of a town or city, forest, etc), the wind profile changes in character but such changes are gradual and start at ground level, spreading or penetrating upwards with increasing fetch length.

4.3.1.12 Gradient height — It 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. For the purpose of this Section, the gradient height is taken as the height above the mean ground level, above which the variation of wind speed with height need not be considered.

4.3.1.13 Tall building — A building with a height more than or equal to 50 m or having a height to smaller

lateral dimension more than 6.

4.3.1.14 Mean ground level — The mean ground level is the average horizontal plane of the area enclosed by the boundaries of the structure.

4.3.1.15 Pressure coefficient — It is the ratio of the difference between the pressure acting at a point on the surface and the static pressure of the incident wind to the design wind pressure, where the static and design wind pressures are determined at the height of the point considered after taking into account the geographical location, terrain conditions and shielding effect. The pressure coefficient is also equal to $[1-(V_{\mathrm{p}}/V_{\mathrm{z}})^{2}]$ , where $V_{p}$ is the actual wind speed at any point on the structure at a height corresponding to that of $V_{z}$ .

NOTE — Positive sign of the pressure coefficient indicates pressure acting towards the surface and negative sign indicates pressure acting away from the surface.

4.3.1.16 Return period — It is the number of years, reciprocal of which gives the probability of extreme wind exceeding a given wind speed in anyone year.

4.3.1.17 Shielding effect — Shielding effect or shielding refers to the condition where wind has to pass along some structure(s) or structural element(s) located on the upstream wind side, before meeting the structure or structural element under consideration. A factor called ‘shielding factor’ is used to account for such effects in estimating the force on the shielded structures.

4.3.1.18 Suction — It means pressure less than the atmospheric (static) pressure and is taken to act away from the surface.

4.3.1.19 Solidity ratio — It is equal to the effective area (projected area of all the individual elements) of a frame normal to the wind direction divided by the area enclosed by the boundary of the frame normal to the wind direction.

NOTE — Solidity ratio is to be calculated for individual frames.

4.3.1.20 Terrain category — It means the characteristics of the surface irregularities of an area which arise from natural or constructed features. The categories are numbered in increasing order of roughness.

4.3.1.21 Topography — The nature of the earth's surface as influenced by the hill and valley configurations.

4.3.1.22 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.

4.4Wind Speed

4.4.1 Nature of Wind in Atmosphere

In general, wind speed in the atmospheric boundary layer increases with height from zero at ground level to maximum at a height called the gradient height. There is usually a slight change in direction (Ekman effect) but this is ignored in this Section. The variation with height depends primarily on the terrain conditions. However, the wind speed at any height never remains constant and it has been found convenient to resolve its instantaneous magnitude into an average or mean value and a fluctuating component around this average value. The average value depends on the average time employed in analyzing the meteorological data and this averaging time varies from few seconds to several minutes. The magnitude of fluctuating component of the wind speed, which is called gust, depends on the averaging time. In general, smaller the averaging interval more is the magnitude of the gust speed.

4.4.2 Basic Wind Speed

Figure 1 gives 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 peak gust velocity averaged over a short time interval of about 3 s and corresponds to mean heights above ground level in an open terrain (Category 2). Basic wind speeds presented in Fig. 1 have been worked out for a 50 year return period. Basic wind speed for some important cities/towns is also given in Annex C.

4.4.3Design Wind Speed $(V{mathrm{z}})$

The basic wind speed $(V_{\mathrm{b}})$ for any site shall be obtained from Fig. 1 and shall be modified to include the following effects to get design wind speed, $V_{z}$ at any height $z$ , for the chosen structure:

a) Risk level,

b) Terrain roughness and height of structure,

c) Local topography, and

d) Importance factor for the cyclonic region.

It can be mathematically expressed as follows:

V _ {\mathrm{z}} = V _ {\mathrm{b}} k _ {1} k _ {2} k _ {3} k _ {4}

where

$V_{z} =$ design wind speed at height $z$ , in $\mathrm{m / s}$ ;

$k_{1}$ = probability factor (risk coefficient) (see 4.4.3.1);

$k_{2}$ = terrain roughness and height factor (see 4.4.3.2);

$k_{3} =$ topography factor (see 4.4.3.3); and

$k_{4} =$ importance factor for the cyclonic region (see 4.4.3.4).

NOTE — Wind speed may be taken as constant up to a height of 10 m. However, pressures for buildings less than 10 m high may be reduced by 20 percent for evaluating stability and design of the framing.

4.4.3.1 Risk coefficient $(k\{1\}$ factor)

Figure 1 gives basic wind speeds for Terrain Category 2 as applicable at 10 m above ground level based on 50 years mean return period. The suggested life period to be assumed in design and the corresponding $k_{1}$ factors for different class of structures for the purpose of design are given in Table 4. In the design of buildings and structures, a regional basic wind speed having a mean return period of 50 years shall be used except as specified in Note of Table 4.

4.4.3.2 Terrain, height factor $(k\{2\}$ factor)

4.4.3.2.1 Terrain

Selection of terrain categories shall be made with due regard to the effect of obstructions which constitute the ground surface roughness. The terrain category used in the design of a structure may vary depending on the direction of wind under consideration. Wherever sufficient meteorological information is available about the nature of wind direction, the orientation of any building or structure may be suitably planned.

Terrain in which a specific structure stands shall be assessed as being one of the following terrain categories:

a) Category 1 — Exposed open terrain with few or no obstructions and in which the average height of any object surrounding the structure is less than 1.5 m. The equivalent aerodynamic roughness height, $(z_{0,1})$ for this terrain is 0.002 m. Typically this category represents open sea-coasts and flat plains without trees.

b) Category 2 — Open terrain with well scattered obstructions having heights generally between 1.5 m and 10 m. The equivalent aerodynamic roughness height, $(z_{0,2})$ for this terrain is 0.02 m. This is the criterion for measurement of regional basic wind speeds and represents airfields, open park lands and undeveloped spar-sely built-up outskirts of towns and suburbs. Open land adjacent to sea coast may also be classified as Category 2 due to roughness of large sea waves at high winds.

c) Category 3 — Terrain with numerous closely spaced obstructions having the size of buildings/structures up to 10 m in height with or without a few isolated tall structures. The equivalent aerodynamic roughness height, $(z_{0,3})$ for this terrain is 0.2 m. This category represents well wooded areas, and shrubs, towns and industrial areas full or partially

PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS

Based upon Survey of India Political map printed in 2002.

The territorial waters of India extend into the sea to a distance of twelve nautical miles measured from the appropriate baseline.

The interstate boundaries between Arunachal Pradesh, Assam and Meghalaya shown on this map are as interpreted from the

North-Eastern Areas (Reorganization) Act, 1971, but have yet to be verified.

The state boundaries between Uttarakhand & Uttar Pradesh, Bihar & Jharkhand, and Chhattisgarh & Madhya Pradesh have not been verified by the Governments concerned.

The administrative headquarters of Chandigarh, Haryana and Punjab are at Chandigarh.

The external boundaries and coastlines of India agree with the Record/Master Copy certified by Survey of India.

The responsibility for the correctness of internal details rests with the publisher.

1 The occurrence of a tornado is possible in virtually any part of India. They are particularly more severe in the northern parts of India. The recorded number of these tornadoes is too small to assign any frequency. The devastation caused by a tornado is due to exceptionally high winds about its periphery, and the sudden reduction in atmospheric pressure at its centre, resulting in an explosive outward pressure on the elements of the structure. The regional basic wind speeds do not include any specific allowance for tornadoes. It is not the usual practice to allow for the effect of tornadoes unless special requirements are called for as in the case of important structures such as, nuclear power reactors and satellite communication towers.

2 The total number of cyclonic storms that have struck different sections of east and west coasts are included in Fig. 1, based on available records for the period from 1877 to 1982. The figures above the lines (between the stations) indicate the total number of severe cyclonic storms with or without a core of hurricane winds (speeds above $87\mathrm{km / h}$ ) and the figures in the brackets below the lines indicate the total number of cyclonic storms. Their effect on land is already reflected in the basic wind speeds specified in Fig. 1. These have been included only as additional information.

FIG. 1 Basic Wind Speed in M/s (based on 50-YEARS Return Period)

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N = mean probable design life of structure in years;

Table 4Risk Coefficients for Different Classes of Structures in Different Wind Speed Zones (Clause 4.4.3.1)

Sl No.	Class of Structure	Mean Probable Design Life of Structure in Years	$K_1$ Factor for Basic Wind Speed (in m/s) of
Sl No.	Class of Structure	Mean Probable Design Life of Structure in Years	33	39	44	47	50	55
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)
i)	All general buildings and structures	50	1.0	1.0	1.0	1.0	1.0	1.0
ii)	Temporary sheds, structures such as those used during construction operations (for example form-work and false work), structures during construction stages and boundary walls	5	0.82	0.76	0.73	0.71	0.70	0.67
iii)	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
iv)	Important buildings and structures, such as hospitals, communication buildings/towers, and power plant structures	100	1.05	1.06	1.07	1.07	1.08	1.08

NOTE — The factor $k_{1}$ is based on statistical concepts which take into account the degree of reliability required and period of time in years during which there will be exposure to wind, that is, life of the structure. Whatever wind speed is adopted for design purposes, there is always a probability (however small) that it may be exceeded in a storm of exceptional violence; more the period of years over which there is exposure to the wind, more is the probability. Larger return periods ranging from 100 to 1 000 years (implying lower risk level) in association with larger periods of exposure may have to be selected for exceptionally important structures, such as, nuclear power reactors and satellite communication towers. Equation given below may be used in such cases to estimate $k_{1}$ factors for different periods of exposure and chosen probability of exceedance (risk level). The probability level of 0.63 is normally considered sufficient for design of buildings and structures against wind effects and the values of $k_{1}$ corresponding to this risk level are given above.

k _ {1} = \frac {X _ {\mathrm{N,P}}}{X _ {5 0 , 0 . 6 3}} = \frac {A - B \left[ l _ {\mathrm{n}} \left\{- \frac {1}{N} l _ {\mathrm{n}} \left(1 - P _ {\mathrm{N}}\right) \quad \right\} \right]}{A + 4 B}

$P_{\mathrm{N}}$ = risk level in $N$ consecutive years (probability that the design wind speed is exceeded at least once in $N$ successive years), nominal value = 0.63;

X _ {\mathrm{N,P}}

P _ {\mathrm{N}};

$X_{50,0.63} =$ extreme wind speed for $N = 50$ years and $P_{\mathrm{N}} = 0.63$

$A$ and $B$ have the following values for different basic wind speed zones:

Zone	A*	B*
m/s	m/s	m/s
33	23.1 (83.2)	2.6 (9.2)
39	23.3 (84.0)	3.9 (14.0)
44	24.4 (88.0)	5.0 (18.0)
47	24.4 (88.0)	5.7 (20.5)
50	24.7 (88.8)	6.3 (22.8)
55	25.2 (90.8)	7.6 (27.3)
* Values of A and B, in km/h are given within bracket

developed. It is likely that the, next higher category than this will not exist in most design situations and that selection of a more severe category will be deliberate.

d) Category 4 — Terrain with numerous large high closely spaced obstructions. The equivalent aerodynamic roughness height, $(z_{0,4})$ for this terrain is 2.0 m. This category represents large city centers, generally with obstructions above 25 m and well developed industrial complexes.

4.4.3.2.2 Variation of wind speed with height in different terrains $(k\{2\})$

Table 5 gives multiplying factors $(k_{2})$ by which the basic wind speed given in Fig. 1 shall be multiplied to obtain the wind speed at different heights, in each terrain category.

4.4.3.2.3 Terrain categories in relation to the direction of wind

The terrain category used in the design of a structure

Table 5Factors to Obtain Design Wind Speed Variation with Height in Different Terrains (Clause 4.4.3.2.2)

Sl No.	Height (z)	Terrain and Height Multiplier ( $k_2$ )
Sl No.	m	Terrain Category 1	Terrain Category 2	Terrain Category 3	Terrain Category 4
(1)	(2)	(3)	(4)	(5)	(6)
i)	10	1.05	1.00	0.91	0.80
ii)	15	1.09	1.05	0.97	0.80
iii)	20	1.12	1.07	1.01	0.80
iv)	30	1.15	1.12	1.06	0.97
v)	50	1.20	1.17	1.12	1.10
vi)	100	1.26	1.24	1.20	1.20
vii)	150	1.30	1.28	1.24	1.24
viii)	200	1.32	1.30	1.27	1.27
ix)	250	1.34	1.32	1.29	1.28
x)	300	1.35	1.34	1.31	1.30
xi)	350	1.35	1.35	1.32	1.31
xii)	400	1.35	1.35	1.34	1.32
xiii)	450	1.35	1.35	1.35	1.33
xiv)	500	1.35	1.35	1.35	1.34

may vary depending on the direction of wind under consideration. Where sufficient meteorological information is available, the basic wind speed may be varied for specific wind direction.

4.4.3.2.4 Changes in terrain categories

The velocity profile for a given terrain category does not develop to full height immediately with the commencement of that terrain category but develop gradually to height $(h_{\mathrm{x}})$ which increases with the fetch or upwind distance $(x)$ .

a) Fetch and developed Height relationship — The relation between the developed height ( $h_{x}$ ) and the fetch (x) for wind-flow over each of the four terrain categories may be taken as given in Table 6.

b) For structures of heights more than the developed height $(h_{\mathrm{x}})$ in Table 6, the velocity profile may be determined in accordance with the following:

The less or least rough terrain, or
The method described in Annex D.

4.4.3.3 Topography $(k\{3\}$ factor)

The basic wind speed $V_{b}$ given in Fig. 1 takes into account 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 cliffs, steep escarpments, or ridges.

Table 6Fetch and Developed Height Relationship (Clause 4.4.3.2.4)

Sl No.	Fetch (x) km	Developed Height ( $h_x$ ) m
Sl No.	Fetch (x) km	Terrain Category 1	Terrain Category 2	Terrain Category 3	Terrain Category 4
(1)	(2)	(3)	(4)	(5)	(6)
i)	0.2	12	20	35	60
ii)	0.5	20	30	35	95
iii)	1	25	45	80	130
iv)	2	35	65	110	190
v)	5	60	100	170	300
vi)	10	80	140	250	450
vii)	20	120	200	350	500
viii)	50	180	300	400	500

The effect of topography shall be significant at a site when the upwind slope $(\theta)$ is more than about $3^{\circ}$ , and below that, the value of $k_{3}$ may be taken to be equal to 1.0. The value of $k_{3}$ is confined in the range of 1.0 to 1.36 for slopes more than $3^{\circ}$ . A method of evaluating the value of $k_{3}$ for values more than 1.0 is given in Annex E. It may be noted that the value of $k_{3}$ varies with height above ground level, with a maximum near the ground, and reducing to 1.0 at higher levels.

4.4.3.4 Importance factor for cyclonic region $(k\{4\}$ factor)

The east coast of India is relatively more vulnerable for occurrences of severe cyclones. On the west coast, Gujarat is vulnerable for severe cyclones. Studies of wind speed and damage to buildings and structures point to the fact that the speeds given in the basic wind speed map are often exceeded during the cyclones. The effect of cyclonic storms is largely felt in a belt of approximately 60 km width at the coast. In order to ensure better safety of structures in this region (60 km wide on the east coast as well as on the Gujarat Coast), the following values of $k_{4}$ , as recommended in good practice [6-1(5)] are stipulated as applicable according to the importance of the structure:

	$k_{4}$
Structures of post-cyclone importance for emergency services(such as cyclone shelters, hospitals, schools, communication towers, etc)	1.30
Industrial structures	1.15
All other structures	1.00

4.4.4Hourly Mean Wind Speed

The hourly mean wind speed at height $z$ , for different terrains can be obtained as:

PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS

\overline {{V}} _ {\mathrm{z,H}} = \bar {k} _ {2, \mathrm{i}} V _ {\mathrm{b}}

where

$\overline{k}_{2,i} =$ hourly mean wind speed factor for terrain category ‘i’

= 0. 1 4 2 3 \left[ \ln \left(\frac {z}{z _ {0 , \mathrm{i}}}\right) \right] \left(z _ {0, \mathrm{i}}\right) ^ {0. 0 7 0 6}

The design hourly mean wind speed at height $z$ can be obtained as:

\begin{array}{r} \overline {{V}} _ {\mathrm{z,d}} = \overline {{V}} _ {\mathrm{z,H}} k _ {1} k _ {3} k _ {4} \\ = V _ {\mathrm{b}} k _ {1} \overline {{k}} _ {2, \mathrm{i}} k _ {3} k _ {4} \end{array}

4.4.5 Turbulence Intensity

The turbulence intensity variations with height for different terrains can be obtained using the relations given below:

a) Terrain category 1

I _ {z, 1} = 0. 3 5 0 7 - 0. 0 5 3 5 \log_ {1 0} \left(\frac {z}{z _ {0 , 1}}\right)

b) Terrain category 2

I _ {z, 2} = I _ {z, 1} + \frac {1}{7} \left(I _ {z, 4} - I _ {z, 1}\right)

c) Terrain category 3

I _ {\mathrm{z}, 3} = I _ {\mathrm{z}, 1} + \frac {3}{7} \left(I _ {\mathrm{z}, 4} - I _ {\mathrm{z}, 1}\right)

d) Terrain category 4

I _ {z, 4} = 0. 4 6 6 - 0. 1 3 5 8 \log_ {1 0} \left(\frac {z}{z _ {0 , 4}}\right)

4.4.6 Off-Shore Wind Velocity

Cyclonic storms form far away from the sea coast and gradually reduce in speed as they approach the sea coast. Cyclonic storms generally extend up to about $60\mathrm{km}$ inland after striking the coast. Their effect on land is already reflected in basic wind speeds specified in Fig. 1. The influence of wind speed off the coast up to a distance of about $200\mathrm{km}$ may be taken as 1.15 times the value on the nearest coast in the absence of any definite wind data. The factor 1.15 shall be used in addition to $k_{4}$ .

4.5Wind Pressures and Forces on Buildings/Structures

4.5.1 General

The wind load on a building/structure shall be

calculated for,

a) the building/structure as a whole;

b) individual structural elements as roofs and walls; and

c) Individual cladding units including glazing and their fixings.

4.5.2Design Wind Pressure

The wind pressure at any height above mean ground level shall be obtained by the following relationship between wind pressure and wind speed:

p _ {\mathrm{z}} = 0. 6 V _ {\mathrm{z}} ^ {2}

where

p _ {z} = \text {wind pressure at height} z, \text {in N / m} ^ {2}; \text {and}

V _ {z} = \text {design wind speed at height} z, \text {in m / s}.

The design wind pressure $p_{\mathrm{d}}$ can be obtained as,

p _ {\mathrm{d}} = K _ {\mathrm{d}} K _ {\mathrm{a}} K _ {\mathrm{c}} p _ {\mathrm{z}}

where

K _ {\mathrm{d}} = \text { wind directionality factor },

K _ {\mathrm{a}} = \text {area averaging factor, and}

K _ {\mathrm{c}} = \text {combination factor [see 4.5.3.3.13]}

The value of $p_{\mathrm{d}}$ , however shall not be taken as less than 0.70 $p_{z}$ .

NOTES

1 The coefficient 0.6 (in SI units) in the above formula depends on a number of factors and mainly on the atmospheric pressure and air temperature. The value chosen corresponds to the average Indian atmospheric conditions.

2 $K_{\mathrm{d}}$ should be taken as 1.0 when considering local pressure coefficients.

4.5.2.1 Wind directionality factor, $K\{mathrm\{d\}\}$

Considering the randomness in the directionality of wind and recognizing the fact that pressure or force coefficients are determined for specific wind directions, it is specified that for buildings, solid signs, open signs, lattice frameworks, and trussed towers (triangular, square, rectangular) a factor of 0.90 may be used on the design wind pressure. For circular or near-circular forms this factor may be taken as 1.0.

For the cyclone affected regions also the factor $K_{\mathrm{d}}$ shall be taken as 1.0.

4.5.2.2 Area averaging factor, $K\{mathrm\{a\}\}$

Pressure coefficients given in 4.5.3 are a result of averaging the measured pressure values over a given area. As the area becomes larger, the correlation of measured values decrease and vice-versa. The decrease in pressures due to larger areas may be taken into account as given in Table 7.

Table 7Area Averaging Factor ( K_{a} ) (Clause 4.5.2.2)

Sl No.	Tributary Area (A) $m^2$	Area Averaging Factor ( $K_a$ )1)
(1)	(2)	(3)
i)	≤ 10	1.0
ii)	25	0.9
iii)	≥ 100	0.8

$^{1)}$ Linear interpolation for intermediate values of tributary area, A is permitted.

4.5.2.2.1 Tributary area

a) Overall structure — For evaluating loads on frames the tributary area shall be taken as the centre-to-centre distances between frames multiplied by the individual panel dimension in the other direction together with overall pressure coefficients.

b) Individual elements — For beam type elements, purlins, etc, the tributary area shall be taken as effective span multiplied by spacing. The effective span is the actual span for mid span and cantilever load effects; and half the sum of adjacent spans for support moments and reactions.

c) For plate type elements, the area of individual plates between supports is taken as the tributary area.

d) For glass cladding, individual pane area of glass is the tributary area.

4.5.4Force Coefficients

The value of force coefficients $(C_{\mathrm{f}})$ apply to a building or structure as a whole, and when multiplied by the effective frontal area $(A_{e})$ of the building or structure and design wind pressure, $p_{d}$ gives the total wind load $(F)$ on that particular building or structure, expressed as:

F = C _ {\mathrm{f}} A _ {\mathrm{e}} p _ {\mathrm{d}}

where F is the force acting in a direction specified in the respective tables and $C_{f}$ is the force coefficient for the building.

NOTES

1 The value of the force coefficient differs for the wind acting

Table 24 External Pressure Coefficients $(C_{\mathrm{pe}})$ for Combined Roofs (Clause 4.5.3.3.9)

Chart

Values of Coefficients ( $C_{pe}$ )
Portion	Direction 1	Direction 2
a	From the Diagram	-0.4
b	$C_{pe} = -0.5, \frac{h_1}{h_2} \leq 1.5$ $C_{pe} = +0.7; \frac{h_1}{h_2} > 1.5$
c and d	See Table 9
e	See 4.5.3.3

PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS

Table 25 External Pressure Coefficients ( $C_{pe}$ ) for Roofs with a Sky Light (Clause 4.5.3.3.10)

b) ROOFS WITH A SKY LIGHT

Values of Coefficients ( $C_{pe}$ )
Portion	$b_1 > b_2$		$b_1 \leq b_2$
Portion	a	b	a and b
$C_{pe}$	-0.6	+0.7	See Table 24 for combined roofs

Table 26 Pressure Coefficients at Top and Bottom Roof of Grand Stands Open Three Sides (Roof Angle up to 5°) (Clause 4.5.3.3.11)

( Shaded area to scale )

Front and Back of Wall
θ	J	K	L	M
0°	+ 0.9	- 0.5	+ 0.9	- 0.5
45°	+ 0.8	- 0.6	+ 0.4	- 0.4
135°	- 1.1	+ 0.6	- 1.0	+ 0.4
180°	- 0.3	+ 0.9	- 0.3	+ 0.9
60°	$M_{w} - C_p$ of K = - 1.0
60°	$M_{w} - C_p$ of J = + 1.0

Top and Bottom Roof
θ	A	B	C	D	E	F	G	H
0°	-1.0	+0.9	-1.0	+0.9	-0.7	+0.9	+0.7	+0.9
45°	-1.0	+0.7	-0.7	+0.4	-0.5	+0.8	-0.5	+0.3
135°	-0.4	-1.1	-0.7	-1.0	-0.9	-1.1	-0.9	-1.0
180°	-0.6	-0.3	-0.6	-0.3	-0.6	-0.3	-0.6	-0.3
45°	'MR' - Cp (top) = -2.0
45°	'MR' - Cp (bottom) = +1.0

PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS

Table 27 External Pressure Coefficients ( $C_{pe}$ ) Around Spherical Structures (Clause 4.5.3.3.12)

Position of Periphery, θ Degree	$C_{pe}$
0	+ 1.0
15	+ 0.9
30	+ 0.5
45	- 0.1
60	- 0.7
75	- 1.1
90	- 1.2
105	- 1.0
120	- 0.6
135	- 0.2
150	+ 0.1
165	+ 0.3
180	+ 0.4

on different faces of a building or structure. In order to determine the critical load, the total wind load should be calculated for each wind direction.

2 If surface design pressure varies with height, the surface area of the building/structure may be sub-divided so that specified pressures are taken over appropriate areas.

3 In tapered buildings/structures, the force coefficients shall be applied after sub-dividing the building/structure into suitable number of strips and the load on each strip calculated individually, taking the area of each strip as $A_{e}$ .

4 For force coefficients for structures not covered herein,

reference may be made to specialist literature on the subject or advice may be sought from specialist in the subject.

4.5.4.1Frictional drag

In certain buildings of special shape, a force due to frictional drag shall be taken into account in addition to those loads specified in 4.5.3. For rectangular clad buildings, this addition is necessary only where the ratio $d / h$ or $d / b$ is more than 4. The frictional drag force, $F'$ , in the direction of the wind is given by the following

formulae:

\begin{array}{l} \text {If} h \leq b, F ^ {\prime} = C _ {\mathrm{f}} ^ {\prime} (d - 4 h) b p _ {\mathrm{d}} + C _ {\mathrm{f}} ^ {\prime} (d - 4 h) 2 h p _ {\mathrm{d}}, \\ \text {and} \end{array}

\text {If} h > b, F ^ {\prime} = C _ {\mathrm{f}} ^ {\prime} (d - 4 b) b p _ {\mathrm{d}} + C _ {\mathrm{f}} ^ {\prime} (d - 4 b) 2 h p _ {\mathrm{d}}

The first term in each case gives the drag on the roof and the second on the walls. The $C_{\mathrm{f}}'$ has the following values:

a) 0.01 for smooth surfaces without corrugations or ribs across the wind direction,

b) 0.02 for surfaces with corrugations across the wind direction, and

c) 0.04 for surfaces with ribs across the wind direction.

For other buildings, the frictional drag has been indicated, where necessary, in the tables of pressure coefficients and force coefficients.

4.5.4.2Force coefficients for clad buildings

4.5.4.2.1 Clad buildings of uniform section

The overall force coefficients for rectangular clad buildings of uniform section with flat roofs in uniform flow shall be as given in Fig. 4 and for other clad buildings of uniform section (without projections, except where otherwise shown) shall be as given in Table 28.

NOTE — Structures that are in the supercritical flow regime, because of their size and design wind velocity, may need further calculation to ensure that the greatest loads do not occur at some wind speed below the maximum, when the flow will be sub-critical.

The coefficients are for buildings without projections, except where otherwise shown.

In Table 28, $V_{\mathrm{d}}b$ is used as an indication of the airflow regime.

4.5.4.2.2 Buildings of circular shapes

Force coefficients for buildings of circular cross-section shapes shall be as given in Table 28. However, more precise estimation of force coefficients for circular shapes of infinite length can be obtained from Fig. 5 taking into account the average height of surface roughness $\varepsilon$ . When the length is finite the values obtained from Fig. 5 shall be reduced by the multiplication factor K (see also Table 31 and Annex F).

4.5.4.2.3 Free standing walls and hoardings

Force coefficients for free standing walls and hoardings shall be as given in Table 29.

To allow for oblique winds, the design shall also be checked for net pressure normal to the surface varying linearly from a maximum of $1.7C_{\mathrm{f}}$ at the windward edge to $0.44C_{\mathrm{f}}$ at the leeward edge.

The wind load on appurtenances and supports for hoardings shall be accounted for separately by using the appropriate net pressure coefficients. Allowance shall be made for shielding effects of one element on another.

4.5.4.2.4 Solid circular shapes mounted on a surface

The force coefficients for solid circular shapes mounted on a surface shall be as given in Table 30.

4.5.4.3Force coefficients for unclad buildings

4.5.4.3.1 This Section applies to permanently unclad buildings and to frameworks of buildings while temporarily unclad. In the case of buildings whose surfaces are well-rounded, such as those with elliptic, circular or oval cross-sections, the total force can be more at a wind speed much less than maximum due to transition in the nature of boundary layer on them. Although this phenomenon is well known in the case of circular cylinders, the same phenomenon exists in the case of many other well-rounded structures, and this possibility shall be checked.

4.5.4.3.2 Individual members

a) The force coefficient given in Table 32 refers to members of infinite length. For members of finite length, the coefficients should be multiplied by a factor $K$ that depends on the ratio $l / b$ where $l$ is the length of the member and $b$ is the width across the direction of wind. Table 31 gives the required values of $K$ . The following special cases shall be noted while estimating $K$ :

When any member abuts on to a plate or wall in such a way that free flow of air around that end of the member is prevented, then the ratio of $l / b$ shall be doubled for the purpose of determining $K$ ; and
When both ends of a member are so obstructed, the ratio shall be taken as infinity for the purpose of determining $K$ .

b) Flat-sided members — Force coefficients for wind normal to the longitudinal axis of flat-sided structural members shall be as given in Table 32.

The force coefficients are given for two mutually perpendicular directions relative to a reference axis on the structural member. They are denoted by $C_{\mathrm{fn}}$ and $C_{\mathrm{ft}}$ and give the forces normal and transverse, respectively to the reference plane as shown in Table 32.

Normal force, $F_{\mathrm{n}} = (C_{\mathrm{fn}}p_{\mathrm{d}}K) / b$

Transverse force, $F_{\mathrm{t}} = (C_{\mathrm{ft}}p_{\mathrm{d}}K) / b$

PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS

Chart

4A VALUES OF $C_f$ Versus $a / b$ for $h / b \geq 1$

Chart

4B VALUES OF $C_{\mathrm{f}}$ Versus $a / b$ for $h / b < 1$

FIG. 4 FORCE COEFFICIENT FOR RECTANGULAR CLAD BUILDING IN UNIFORM FLOW

c) Circular sections — Force coefficients for members of circular section shall be as given in Table 28 (see also Annex F).

d) Force coefficients for wires and cables shall be as given in Table 33 according to the diameter (D), the design wind speed ( $V_{d}$ ) and the surface roughness.

4.5.4.3.3 Single frames

Force coefficients for a single frame having either,

a) all flat sided members; or

b) all circular members in which all the members of the frame have either,

Table 28Force Coefficients, C_{\mathrm{f}} for Clad Buildings of Uniform Section (Acting in the Direction of Wind) (Clause 4.5.4.2.1)

Plan Shape	$\overline{V}_d b$ $m^2/s$	$C_f$ for Height / Breadth Ratio
Plan Shape	$\overline{V}_d b$ $m^2/s$	Up to 1/2	1	2	5	10	20	∞
See also Annex D	All Surfaces < 6	0.7	0.7	0.7	0.8	0.9	1.0	1.2
	Rough or with Projection ≥ 6	0.7	0.7	0.7	0.8	0.9	1.0	1.2
	Smooth ≥ 6	0.5	0.5	0.5	0.5	0.5	0.6	0.6
Ellipse b/d = 1/2	< 10	0.5	0.5	0.5	0.5	0.6	0.6	0.7
Ellipse b/d = 1/2	≥ 10	0.2	0.2	0.2	0.2	0.2	0.2	0.2
Ellipse b/d = 2	< 8	0.8	0.8	0.9	1.0	1.1	1.3	1.7
Ellipse b/d = 2	≥ 8	0.8	0.8	0.9	1.0	1.1	1.3	1.5
$b/d = 1$ $r/b = 1/3$	< 4	0.6	0.6	0.6	0.7	0.8	0.8	1.0
$b/d = 1$ $r/b = 1/3$	≥ 4	0.4	0.4	0.4	0.4	0.5	0.5	0.5
	< 10	0.7	0.8	0.8	0.9	1.0	1.0	1.3
	≥ 10	0.5	0.5	0.5	0.5	0.6	0.6	0.6
	< 3	0.3	0.3	0.3	0.3	0.3	0.3	0.4
	≥ 3	0.2	0.2	0.2	0.2	0.3	0.3	0.3
	All values	0.5	0.5	0.5	0.5	0.6	0.6	0.7
→ $b/d = 2$ $r/b = 1/12$	All values	0.9	0.9	1.0	1.1	1.2	1.5	1.9

PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS

Table 28 — (Continued)

<table><tr><td rowspan="2">Plan Shape</td><td rowspan="2"> $\overline{V}_d b$ $m^2/s$ </td><td colspan="7"> $C_f$ for Height / Breadth Ratio</td></tr><tr><td>Up to 1/2</td><td>1</td><td>2</td><td>5</td><td>10</td><td>20</td><td>∞</td></tr><tr><td rowspan="2"> $_{b/d = 2 \atop r/b = 1/4}$ </td><td>< 6</td><td>0.7</td><td>0.8</td><td>0.8</td><td>0.9</td><td>1.0</td><td>1.2</td><td>1.6</td></tr><tr><td>≥ 6</td><td>0.5</td><td>0.5</td><td>0.5</td><td>0.5</td><td>0.5</td><td>0.6</td><td>0.6</td></tr><tr><td rowspan="2"> $_{r/a = 1/3}$ </td><td>< 10</td><td>0.8</td><td>0.8</td><td>0.9</td><td>1.0</td><td>1.1</td><td>1.3</td><td>1.5</td></tr><tr><td>≥ 10</td><td>0.5</td><td>0.5</td><td>0.5</td><td>0.5</td><td>0.5</td><td>0.6</td><td>0.6</td></tr><tr><td> $</td><td>All values</td><td>0.9</td><td>0.9</td><td>0.9</td><td>1.1</td><td>1.2</td><td>1.3</td><td>1.6</td></tr><tr><td> $_{r/a = 1/48}$ </td><td>All values</td><td>0.9</td><td>0.9</td><td>0.9</td><td>1.1</td><td>1.2</td><td>1.3</td><td>1.6</td></tr><tr><td rowspan="2"> $_{r/b = 1/4}$ </td><td>< 11</td><td>0.7</td><td>0.7</td><td>0.7</td><td>0.8</td><td>0.9</td><td>1.0</td><td>1.2</td></tr><tr><td>≥ 11</td><td>0.4</td><td>0.4</td><td>0.4</td><td>0.4</td><td>0.5</td><td>0.5</td><td>0.5</td></tr><tr><td> $_{r/b = 1/12}$ </td><td>All values</td><td>0.8</td><td>0.8</td><td>0.8</td><td>1.0</td><td>1.1</td><td>1.2</td><td>1.4</td></tr><tr><td> $_{r/b = 1/48}$ </td><td>All values</td><td>0.7</td><td>0.7</td><td>0.8</td><td>0.9</td><td>1.0</td><td>1.1</td><td>1.3</td></tr><tr><td rowspan="2"> $_{r/b = 1/4}$ </td><td>< 8</td><td>0.7</td><td>0.7</td><td>0.8</td><td>0.9</td><td>1.0</td><td>1.1</td><td>1.3</td></tr><tr><td>≥ 8</td><td>0.4</td><td>0.4</td><td>0.4</td><td>0.4</td><td>0.5</td><td>0.5</td><td>0.5</td></tr></table> Table 28 — (Concluded) <table><tr><td rowspan="2">Plan Shape</td><td rowspan="2"> $\overline{V}_{d}$ bm2/s</td><td colspan="7"> $C_{f}$ for Height / Breadth Ratio</td></tr><tr><td>Up to 1/2</td><td>1</td><td>2</td><td>5</td><td>10</td><td>20</td><td>∞</td></tr><tr><td>1/48 < r/b < 1/12</td><td>All values</td><td>1.2</td><td>1.2</td><td>1.2</td><td>1.4</td><td>1.6</td><td>1.7</td><td>2.1</td></tr><tr><td rowspan="2">12 Sided Polygon</td><td>< 12</td><td>0.7</td><td>0.7</td><td>0.8</td><td>0.9</td><td>1.0</td><td>1.1</td><td>1.3</td></tr><tr><td>≥ 12</td><td>0.7</td><td>0.7</td><td>0.7</td><td>0.7</td><td>0.8</td><td>0.9</td><td>1.1</td></tr><tr><td>Octagon</td><td>All values</td><td>1.0</td><td>1.0</td><td>1.1</td><td>1.2</td><td>1.2</td><td>1.3</td><td>1.4</td></tr><tr><td></td><td>All values</td><td>1.0</td><td>1.1</td><td>1.2</td><td>1.3</td><td>1.4</td><td>1.4</td><td>1.5</td></tr></table> ![Chart](/codes/figures/9d65449f-c691-4c6f-b5bf-068aee577943/p0472_0190.png) FIG. 5 VARIATION OF $\frac{C_{f}}{1+\frac{2\varepsilon}{D}}$ WITH $R_{e}>3\times10^{4}$ FOR CIRCULAR SECTIONS PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS Table 29 Force Coefficients, $C_{\mathrm{f}}$ for Low Walls or Hoardings (<15m High) (Clause 4.5.4.2.3) ![Image](/codes/figures/9d65449f-c691-4c6f-b5bf-068aee577943/p0473_0191.png) ![Image](/codes/figures/9d65449f-c691-4c6f-b5bf-068aee577943/p0473_0192.png) (WIND NORMAL TO FACE) <table><tr><td colspan="2">Width to Height Ratio, $b/h$ </td><td rowspan="2">Force Coefficient $C_f$ </td></tr><tr><td>Wall Above Ground</td><td>Wall on Ground</td></tr><tr><td>From 0.5 TO 6</td><td>From 1 TO 12</td><td>1.2</td></tr><tr><td>10</td><td>20</td><td>1.3</td></tr><tr><td>16</td><td>32</td><td>1.4</td></tr><tr><td>20</td><td>40</td><td>1.5</td></tr><tr><td>40</td><td>80</td><td>1.75</td></tr><tr><td>60</td><td>120</td><td>1.8</td></tr><tr><td>80 or More</td><td>160 or More</td><td>2.0</td></tr></table> 1) $D\overline{V}_{\mathrm{d}}$ less than $6\mathrm{m}^2 /\mathrm{s}$; or 2) $D\overline{V}_{d}$ more than or equal to 6 m²/s. shall be as given in Table 31 according to the type of the member, the diameter (D), the design hourly mean wind speed ( $D\overline{V}_{d}$ ) and the solidity ratio ( $\Phi$ ). Force coefficients for a single frame not complying with the above requirements shall be calculated as follows:$ C _ {\mathrm{f}} = \gamma C _ {\mathrm{f} _ {\text {super}}} + (1 - \gamma) \frac {A _ {\text {circ sub}}}{A _ {\text {sub}}} C _ {\mathrm{f} _ {\text {sub}}} + (1 - \gamma) \frac {A _ {\text {flat}}}{A _ {\text {sub}}} C _ {\mathrm{f} _ {\text {flat}}}

where $C_{f super} = \text{force coefficient for the supercritical circular members as given in Table 34 or Annex F,}$ $C_{f sub}$ = force coefficient for subcritical circular members as given in Table 34 or Annex F, $C_{f flat}$ = force coefficient for the flat sided members as given in Table 34, $A_{circ sub} = \text{effective area of subcritical circular members,}$ $A_{\mathrm{flat}} =$ effective area of flat-sided members,

\begin{array}{r c l} A _ {\text {sub}} & = & A _ {\text {circ sub}} + A _ {\text {flat}}, \end{array}

$\gamma =$ (Area of the frame in a supercritical flow)/ $A_{\mathrm{e}}$ , and $A_{\mathrm{e}}$ = Effective frontal area. ### 4.5.4.3.4 Multiple frame buildings This Section applies to structures having two or more parallel frames where the windward frames may have a shielding effect upon the frames to leeward side. The windward frame and any unshielded parts of other frames shall be calculated in accordance with 4.5.4.3.3, but the wind load on the parts of frames that are sheltered should be multiplied by a shielding factor which is dependent upon the solidity ratio of the windward frame, the types of members comprising the frame and the spacing ratio of the frames. The values of the shielding factors are given in Table 35. Where there are more than two frames of similar geometry and spacing, the wind load on the third and subsequent frames should be taken as equal to that on Table 30 Force Coefficients, $C_{\mathrm{f}}$ for Solid Shapes Mounted on a Surface (Clause 4.5.4.2.4) <table><thead><tr><th>Side Elevation</th><th>Description of Shape</th><th>$C_f$</th></tr></thead><tbody><tr><td></td><td>Circular disc</td><td>1.2</td></tr><tr><td></td><td>Hemispherical bowl</td><td>1.4</td></tr><tr><td></td><td>Hemispherical bowl</td><td>0.4</td></tr><tr><td></td><td>Hemispherical solid</td><td>1.2</td></tr><tr><td></td><td>Spherical solid</td><td>0.5 FOR $V_Z$ D &lt; 70.2 FOR $V_Z$ D ≥ 7</td></tr></tbody></table> the second frame. The loads on the various frames shall be added to obtain total load on the structure. a) The frame spacing ratio is equal to the centre-to-centre distance between the frames, beams or girders divided by the least overall dimension of the frames, beam or girder measured in a direction normal to the direction of wind. For triangular framed structures or rectangular framed structures diagonal to the wind, the spacing ratio should be calculated from the mean distance between the frames in the direction of the wind. b) Effective solidity ratio, $\Phi \mathrm{e}$: $\Phi \mathrm{e} = \Phi$ for flat-sided members. $\Phi \mathrm{e}$ is to be obtained from Fig. 6 for members of circular cross-sections. ![Chart](/codes/figures/9d65449f-c691-4c6f-b5bf-068aee577943/p0474_0193.png) FIG. 6 EFFECTIVE SOLIDITY RATIO, FOR CIRCULAR SECTION MEMBERS PART 6 STRUCTURAL DESIGN — SECTION 1 LOADS, FORCES AND EFFECTS Table 31 Reduction Factor $K$ for Individual Members [Clauses 4.5.4.2.2 and 4.5.4.3.2(a)] <table><thead><tr><th>Sl No.</th><th>l/b or l/D</th><th>2</th><th>5</th><th>10</th><th>20</th><th>40</th><th>50</th><th>100</th><th>$\square$</th></tr></thead><tbody><tr><td>(1)</td><td>(2)</td><td>(3)</td><td>(4)</td><td>(5)</td><td>(6)</td><td>(7)</td><td>(8)</td><td>(9)</td><td>(10)</td></tr><tr><td>i)</td><td>Circular cylinder, subcritical flow</td><td>0.58</td><td>0.62</td><td>0.68</td><td>0.74</td><td>0.82</td><td>0.87</td><td>0.98</td><td>1.00</td></tr><tr><td>ii)</td><td>Circular cylinder, supercritical flow ($\overline{D\overline{V}_{d}} \geq 6\mathrm {m}^{2}/\mathrm {s}$)</td><td>0.80</td><td>0.80</td><td>0.82</td><td>0.90</td><td>0.98</td><td>0.99</td><td>1.00</td><td>1.00</td></tr><tr><td>iii)</td><td>For plate perpendicular to wind ($b\overline{V}_{d} \geq 6\mathrm {m}^{2}/\mathrm {s}$)</td><td>0.62</td><td>0.66</td><td>0.69</td><td>0.81</td><td>0.87</td><td>0.90</td><td>0.95</td><td>1.00</td></tr></tbody></table> ### 4.5.4.3.5 Lattice towers a) Force coefficient for lattice towers of square or equilateral triangle section with flat-sided members for wind blowing against any face shall be as given in Table 36. b) For square lattice towers with flat-sided members the maximum load, which occurs when the wind blows into a corner, shall be taken as 1.2 times the load for the wind blowing against a face. c) For equilateral triangle lattice towers with flat-sided members, the load may be assumed to be constant for any inclination of wind to a face. d) Force coefficients for lattice towers of square section with circular members, all in the same flow regime, may be as given in Table 37. e) Force coefficients for lattice towers of equilateral-triangle section with circular members all in the same flow regime may be as given in Table 38. ### 4.5.4.3.6 Tower appurtenances The wind loading on tower appurtenances, such as ladders, conduits, lights, elevators, etc, shall be calculated using appropriate net pressure coefficients for these elements. Allowance may be made for shielding effect from other elements.

a) Risk level,

b) Terrain roughness and height of structure,

c) Local topography, and

d) Importance factor for the cyclonic region.

It can be mathematically expressed as follows:

V _ {\mathrm{z}} = V _ {\mathrm{b}} k _ {1} k _ {2} k _ {3} k _ {4}