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IS 875 part-3 | Wound | Wind Speed
IS 875 part-3Uploaded by Saurav MukherjeeRelated InterestsWoundWind SpeedStructural LoadTropical CyclonesStormsRating and Stats0.0 (0)Document ActionsDownloadShare or Embed DocumentEmbedView MoreCopyright: Attribution Non-Commercial (BY-NC)List price: $0.00Download as PDF, TXT or read online from ScribdFlag for inappropriate contentDocument No. :: IITK-GSDMA-Wind02-V5.0 :: IITK-GSDMA-Wind04-V3.0 Final Report :: B - Wind Codes IITK-GSDMA Project on Building Codes
Canada. should be considered along with the above loads.0
. Live load surveys have been carried out in America. a number of comments were received on provisions on live load values adopted for different occupancies.
IITK-GSDMA-Wind02-V5. the following important modifications were made from those covered in the 1964 version of IS: 875:
*Criteria for Earthquake Resistant Design of Structures (2002 revision).2 This part (Part 3) deals with wind loads to be considered when designing buildings. In its second revision in 1987. structures and components thereof.2002*.Code & Commentary IS 875 (Part 3)
sheeted roofs.0
IITK-GSDMA-Wind04-V3. IS:1893(Part 1). the Structural Safety Sectional Committee decided to prepare the second revision of IS: 875 in the following five parts: Part 1: Dead loads Part 2: Imposed loads Part 3: Wind loads Part 4: Snow loads Part 5: Special loads and load combinations Earthquake load being covered in a separate standard. seismic load provisions were deleted (separate Code having been prepared) and metric system of weights and measurements was adopted. Keeping this in view and other developments in the field of wind engineering.3. both curved and sloping were modified.3. 0.1 With the increased adoption of this Code. 0. UK and in India to arrive at realistic live loads based on actual determination of loading (movable and immovable) in different occupancies. namely.
(c) Terrain was classified into four categories based on characteristics of the ground surface irregularities. etc. The map and related recommendations were provided in the Code with the active cooperation of Indian Meteorological Department (IMD). (g) The external and internal pressure coefficients for gable roofs. lattice towers. Isotachs (lines of equal wind speed) were not given. (b) Modification factors to modify the basic wind speed to take into account the effects of terrain.Code & Commentary IS 875 (Part 3)
(a) The earlier wind pressure maps (one giving winds of shorter duration and other excluding winds of shorter duration) were replaced by a single wind map giving basic maximum wind speed in m/s (peak gust speed averaged over a short time interval of about 3 seconds duration). were included.0
IITK-GSDMA-Wind04-V3. The wind speeds were worked out for 50 years return period based on the up-to-date wind data of 43 dines pressure tube (DPT) anemograph stations and study of other related works available on the subject since 1964. (d) Force and pressure coefficients were included for a large range of clad and unclad buildings and for individual structural elements. (e) Force coefficients (drag coefficients) were given for frames. canopy roofs (butterfly type structures) and multispan roofs were rationalized. lean-to roofs. as in the opinion of the committee there was still not enough extensive meteorological data at close enough stations in the country to justify drawing of isotachs. (f) The calculation of force on circular sections was included incorporating the effects of Reynolds number and surface roughness. IITK-GSDMA-Wind02-V5. size of structures. curved roofs. local topography. walls and hoardings.0
etc.3.0
.3 though additional data has become available through measurements of wind speed at the meteorological stations. chimneys. buildings. felt the paucity of data on which to base wind maps for Indian conditions on statistical analysis. 0. (i) An analysis procedure for evaluating the dynamic response of flexible structures under wind loading using gust response factor was included. etc). strain gauges. The Committee.) at different elevations (at least at two levels) to continuously measure and monitor wind data.3. roofs with sky light. It was noted that instruments were required to collect data on wind direction. and the data so collected will be very valuable in assessing more accurate wind loading on structures. wind speed and structural response of the structure due to wind (with the help of accelerometers.4 It is seen at the time of undertaking the third revision of this Code (during 20032004) that: (i) Not much progress has yet been made in regard to instrumentation and collection of data in India as mentioned in 0.Code & Commentary IS 875 (Part 3)
(h) Pressure coefficients were given for combined roofs. It was also the opinion of the committee that such instrumentation in tall structures will not in any way affect or alter the functional behaviour of such structures. etc. In addition there is a need to address the issue of cyclonic winds and the damage caused by these winds. while reviewing available meteorological wind data and response of structures to wind. recommended to all individuals and organizations responsible for putting-up of tall structures to provide instrumentation in their existing and new structures (transmission towers. circular silos. grandstands. cooling towers. 0.
IITK-GSDMA-Wind02-V5. cylindrical elevated structures.3 The Committee responsible for the revision of wind maps.3. therefore.
4 The Sectional Committee responsible for the preparation of this Standard has taken into account the prevailing practice in regard to Loading Standards followed in this country by the various authorities and has also taken note of the developments in a number of other countries. There is a better appreciation about the randomness that prevails in the directionality of wind. thus making available additional information of improved quality. There is an appreciation of the fact that wind loads on different parts of the structure are not fully correlated.0
IITK-GSDMA-Wind04-V3. Changes are therefore warranted only where these would bring about an improvement in the quality of the standard. 0. the following overseas Standards have also been examined: (a) BS 6399-2:1997 Loading for
IITK-GSDMA-Wind02-V5. the above observations have been taken into account. In the preparation of this Code. (iii) A better understanding has developed concerning peak suctions/pressures. during the past couple of decades. and the degree of correlation amongst pressures that it causes on a surface. There is a significant effect possible on the wind forces in a building on account of interference between similar or dissimilar buildings.Code & Commentary IS 875 (Part 3)
(ii) There has been a substantial research effort on determination of wind effects on buildings and structures. the standard produced was on contemporary lines. There is a better understanding of the significant influence of the averaging area used on the pressures evaluated. the world over.
In carrying out this revision. It is realized that as a result of the second revision.0
IITK-GSDMA-Wind04-V3. (e) Architectural Institute of Japan Recommendations for Loads on Buildings. 0. the final value.Code & Commentary IS 875 (Part 3)
Buildings. Tokyo. Part 2: Code of Practice for Wind loads.5 For the purpose of deciding whether a particular requirement of this Standard is complied with. shall be rounded off in accordance with IS:21960*. (c) ASCE 7-02 American Society of Civil Engineers: Minimum Design Loads for Buildings and Other Structures. The number of significant places retained in the rounded off value should be the same as that of the specified value in this Standard. Wind Resistant Design Regulations. Association for Science Documents Information.
*Rules for Rounding-off Numerical Values (Revised).
IITK-GSDMA-Wind02-V5. 1996. A World List. observed or calculated. expressing the result of a test or analysis.2: 2002 Structural Design Actions-Part 2: Wind Actions. (b) AS/NZS1170.0
. (d) National Building Code of Canada 1995.
1. chimneys. loads on structural members. – Scope
IITK-GSDMA-Wind04-V3. However. Superimposed on the mean/static component is the time varying component having multiple frequencies spread over a wide band. With substantial work being done worldwide in the area of wind engineering. a smaller return period of 5 to 10 years or longer may be considered for arriving at the design factor (factor k1) for construction stages/period of a structure depending on its importance. transmission line towers and bridges are outside the scope of this Code.1. and their components. guyed masts that need to be examined for aerodynamic effects. Structures such as chimneys. For aerodynamics of bridges. Methods for computing the dynamic effect of wind on buildings have been introduced in this Standard. A structure may be deemed to be short and rigid if its natural time period is less than one second. and therefore can be satisfactorily treated as having an equivalent static pressure. which can be seen as a mean plus a fluctuating component.1 This Standard gives wind forces and their effects (static and dynamic) that should be taken into account while designing buildings. Strictly speaking all structures will experience dynamic oscillations due to the fluctuating component (gustiness) of wind.2 –
This Code also applies to buildings or other structures during erection/ construction and the same shall be considered carefully during various stages of erection/construction. In short rigid structures these oscillations are insignificant. a mean component of wind speed can be defined which would produce a static force on a structure. The more flexible systems such as tall buildings undergo a dynamic response to the gustiness of wind.1 –
Wind is not a steady phenomena due to natural turbulence and gustiness present in it.1. This is the approach taken by most Codes and Standards.1–
Wind causes a random time-dependent load. Information on bridges (only static forces) is given in IRS and IRC Specifications. In snowfall areas where icing occurs. cooling towers.Code & Commentary IS 875 (Part 3)
CODE 1. In locations where the strongest winds and icing may occur simultaneously.
This Code provides information on wind effects for buildings and structures.0
. specialist literature may be consulted.2 –
The construction period of a structure is much smaller than its expected life. The user of this Code is advised to consult specialist literature for the design of large or important projects involving various types of structures. when averaged over a sufficiently long time duration (from a few minutes to an hour). wind loads have to be
IITK-GSDMA-Wind02-V5. There are Indian Standards dealing with chimneys and cooling towers separately. there is growing body of new information. as is also the case in this Standard. structures and components thereof. cables and ropes shall be calculated by assuming an ice covering
C1. Therefore. Apart from tall buildings there are several other structural forms (though outside the scope of this Standard) such as tall latticed towers.
1. the designs ought to be checked for torsional effects due to wind pressure. etc.1. unusual locations.1
IITK-GSDMA-Wind02-V5. such as chimneys.
assessed accordingly.3–
In the design of special structures. overhead transmission line towers.
C1. Some of the Indian Standards available for the design of special structures are: IS: 4998 (Part 1) –1992 Criteria for design of reinforced concrete chimneys: Part 1 Design Criteria (first revision) IS:6533 –1989 Code of practice for design and construction of steel chimneys IS:5613 (Part 1/Sec 1).
1. installation and maintenance of overhead power lines: Part 1 Lines up to and including 11 kV. 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.. Elements such as cables and ropes can undergo a dynamic response in such cases and have to be examined accordingly.3 – See C1.0
IITK-GSDMA-Wind04-V3. specific requirements as given in the respective Codes shall be adopted in conjunction with the provisions of this Code as far as they are applicable. Wind tunnel studies may also be required in such situations.1985 Code of practice for design. Section 1 Design IS:802 (Part 1)-1995 Code of practice for use of structural steel in overhead transmission line towers: Part 1 Loads and permissible stresses (second revision) IS:11504-1985 Criteria for structural design of reinforced concrete natural draught cooling towers
NOTE: 1 – This standard IS:875 (Part 3)-1987 does not apply to buildings or structures with unconventional shapes. NOTE: 2 – In the case of tall structures with unsymmetrical geometry.0
based on climatic and local experience.
IITK-GSDMA-Wind04-V3. Depth of structure Wind energy factor First mode natural frequency of vibration Force on a surface Normal force Transverse force Frictional force
IITK-GSDMA-Wind02-V5. having been used for denoting different variables : A= Ae = Az = b= Surface area of a structure or part of a structure Effective frontal area Frontal contributory area at height z Breadth of a structure or structural member normal to the wind stream in the horizontal plane Background factor Drag coefficient Lift coefficient Force coefficient Normal force coefficient Transverse force coefficient Frictional drag coefficient Dynamic response factor Pressure coefficient External pressure coefficient Internal pressure coefficient Cross-wind force spectrum coefficient Depth of a structure or structural member parallel to wind stream in the horizontal plane Diameter of cylinder or sphere.0
. A few of the notations have more than one definition.1–
The following notations shall be followed unless otherwise specified in relevant clauses. – Notations
2.Code & Commentary IS 875 (Part 3)
CODE 2. Notions have been defined in the text at their first appearance.
having been used for denoting different parameters.
. A few of the notations have more than one definition.0
Notations have been defined also in the text at their first appearance.
IITK-GSDMA-Wind04-V3. Equivalent cross-wind static force Distance downwind from a change in terrain category. Damping ratio Average height of surface roughness Solidity ratio Shielding factor or eddy shedding frequency Wind direction in plan from a given axis. Bay width in a multi-bay building.0
. or in the cross-section a structural member. fetch length Height above average ground level Inclination of roof to the horizontal plane Effective solidity ratio. upwind ground / hill slope
W= Hourly mean wind speed at height z Lesser horizontal dimension of a building in plan.
At lesser fetch lengths. the dynamic pressure due to the incident design wind speed and the reference area over which the force is required. When the force is perpendicular to the direction of incident wind. At large fetch lengths. – Terminology
For the purpose of this Code.
Note – Breadth and depth are dimensions measured in relation to the direction of wind. Effective Frontal Area (Ae) – The projected area of the structure normal to the direction of the wind. such penetration reaches the gradient height. above which the wind speed may be taken to be constant. Depth (D) – Depth means the horizontal dimension of the building measured in the direction of the wind.0
. the non-dimensional coefficient will be called as lift coefficient (CL).Angle in vertical plane between the direction of wind and a reference axis of the structure.A non-dimensional coefficient such that the total wind force on a body is the product of the force coefficient. IITK-GSDMA-Wind02-V5.
Developed Height – Developed height is the height of upward penetration of the wind speed profile in a new terrain.0
CODE 3. Element of Surface Area – The area of surface over which the pressure coefficient is taken to be constant. the following definitions shall apply. NOTE – When the force is in the direction of the incident wind. a wind speed profile of a smaller height but similar to that of the fully developed profile of the terrain category has to be taken. Breadth (b) – Breadth means horizontal dimension of the building measured normal to the direction of wind. Force Coefficient (Cf) . the nondimensional coefficient will be called as drag coefficient (CD). whereas length and width are dimensions related to the plan. Angle of Attack / Incidence (α) . with the additional provision that wind speed at the top of this shorter profile equals that of the unpenetrated earlier profile at that height.
above which the variation of wind speed with height need not be considered.0
IITK-GSDMA-Wind04-V3. For example. Gust – A positive or negative departure of wind speed from its mean value. 3 second gust wind speed. the wind profile changes in character but such changes are gradual and start at ground level. Interference Factor – Ratio of the value of a typical response parameter for a structure due to interference divided by the corresponding value in the ‘stand alone’ case. Coriolis force and centrifugal force.). the gradient height is taken as the height above the mean ground level. forest.Code & Commentary IS 875 (Part 3)
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.0
. where the static and design wind pressures are determined at the height of the point considered after taking into account the IITK-GSDMA-Wind02-V5. Pressure Coefficient – Pressure coefficient 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. etc. say. Gradient Height – Gradient height is the height above the mean ground level at which the gradient wind blows as a result of balance among pressure gradient force. lasting for not more than. Fetch Length (X) – Fetch length is the distance measured along the wind from a boundary at which a change in the type of terrain occurs. For the purpose of this Code. Peak Gust – Peak gust or peak gust speed is the wind speed associated with the maximum value. the boundary of a town or city. spreading or penetrating upwards with increasing fetch length. Mean Ground Level – The mean ground level is the average horizontal plane of the area in the close vicinity and immediately surrounding the structure. 2 minutes with the peak occurring over a specified interval of time. When the changes in terrain types are encountered (such as.
The categories are numbered in increasing order of roughness.0
NOTE – Solidity ratio is to be calculated for individual frames.0
. Suction – Suction means pressure less than the atmospheric (static) pressure and is considered to act away from the surface. Solidity Ratio – Solidity ratio 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. Topography – The nature of the earth’s surface as influenced by the hill and valley configurations in the vicinity of the existing / proposed structure. the reciprocal of which gives the probability of an extreme wind exceeding a given speed in any one year. Speed Profile – The variation of the horizontal component of the atmospheric wind speed with height above the mean ground level is termed as speed profile.
NOTE: Positive sign of the pressure coefficient indicates pressure acting towards the surface and negative sign indicates pressure acting away from the surface. terrain conditions and shielding effect. before meeting the structure or structural element under consideration. which arise from natural or constructed features. 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.
Terrain Category – Terrain category means the characteristics of the surface irregularities of an area. A factor called shielding factor is used to account for such effects in estimating the force on the shielded structure(s).
Return Period – Return period is the number of years.
which may last for a few seconds. keeping in view the importance of the structure. however. often higher than in the severest cyclones.0
IITK-GSDMA-Wind02-V5. – GENERAL
4. have extremely high wind speeds.0
. in general exceed about 60 kilometers. The wind speeds are assessed with the aid of anemometers or anemographs.1 Wind is air in motion relative to the surface of the earth. However. mostly in Northern parts of India.
4. often occur during the summer. which are a narrow band phenomenon of limited time duration. These can last several hours. there are effects of gusts. The current revised draft has recognized the fact that the high wind speeds that occur in cyclones far exceed the wind speeds for design given in the Code at present. Very short duration hurricanes of very high wind speeds called Kal Baisaki or Norwesters occur fairly frequently during summer months over North East India. are the most devastating due to extremely high wind speeds in these storms accompanied by sea surge and flooding. that hit some of the coastal regions of India. Cyclonic storms. or in other words. The effect of reduction in the average wind pressure with increase in the area over which the pressure is considered (the IITK-GSDMA-Wind04-V3. though sometimes. dust storms or vigorous monsoons. The radiation effects are mainly responsible for convection current either upwards or downwards. A feature of the cyclonic storms over the Indian region is that they rapidly weaken after crossing the coasts and move as depressions/ lows inland. Several new recording stations have been established in the country by the Indian Meteorological Department over the last two decades. These gusts cause increase in air pressure but their effect on stability of the building may not be
C4. and addresses the problem vis-àvis the 60 km strip in the east coast and the Gujarat coast by including suitable factors to enhance the design wind speed.2 –
Very strong winds are generally associated with cyclonic storms.3 –
The wind speeds recorded at any locality are extremely variable and in addition to steady wind at any time. more extensive data are needed to make this exercise meaningful. thunderstorms. The primary cause of wind is traced to earth’s rotation and differences in terrestrial radiation.
C4. The Code specifies the basic wind speed as that of a gust of 3 second duration. The influence of a severe storm after striking the coast does not.3 Higher the intensity of a gust.Code & Commentary IS 875 (Part 3)
C4. which are installed at meteorological observatories at heights generally varying from 10 to 30 meters above ground. These.2 Several atmospheric phenomena are responsible for wind storms. it may extend even up to 120 kilometers.
4. the wind speed averaged over a 3second period. the information from which can help upgrade the wind zoning map of India. the term ‘wind’ denotes almost exclusively the horizontal wind while ‘vertical winds’ are always identified as such. lower is its duration. The wind generally blows horizontal to the ground at high speeds.1 For the purpose of this Code wind speed has been considered as that occurring at 10 m height above the general ground level. Since vertical components of atmospheric motion are relatively small. Tornados.
4. which itself gets modified due to presence of other structures/ obstructions.0
. roofs and other parts of the building above average roof level.0
IITK-GSDMA-Wind04-V3.4 –
The response of a building to high wind pressures depends not only upon the geographical location and proximity of other obstructions to airflow but also upon the characteristics of the structure itself. roof sheeting and individual cladding units (glass panels) and their supporting members such as purlins. particularly those in the close vicinity of the structure.5 4. Because of the inertia of the building. The dynamic characteristics of a flexible structure defined by its time period of vibration and damping would affect its response to the gustiness or turbulence in wind.6 4. A maximum reduction of 20% in wind pressures is specified for tributary area beyond 100 m2. the external pressures (or forces) acting on different parts of a framework do not correlate fully. where large suctions occur due to separation of flow and generation of eddies. The stability of a structure shall be checked both with and without the wind loads. Hence there is a reduction in the overall effect. Ka’ defined in Section 5. often. one may consider wind effects over a limited (small) area of the surface.
Contrary to this. This has been allowed for in clause 6. The area of influence being small. 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.5 –
The effect of wind on the structure as a whole is determined by the combined action of external and internal pressures acting upon it.
IITK-GSDMA-Wind02-V5.3. These local area effects are treated elsewhere in the Code.13. The pressures created inside a building due to access of wind through openings could be suction (negative) or pressure (positive) of the same order of intensity while those outside may also vary in magnitude with possible reversals.
C4. as there may be reversal of the forces under wind besides a reduced factor of safety considered with the wind loads. The effect of the latter is difficult to evaluate and a simplified approach has been added for limited cases for the first time in the Code to approximate these so called interference effects in Section 7. Gusts can also be extremely important for design of structures with high slenderness ratios.
C4. Thus the design value shall be taken as the algebraic sum of the two in appropriate/concerned direction.6 –
The stability calculations as a whole shall be done with and without the wind loads on vertical surfaces. short period gusts may not cause any appreciable increase in stress in main components of the building although the walls. In all cases. the calculated wind loads act normal to the surface to which they apply.
C4. sheeting rails and glazing bars may be more seriously affected.2. Furthermore.4 4.2. there is better correlation within these areas.Code & Commentary IS 875 (Part 3)
so important. This is particularly important near the edges and ridge of a structure or sharp corners elsewhere in a building.
tributary area) is accounted for by the ‘Area Averaging Factor.
particularly if the building has other similar structures adjacent to it. Likewise. there may be accentuated flow conditions.0
C4. particularly the latter.7 –
Buildings shall also be designed with due attention to the effects of wind on the comfort of people inside and outside the buildings.Code & Commentary IS 875 (Part 3)
4. Thus the pedestrians at the plaza level can be put to inconvenience. A wind tunnel model study is required to determine the flow pattern and to carryout the design accordingly. the problem has not attained importance. at the plaza level around a tall building. There is no criterion included in this Code for control on these parameters.7 Comfort of the inhabitants of a tall flexible building can be affected by large wind induced deflections or accelerations.0
IITK-GSDMA-Wind02-V5. Since there is no real tall building activity yet in India.
To obtain hourly mean wind speed. The basic wind speed for some important cities/towns is also given in Appendix A. which represents the gustiness of wind. The country has been divided into six wind zones and certain coastal regions affected by cyclonic storms as defined in clause 5.
5. 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.1 . Basic wind speeds presented in Fig.4. The average value depends on the averaging time employed in analyzing the meteorological data and this averaging time can be taken to be from a few seconds to several minutes. since wind speed varies with height. Smaller the averaging interval.65.0
IITK-GSDMA-Wind04-V3. besides the region of the country. wind speed in the atmospheric boundary layer increases with height from zero at ground level to a maximum at a height called the gradient height.2 Code defines the basic wind speed as the peak gust wind speed averaged over a period of 3 seconds.Nature of wind in Atmosphere
In general. In the open terrain category. – WIND SPEED AND PRESSURE
C5. Thus with reduction in the averaging time. with the magnitude of the latter varying with time interval over which the gust is averaged. Basic wind speed is based on peak gust speed averaged over a short time interval of about 3 seconds and corresponds to 10m height above the mean ground level in an open terrain (Category 2). the conditions for which Vb is defined have been specified in this clause. The variation with height depends primarily on the terrain conditions.0
. 1 have been worked out for a 50-year return period. greater is the magnitude of the wind speed. The magnitude of fluctuating component of the wind speed. ground roughness. as applicable at 10 m height above mean ground level for different zones of the country. local topography and return period of the storm. the 3-second value may be multiplied by factor 0. wind speed can be taken to comprise of a static (mean) component and a fluctuating component.Code & Commentary IS 875 (Part 3)
CODE 5. There is usually a slight change in direction (Ekman effect) but this is ignored in the Code.
C5. It includes both the mean and the fluctuating components of the turbulent wind. the fluctuating wind speed would increase. depends on the averaging time.3. The fluctuating velocity is normally expressed in terms of turbulence intensity which is the ratio of the standard deviation to the mean wind speed and is expressed in percentage. However.1 As is explained in Code.2 – Basic Wind Speed (Vb)
Figure 1 gives basic wind speed map of India.
This has been termed as the risk level PN in N consecutive years (Table –1) and the corresponding value of the risk coefficient. with Vb defined for 50-years return period considering the generally acceptable value of probability of exceedence as 0. k3 = topography factor (see 5.2).0. The designer may. k1. if it is considered necessary to reduce the risk level of an important structure. use a higher value of N or k1. Thus for economical design of structures.
C5. the design wind speed has been related to the return-period of storms. and k4 = importance factor for the cyclonic region (see 5. (c) Local topography. 1 and shall be modified to include the following effects to get design wind speed. k2. The values of k1 for N taken as 5.
NOTE: The wind speed may be taken as constant upto a height of 10 m. Z for the chosen structure: (a) Risk level. The suggested life span to be assumed in design and the corresponding k1 factors for different class of structures for the purpose of design are given in Table 1. However.4).
Fig.63 for the design wind speed over the life of the structure.0
IITK-GSDMA-Wind02-V5. k1 = probability factor (risk coefficient) (see 5. Vb corresponds to certain reference conditions.3).3. are given in Table-1. Vz at any height. for N taken as 50 years. to account for various effects governing the design wind speed in any terrain condition.3. a regional basic wind speed having a mean return period of 50 years shall be used except as specified in the note of Table 1.1 – Risk Coefficient (k1)
C5.3. Hence. that is. where Vz = design wind speed at any height z in m/s. k3.3 The basic wind speed. 25 and 100 years.1).3 –Design Wind Speed (Vz)
The basic wind speed for any site shall be obtained from Fig. k2 = terrain roughness and height factor (see 5. 1 gives basic wind speeds for terrain category 2 as applicable at 10 m height above mean ground level based on 50 years mean return period. It can be mathematically expressed as follows: Vz = Vb k1 k2 k3 k4.3.0
. and (d) Importance factor for the cyclonic region. and k4 are specified. pressures for buildings less than 10m high may be reduced by 20% for stability and design of the framing. would be 1. It is known that storms of greater severity are less frequent. modifications in the form of factors k1. however.1 The peak wind speed considered for design is based on the probability of occurrence of the maximum/severest storm over the design life of the structure. and for various zones of the country. (b) Terrain roughness and height of structure. such storms have a longer return period. In the design of all buildings and structures.
XN.76
0. The probability level of 0.82
0. life of the structure.89
0. such as. such as isolated towers in wooded areas.70
0. structures such as those used during construction operations (for example.90
0. communication buildings.0 44 1.63 = extreme wind speed for N = 50 years and PN = 0.2 84.63 is normally considered sufficient for design of buildings and structures against wind effects and the values of k1 corresponding to this risk level are given above.5 22. that is.73
0.0 20.P ⎣ ⎩ N N = k1 = X 50.Code & Commentary IS 875 (Part 3)
Table 1: Risk coefficients for different classes of structures in different wind speed zones [Clause 5.92
0. etc. which take account of the degree of reliability
required.0 88.0.91
0. nuclear power reactors and satellite communication towers. formwork and false work).07
1.3. and period of time in years during which there will be exposure to wind.8 B 9.63 A + 4B
where N PN = mean probable design life of the structure in years.67
0.0 50 1.3
IITK-GSDMA-Wind02-V5.8 27.0 55 1. and
X50.94
0.8 90. there is always a probability (howsoever small) that it may be exceeded in a storm of exceptional violence.2 14.06
1. = risk level in N consecutive years (probability that the design wind speed is exceeded at least once in N successive years).63. Important buildings and structures such as hospitals.08
1.P = extreme wind speed for given value of N and PN. Equation given below may be used in such cases to estimate k1 factors for different periods of exposure and chosen probability of exceedence (risk level).0
.0 39 1.08
NOTE – The factor k1 is based on statistical concepts. nominal value = 0.0 88.1]
Mean Probable design life of structure in years 50 k1 factor for Basic Wind Speed (m/s) of 33 All general buildings and structures Temporary sheds. towers and power plant structures 1.90
0. and boundary walls Buildings and structures presenting a low degree of hazard to life and property in the event of failure.
⎡ ⎧ 1 A − B ⎢ln ⎨− ln 1 − PN X N .0 88. farm buildings other than residential buildings.07
1. 0. the greater the number of years over which there will be exposure to wind. Whatever wind speed is adopted for design purposes. High return periods ranging from 100 to 1000 years (implying lower risk level) in association with greater period of exposure may have to be selected for exceptionally important structures.63 A and B are coefficients having the following values for different basic wind speed zones: Zone 33 m/s 39 m/s 44 m/s 47 m/s 50 m/s 55 m/s A 83. structures during construction stages.0 47 1.0
IITK-GSDMA-Wind04-V3. the greater is the probability.71
34 1.38 1.26 1.20 1.30 1.80 0.28 1.32 1.09 1.91 0.06 1.36 1.05 1.32 1.30 1.80 0.17 1.2] Height (z) Terrain Category 1 1.24 1.3.27 1.12 1.32 1.12 1.15 1.2.24 1.97 1.31 1.34 1.30 1.28 1.38 1.39 Terrain Category 3 0.80 0.27 1.37 1. use linear interpolation.34
NOTE: For intermediate values of height z and terrain category.20 1.32 1.33 1.24 1.29 1.97 1.20 1.10 1.35 1.0
.35 1.31 1.36 Terrain Category 4 0.34 1.01 1.
k2 factors to obtain design wind speed variation with height in different terrains [Clause 5.0
IITK-GSDMA-Wind04-V3.07 1.05 1.00 1.40 Terrain and height multiplier (k2) Terrain Category 2 1.37 1.39 1.12 1.
NOTE – This category includes open sea coasts and flat treeless plains. which is likely to be blown down or defoliated.0
. the orientation of any building or structure may be suitably planned.2. if deviated from the above reference terrain.Code & Commentary IS 875 (Part 3)
5. towns and industrial areas fully or partially developed. The terrain category used in the design of a structure may vary depending on the direction of wind under consideration. In addition. Where such a situation exists. NOTE 3 – Particular attention must be given to performance of obstructions in areas affected by fully developed tropical cyclones.1 The Code defines 4 types of terrains and explains that a structure may effectively lie in two different types of terrain for two different wind directions. and shrubs.
NOTE – This is the criterion for measurement of regional basic wind speeds and includes airfields. open parklands and undeveloped sparsely built-up outskirts of towns and suburbs.
NOTE 1 – This category includes well-wooded areas.
Category 3 – Terrain with numerous closely spaced obstructions having the size of building-structures up to 10 m in height with or without a few isolated tall structures. cannot be relied upon to maintain Category 3 conditions. NOTE 2 – 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. is accounted for through the factor.5 and 10 m.
Category 2 – Open terrain with wellscattered obstructions having height generally between 1. Photographs CP1 to CP4 (Cook 1985) are given to demonstrate how terrain categories 1 to 4 may be assigned. the designer shall keep in mind.3. k2.
Terrain – Selection of terrain categories shall be made with due regard to the effect of obstructions which constitute the ground surface roughness.3.3. This is merely for guidance purpose. Vegetation. 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 a few or no obstructions and in which the average height of any object surrounding the structure is less than 1.1 –
C5. the effect of terrain condition. the future development of the surrounding area which may alter the ground roughness and hence the terrain category. It may be noted that Category 2 has been considered as the datum with respect to which the other terrain categories have been defined.0
IITK-GSDMA-Wind04-V3. Open land adjacent to seacoast may also be classified as Category 2 due to roughness of large sea waves at high winds. either an
IITK-GSDMA-Wind02-V5.2 – Terrain and Height Factor (k2)
5.2. Wherever sufficient meteorological information is available about the wind direction. In a given situation.
as applicable according to the importance of the structure: Structures of post–cyclone importance 1. The value of k3 is confined in the range of 1. The peak wind speeds in these regions may exceed 70 m/s. In order to ensure greater safety of structures in this region (60 km wide on the east coast as well as on the Gujarat coast). the factor k4 shall obviously be taken as 1.3 The factor k3 is a measure of the enhancement that occurs in wind speeds over hills. that is. other structures may be designed for a k4 value of unity. or ridges.Code & Commentary IS 875 (Part 3)
C5. cliffs and escarpments. hospitals. However. at a maximum near the ground. The effect of topography is to accelerate wind near the summits of hills or crests of cliffs. or ridges.
IITK-GSDMA-Wind02-V5.3. 1 takes account of the general level of site above sea level. and reducing to 1. without considering the effect of the possible higher wind speeds in cyclonic storms.36 for slopes greater than 3o.0 at higher levels.3. which can significantly affect the wind speed in their vicinity. For reasons of economy. The effect of cyclonic storms is largely felt in a belt of approximately 60 km width at the coast.
C5. For non-cyclonic regions.0. power-plant structures.30.0
IITK-GSDMA-Wind04-V3.1 –
The effect of topography will be significant at a site when the upwind slope (θ) is greater than about 3o.3. communication towers.4–
A belt of approximately 60 km width near sea coast in certain parts of the country is identified to be affected by cyclonic storms.0
.3. factor k4 has been introduced with a maximum value of 1. the value of k3 may be taken to be equal to 1.15 All other structures 1.3. while a lower value of 1.3. school and community buildings. Therefore. damage to which can cause serious economic losses.1–
No increase in wind speed is indicated for upwind ground slopes upto 3o. while a maximum increase of 36% is specified for slopes beyond 17o. and below that. 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 following values of k4 are stipulated.3 –
Topography (k3 factor) – The basic wind speed Vb given in Fig. Also.0 to 1.30 Industrial structures 1. for hill slope in excess of 17o.0 is given in Appendix C. A method of evaluating the value of k3 for values greater than 1. Maximum effect is seen to occur at the crest of a cliff or escarpment and reduces gradually with distance from the crest. the highest value may be used only for structures of postcyclone importance such as cyclone shelters.
C5. and water tanks. locally k3 reduces from the base of a structure to its top.4 –
5. escarpments. escarpments or ridges and decelerate the wind in valleys or near the foot of cliffs.15 may be used for industrial structures.
5. steep escarpments. valleys.3. cliffs. It may be noted that the value of k3 varies with height above ground level.0. This does not allow for local topographic features such as hills.3.
3. Kc. Ka.9 has been used in the present revision except for circular. Ka and Kc are to be applied. pd = Kd.4.0. For circular or near – circular forms this factor may be taken as 1.4.1 – Wind Directionality Factor. rectangular) a factor of 0.
IITK-GSDMA-Wind02-V5. irrespective of the direction of wind. it is specified that for buildings.
NOTE 1 – The coefficient 0.Code & Commentary IS 875 (Part 3)
5. square. NOTE 2 – Ka should be taken as 1. the factor Kd shall be taken as 1. Kd
To obtain the design wind pressure. A flat value of 0.6 (in SI units) in the above formula depends on a number of factors and mainly on the atmospheric pressure and air temperature.
5.0 when considering local pressure coefficients.20 kg/m3. This factor has not been included in the 1987 version of the Code.0
The relationship between design wind speed Vz and the pressure produced by it assumes the mass density of air as 1. solid signs. For the cyclone affected regions also. and trussed towers (triangular.1 The factor recognizes the fact of (i) reduced probability of maximum winds coming from any given direction (ii) reduced probability of the
Considering the randomness in the directionality of wind and recognizing the fact that pressure or force coefficients are determined for specific wind directions. pz where Kd = Ka = Kc = Wind directionality factor Area averaging factor Combination factor (See 6. The design wind pressure pd can be obtained as. Some of the other Codes (ASCE/Australian) give varying values of the factor for different situations based on a more detailed study of wind directionality. which changes somewhat with the atmospheric temperature and pressure.13)
C5. The value chosen corresponds to the average Indian atmospheric conditions. These factors are explained in following Sections.90 may be used on the design wind pressure.6 Vz where pz = wind pressure in N/m2 at height z. These have been assigned a value of 1.0 for the factor Kd.0.4 – Design 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: 2 p z = 0.
maximum pressure coefficient occurring for any given wind direction.0
IITK-GSDMA-Wind04-V3. and Vz = design wind speed in m/s at height z. near – circular and axisymmetric sections which offer a uniform resistance. lattice frameworks.2. various modifications through factors Kd. open signs.
Thus. there are local area effects.4.4.0
.2 – Area Averaging Factor. Because of separation of the flow at the edges and the corners. This area is defined as the tributary area for the element/part of the structure. These should not be used for calculating the forces on the roof or the framework as a whole. the lack of correlation amongst the pressures gets modified because of the generation of local eddies and the distortion of those contained in the incoming wind.
C5. the tributary area will be smaller for a purlin as compared to that for a roof truss or a framework.5 The cyclonic storms are formed away from the coasts and have wind speeds much higher than recorded on the coasts. Local area effects are already being taken into account in the 1987 version of the Code for the design of the cladding and its connections to the supporting framework. as an example. shall be the surface area from which the wind pressures/forces get transferred to the element/part of the structure being designed.
Pressure coefficients given in Section 6. In fact.9 0. pressures being directly proportional to the square of the wind speed. As the area becomes larger. At least 15% higher wind speed than at the coast may be considered for distances upto about 200 kilometers into the sea in the affected regions.2] Area Averaging Factor (Ka) 1. This would naturally lead to a lack of correlation amongst pressures induced by the wind impinging on a surface. to be used as a multiplier for the pressures/forces occurring on the structure.8
5. The magnitude of these suctions can be greatly influenced by the geometry of the structure and the angle of wind incidence.0
IITK-GSDMA-Wind04-V3. though the area of influence of such suction peaks is expected to be small.1.
IITK-GSDMA-Wind02-V5.2 are a result of averaging the measured pressure values over a given area.5 – Offshore Wind Speed
Cyclonic storms form far away from the sea coast and gradually reduce in speed as they approach the sea coast. Ka C5. suctions are experienced at these locations. which can be quite high. The decrease in pressures due to larger areas may be taken into account as given in Table 4. Conversely. The reduced correlation is deemed to be accounted for by introducing the area reduction factor. near the edges and corners of a structure.2 -
It is well recognized that the incoming wind becomes increasingly un-correlated as the area considered increases. the correlation of measured values decrease and vice-versa. as the wind flows past a surface. Their effect on land is already reflected in basic wind speeds specified in Fig.
Area averaging factor (Ka) [Clause 5.0 0. Cyclonic storms generally extend up to about 60 kilometers inland after striking the coast. The area to be considered for any part of the building for computing the area reduction factor.15 times the value on the nearest coast in the absence of any definite wind data. The influence of wind speed off the coast up to a distance of about 200 kilometers may be taken as 1.4.Code & Commentary IS 875 (Part 3)
1.2. The average values of these pressure coefficients for some building shapes are given in Sections 6. direction of wind incidence.
6. As mentioned in C 6. For clad buildings. Besides. and c) Individual cladding units glazing and their fixtures. point of separation etc. Coefficients for the local effects should only be used for calculation of forces on these local areas affecting roof
C6. – WIND PRESSURES AND FORCES ON BUILDINGS/STRUCTURES
C6. Areas of high local pressure or suction frequently occur near the edges of walls and roofs. which determine the nature of wind flow over or around a building/structure.0
IITK-GSDMA-Wind04-V3.2. from which the forces get transferred to the framework.1 -
CODE 6. separation of the flow at the edges and corners and formation of vortices generates suctions. including
A major purpose of the Code is to determine forces and pressures on components of a building or a structure as required for design purposes. or an assembly of members or a framework.1 – General
Wind load on a building shall be calculated for: a) The building as a whole. the geometry. Coefficients for these are given separately for the design of cladding in Section 6.3. The Code provides values of pressure coefficients for a variety of cases covered.3. The nature and magnitude of these pressures/suctions is dependent upon a large number of variables. The most common approach to the determination of pressure distribution on different building forms is to test geometrically similar rigid models in a simulated wind environment in wind tunnels. The pressures caused are also often quite sensitive to changes in geometry and the angle of wind incidence. the nature of the incident wind. force coefficients are given for (i) clad buildings and (ii) unclad structures and (iii) elements. These forces are used in designing the framework. b) Individual structural elements such as roofs and walls.3 can be used for individual cladding units including their fixtures. Thus the building frame experiences the cumulative effect of pressures produces forces on different parts of the cladding – both on the walls as well as the roof as the case may be. Local pressure coefficients given in Section 6.3. These coefficients can be used to determine forces on an element. This is generally carried out by making ‘point’ pressure measurements over the model and averaging the
IITK-GSDMA-Wind02-V5. namely.2. In addition.2 . The wind load acting normal to a surface is obtained by multiplying area of the surface or its appropriate portion by the and the pressure coefficient (Cp) corresponding design wind pressure. Both pressure and force coefficients are derived on the basis of models tested in wind tunnels. pressures on the cladding are required in order to design the cladding and its supporting elements..1– Pressure Coefficients
Wind causes pressure or suction normal to the surface of a building or structure.Pressure Coefficients
The pressure coefficients are always given for a particular surface or part of the surface of a building. higher values may also be experienced on small (local) areas of walls.2. often large in magnitude.2/6.2.2.3.2 and 6.
primarily the turbulence. Pressure coefficients are commonly based on the quasi – steady assumption. NOTE 3 – Influence of local values of suction or pressure may not be of much consequence for the overall safety of the structure but can be a cause of local damage to cladding or glazing. This is for two reasons – (i) the increasing lack of correlations over an extended area.0
IITK-GSDMA-Wind04-V3. Early wind tunnel work did not recognize the importance of simulating the ‘boundary layer’ flow of wind and its characteristics. Obviously. may be on the conservative side. the method has the advantage of simplicity.as well as the building geometry.2. The body of information that has thus emerged is expected to better represent the wind effects expected in the field. This in turn may have a ‘chain’ effect and lead to much economic loss. mostly in the walls. namely 6.3. pressure coefficient contours over a gable roof may be as seen in Figure C2. The lack of adequacy of the database. remains because of the large variability involved both. The following sections. This approach implicitly assumes that the fluctuations in pressure follow directly those in the speed.
NOTE 1 – The pressure coefficients given in different tables have been obtained mainly from measurements on models in wind tunnels. They should not be used for calculating forces on entire structural elements such as roof. This of course may not be true.Code & Commentary IS 875 (Part 3)
sheeting. though it may not be suitable for very large structures. The approach followed in the present Indian Code as well as the proposed revision (and several other Codes) is to take V as the peak gust value. and eddies form at separation. This would increase accuracy but will create difficulties in practical design work. in an overall analysis. Pressures are caused both on the exterior as well as the interior surfaces. Some Codes use the mean wind speed averaged over a longer period.
pressure values over a specified tributary area. NOTE 2 – For pressure coefficients for structures not covered here. there has been a realization of the importance of such simulation over the last 3-4 decades. since the wind turbulence gets modified as it approaches the structure. walls or structure as a whole. however. However. reference may be made to specialist literature on the subject or advice may be sought from specialists in the subject.2 and 6. and (ii) the dynamics of a large structural system. However. Here ρ is the mass density of air and V the wind speed. whereby the pressure coefficient is taken to be the ratio of mean pressure measured over a point or pressure averaged over a small tributary area divided by the dynamic pressure ( 1 ρV2) for the mean speed 2 of incident wind. it will be ideal to divide the roof into a large number of zones to specify the pressures for each zone. Making a coarser grid-work will lead to averaged out values such as in Figure C 3. with respect to the wind – its structure and directionality . The approach adopted in practice is to go by the latter and use area averages which. the latter being dependent on openings (or permeability) in the structure.2. individual cladding units including their fixtures. glass panels. respectively give values
IITK-GSDMA-Wind02-V5. Typically.0
F. and individual cladding units and their fittings. acting in a direction normal to the individual structural element or cladding unit is: F = (Cpe – Cpi) A pd where Cpe = external pressure coefficient. therefore. necessary to know the internal pressure as well as the external pressure. the surface areas of the structural element may be sub-divided so that the specified pressures are taken over appropriate areas. Cpi = internal pressure coefficient A = surface area of structural element or cladding unit.
IITK-GSDMA-Wind02-V5. and pd = design wind pressure
NOTE 1 . NOTE 2 – Positive wind load indicates the force acting towards the structural element (pressure) and negative away from it (suction). Then the wind load. For clad structures. it is essential to take account of the pressure difference between opposite faces of such elements or units.1 – Wind Load on Individual Members
When calculating the wind load on individual structural elements such as roofs and walls. it is.Code & Commentary IS 875 (Part 3)
of pressure coefficients for the interior and exterior surfaces.0
IITK-GSDMA-Wind04-V3.2.If the surface design pressure varies with height.0
Cpmean (b) Fig.-)
IITK-GSDMA-Wind02-V5. C3 : Variation of ressure over a Pitched Roof (
) and the Average Value (. C2 : Typical Contours of Pressure Coefficients over a Pitched Roof (a) Cp min (b) Cp
etc. In most situations a simple inspection of the sign of external pressure will at once indicate the proper sign of the internal pressure coefficient to be taken for design. ventilators and through the joints between roof coverings. The internal pressure coefficient is algebraically added to the external pressure coefficient and the analysis which indicates greater distress of the member.2.
6.2. Small permeability implies upto 5% openings and may be deemed to occur even with doors and windows closed.2.Code & Commentary IS 875 (Part 3)
6.2 – Internal Coefficients Cpi Pressure
Internal air pressure in a building depends upon the degree of permeability of cladding to the flow of air.
Note: The term small permeability relates to the flow of air commonly afforded by claddings not only through open windows and doors. Internal pressures vary with the degree of permeability. the total open area being less than 5 percent of area of the walls having the openings. Cpe. Buildings with medium
IITK-GSDMA-Wind02-V5. one with an internal pressure coefficient of +0. Two design conditions shall be examined. specified herein as small.2.0
IITK-GSDMA-Wind04-V3. medium and large.2 –
Internal pressures are not influenced much by the external shape or geometry of the building but are primarily a function of the openings in it.2.1 –
In case of buildings where the claddings permit the flow of air with openings not more than about 5 percent of the wall area but where there are no large openings. but also through the slits round the closed windows and doors and through chimneys. to obtain the critical design combination. These can be positive or negative and have to be combined algebraically with the external values. since flow can take place through slits and recesses in doors. The internal air pressure may be positive or negative depending on the direction of flow of air in relation to openings in the building.2 and other with an internal pressure coefficient of –0.2 –
Buildings with medium and large openings Buildings with medium and large openings may also exhibit either positive or negative internal pressure depending upon the direction of wind. it is necessary to consider the possibility of the internal pressure being positive or negative. 2 in the Code. shall be adopted. Buildings with one large opening may be treated as per Fig.
also a little more accurate. Recent versions of some international Codes have been revised on the basis of these studies.3.
6. though the revised international Codes have become more elaborate. These have further underlined the influence of wind incidence angle particularly on edges and corners. shall be adopted. which produces greater distress in the members. 875 (Part 3)-1987 was written. there have been further studies of wind effects on low buildings.2/6.2.2 indicating values of internal pressure coefficients with respect to the direction of wind. that is. Furthermore.The average external pressure coefficient for the walls of clad buildings of rectangular plan shall be as given in Table 5.S.2. openings larger than 20 percent of the wall area shall be examined once with an internal pressure coefficient of + 0. Since the present version of I.7 and again with an internal pressure coefficient of –0. which have relevance to the design of the cladding and its connections to
openings between about 5 to 20 percent of the wall area shall be examined for an internal pressure coefficient of + 0.0
. and the analysis. shall be adopted.3. these have also become somewhat more complex to use. In buildings with roofs but no walls.2.1 –
C6. A few examples of buildings with one-sided openings are shown in Fig. Most part of this section has therefore been retained as it occurs in the current Code.3. However.5 and later with an internal pressure coefficient of – 0. a comparative analysis has shown that the overall design values as obtained by the present IS Code do not differ by significant enough extent to warrant a revision of these coefficients. and.2. and the analysis.0
IITK-GSDMA-Wind04-V3. Buildings with large openings.1–
Walls . the roofs will be subjected to pressure from both inside and outside and the recommendations are as given in 6.
6.5. Buildings with one open side or openings exceeding 20 percent of the wall area may be assumed to be subjected to internal positive pressure or suction similar to those for buildings with large openings.3It has been explained in C 6.3– External Pressure Coefficients
C6.1 as to how pressure coefficients are obtained. Local pressure coefficients at the edges of the wall.7. local pressure concentration coefficients are also given. In addition.2.2.2. which produces greater distress in the members.
Furthermore..3. Thus a relief of 20% is being permitted in the pressure coefficients for the hipped slopes. Hipped and Monoslope Roofs of Rectangular Clad Buildings– The average external pressure coefficients and pressure concentration coefficients for pitched roofs of rectangular clad buildings shall be as given in Table 6. The values on the leeward slope are not affected much by the variations in geometry. Local pressure coefficients (for the design of cladding and its connections) at the edges and ridge are also given – these act upwards.2 –
This clause provides information on the roofs of clad buildings. and be reduced by 20% for the hip slope. as appropriate from Table 6.3.2. been shown that hipped roofs experience smaller suction as compared to pitched roofs of corresponding geometry (see Fig.0
IITK-GSDMA-Wind04-V3. however.Code & Commentary IS 875 (Part 3)
the supporting framework. values are given here for 0o and 90o only. cornices and 90 degree corners of roofs. pressure coefficients (including local values) may be taken on all the four slopes.2. The roof surface is divided into different zones for the purpose of specifying the design pressure coefficients.
For monoslope roofs of rectangular clad buildings. For monoslope and hipped roofs also the pressure coefficients can be taken from Table 6. clad buildings with monoslope roofs are covered in detail in Table 7. It is now known that wind directions other than 0o and 90o can give values higher than those at 0o and 90o.3.3.
IITK-GSDMA-Wind02-V5.The pressure concentration shall be assumed to act outward (suction pressure) at the ridges. Table 6 gives pressure coefficients for pitched roofs with different aspect ratios and varying roof pitch for two directions of wind incidence . NOTE 2 . are also given in the Table. i. Information on force coefficients for free standing walls is given separately in 6. the average pressure coefficient and pressure concentration coefficient for monoslope (lean-to) roofs of rectangular clad buildings shall be as given in Table 7.2. The pressure coefficients on the underside of any overhanging roof shall be taken in accordance with 6.
NOTE 1 . the average coefficients shall apply. eaves.2–
Pitched.0o and 90o. It has. Pressure coefficients for different angles of wind incidence are given therein.
6. suction. C4 and also C5). which are perhaps the most commonly used. for the applicable roof slope.2. for simplicity in design.e.2. However. NOTE 3 – For hipped roofs. which is not so for the windward slope where values vary from large pressures to suctions.5.The pressure concentration shall not be included with the net external pressure when computing overall load.0
. Where no pressure concentration coefficients are given.
IITK-GSDMA-Wind04-V3. C 4: Worst Peak Negative Pressure Coefficients – all azimuths (Meecham 1992)
Fig. C 5 : Effects of Roof Architecture on Uplifts
(c) For (b/d) = 1.4
IITK-GSDMA-Wind04-V3.Code & Commentary IS 875 (Part 3) (a) For (b/d) > 1 (b) For (b/d) < 1
-0. Figure 2: Large opening in buildings (values of coefficients of internal pressure) with top closed [Clause 6.2.7
-0. use average values Arrows indicate direction of wind.2]
IITK-GSDMA-Wind02-V5.8
.3.2.0
Table 5 External Pressure Coefficients (Cpe) for Walls of Rectangular Clad Buildings (Clause 6.
5 -0.5 -0.7 -0.4 -0.4 -0.1 ---1.8 -0.8 -0.2
-2.7 -0.5 -1.4 0 +0.2 -1.0 -1.0 -2.0
.2 -1.0
IITK-GSDMA-Wind04-V3.4 -0.0 -2.5 -0.9 -0.6 -0.2 +0.5 -1.2 +0.5
-2.2.1 -0.7 -0.1 -1.5 -1.8 -0.8 -0.0 -1.8 -0.1 -1.7 -0.0 -1.7 -0.8 -0.8 -0.5
-2.Code & Commentary IS 875 (Part 3)
External Pressure Coefficients (Cpe) for Pitched Roofs of Rectangular Clad Buildings (Clause 6.4 -0.7 -0.5 Wind angle θ 90o EG -0.6 -0.5 -1.8 -1.8 -0.0 -2.0 -1.0 -0.2 -1.0 -1.7 -1.5 -1.0 ---1.8 FH -0.6 -0.0 -0.8 -0.0 -2.4 -0.8 -0.6 -0.9 -1.6 -0.6 -0.2 -0.2 +0.6 -0.2 +0.2 ---1.8 -0.5 -0.4 -2.0
-2.0 -1.0 -1.9 -0.9 -1.8 -0.5 GH -0.0 -1.6 -0.0 -0.0 -2.6 -0.6 -0.5 -0.7 -0.7 -0.8 -0.7 -2.6 -0.7 -0.6 -0.2 -1.8 -0.0 -2.8 -0.5 -0.6 -0.5
-2.0 -1.8 -0.8 -0.6 -0.0 -1.7 -0.6 -0.0 -1.8 -0.7 -0.8 -0.0 -2.0 -2.0 -1.5
Contd………… IITK-GSDMA-Wind02-V5.0 -1.4 -0.0 -2.6 -0.3 +0.6 -0.8 -0.3.8 -2.5 -1.5 -0.6 -0.5 -0.7 -0.4 -1.4 -0.2) Wind angle θ 0o EF -0.5 -1.8 -0.2 -1.8 -0.
NOTE 3 – For hipped roofs the local coefficient for the hip ridge may be conservatively taken as the central ridge value. whichever is the smaller
.15 W.
w KEY PLAN Y = h or 0. the overall coefficients apply. NOTE 2 – Where no local coefficients are given.
Tables 8 and 9 give pressure coefficients for the limited ratios of h/w ( 1 to 1) and L/W (1 to 3) for 4 free standing canopies for the roof slope varying between 0o and 30o. railway platforms. Values of Cp for intermediate solidities may be linearly interpolated between these two extremes.3. the resultant is to be taken normal to the canopy.
C6. such as those due to trains. φ = 0 for no blockage and φ = 1 for full blockage. φ = 1 represents the canopy fully blocked with contents to the downwind eaves.3.2. In addition to the pressure forces normal to the canopy. φ = 0 represents a canopy with no obstructions underneath. only the greater of these two forces need be taken into account. both areas normal to the wind direction. For any wind direction.The pressure
coefficients are given in Tables 8 and 9 separately for mono pitch and double pitch canopy roofs such as open-air parking garages. outdoor areas. The coefficients take account of the combined effect of the wind exerted on and under the roof for all wind directions. and applied upwind of the position of maximum blockage only.3– Canopy roofs with . Downwind of the position of maximum blockage the coefficients for φ = 0 may be used. buses and stored materials shall be foreseen and taken into account. shelter areas.Code & Commentary IS 875 (Part 3)
6. the greater of the two given values should be taken. Positive values are not affected by the blockage under the roof while the suctions (negative values) are given for two cases. stadiums and theatres.0
IITK-GSDMA-Wind04-V3. there will be horizontal loads on the canopy due to the wind pressure on any fascia and due to friction over the surface of the canopy.3.3. The solidity ratio φ is equal to the area of obstructions under the canopy divided by the gross area under the canopy. Frictional drag should be calculated using the coefficients given in 6. Fascia loads should be calculated on the area of the surface facing the wind. Where the local coefficients overlap. using a force coefficient of 1.0
IITK-GSDMA-Wind02-V5. However. the effect of partial closures of one side and / or both sides.
-1.7 0.0
IITK-GSDMA-Wind04-V3.2) w y He H1 H Le L L1 W
L2 l y = h or 0.5 -1.0 -2.8 -1.5 -0.3 -0.0 -1.8 -1.0 -2.0 -1.0
Note – h is the height to eaves at lower side.6
IITK-GSDMA-Wind02-V5.0 -0.5 -0.4 -0.0 -2.6 -0.0 -0.8 -1.5
-2.9 -0.Code & Commentary IS 875 (Part 3)
External Pressure Coefficients (Cpe) for Monoslope Roofs for Rectangular h Clad Buildings with < 2 (Clause 6.9 -0.8
-1.0 -1.5 -0.5 -0.5 -0.9 -0.0
-0.5 -1.0 -2.0 -0.4 -0.0 -1.7 -0.8 -0.5 -1.9 -0.0 -2.2
-2.0 -1.9 -0.0 -2.8 -0.5 -0.0 -1.0 -1.15 w.5
-0. whichever is the lesser
-1.5 -0.4 -1.5 -0.0 -1.0 -1.6
-2.2 -0.5 -0.8 -0.0 -1.0 -2.6 -0.1 0
-1.6 -0.0 -0.3.5 -0.3 -0.5 -0.9 -0.5 -0.0 -2.0 -1.0 -1.5 -0.8 -0.2 -0.5
-0. l is the greater horizontal dimension of a building and W is the smaller horizontal dimension of a building.8 -1.0
.9 -0.2 -0.8
-0.8 -0.2.8 -0.9 -0.6
-2.0 -1.0 -1.0 -2.0 -0.9 -0.
IITK-GSDMA-Wind04-V3.2.3.3 W from the windward
Note – For monopitch canopies the centre of pressure should be taken to act at 0.Code & Commentary IS 875 (Part 3)
Pressure Coefficients for Monoslope Roofs (Clause 6.
IITK-GSDMA-Wind04-V3.3.3)
IITK-GSDMA-Wind02-V5.2.Code & Commentary IS 875 (Part 3)
2. Y for saw.3. Y for pitched roofs.5. if n is greater than 4. take the local coefficient as – 2. 3) and saw-tooth roofs (Fig. where n is the total number of spans.2. It is to be noted that whereas the roof surfaces of exterior spans (A.0 and for the intermediate spans as – 1.4 ]
IITK-GSDMA-Wind02-V5.2. take n = 4.0
. the roofs of interior spans (M for pitched roof and M. X. N for saw-tooth roof) will experience similar effects. 4) of multi-span buildings.05 (n-1)] shall be added to the roof pressure coefficients in the region 0 to 1h from the leading edge. for some surfaces. over a width equal to h or 0. Where two values are given for a coefficient.3. and A.4 ]
Figure 4: External pressure coefficients (Cpe) for multi-span buildings having saw-tooth roofs [Clause 6.2.1 ds whichever is smaller.1 and 6.3.
Figure 3: External pressure coefficients (Cpe) for multi-span buildings having pitched roofs [Clause 6. both shall be considered for design.3. the external average pressure coefficients shall be as given in Tables 10 and 11 respectively provided that all spans are equal.For pitched (Fig.2.2.3. Local pressures may be taken at the edges and ridge of each span.tooth roofs) will experience different wind pressures.
C6. C. For this calculation.
External pressure coefficients for wind directions of θ = 90o / 270o shall be obtained from clauses 6. B. For the end spans.3.0
IITK-GSDMA-Wind04-V3.2 but [-0. both negative as well as positive values are applicable and should be considered. W.4 –
The Figures and Tables are self-explanatory. C.
Tables 10 and 11 have to be used in conjunction with Tables 5 and 6 to give the complete information on wind forces on such structures.4– Pitched and saw-tooth roofs of multi-span buildings . All pressure coefficients shall be used with the value of wind speed applied at average roof height (h). as found critical in design. D.Code & Commentary IS 875 (Part 3)
6. Furthermore. B.
0. 0.3.5 and 0. 0.4 -0.5. 0.3.7 X -0.2 C -0.2.4] Surface reference (see Figure3) A 0. 0.0
. 0.2 for α < 10
IITK-GSDMA-Wind04-V3.4 W -0.3 and 0.Code & Commentary IS 875 (Part 3)
Table 10: External pressure coefficients (Cpe) for multi-span buildings having pitch roofs [Clause 6.3 D -0.2 B -0.5 -0.2 -0.2 M -0.2 0.9 -0.3 for α ≥10o
Table 11: External pressure coefficients (Cpe) for multi-span buildings having saw tooth roofs [Clause 6.7 -0.5.3. as appropriate
Use Table 6 for (h/w) and α.7
IITK-GSDMA-Wind02-V5.4] Wind Direction θ (degrees) 0 180 Surface reference (see Figure 4) A 0.2.0.7 B C M -0.3 Y -0.5.5 -0.2.2.4 N -0.3 -0.9 -0.
2. such as.5 –
Overhangs from a building are affected by wind pressure acting from underneath.0
C6. saucer shaped. and doubly curved (hyperbolic paraboloid).25 if the overhang slopes downwards. A variety of other curved shapes have been used in roofs.
the external pressure coefficients shall be as given in Table12.2.
6.6 – Curved Roofs -For curved roofs.
C6.3.6 –
This clause specifies values of pressure coefficients on curved convex roof surfaces. Values of wind pressure coefficients are available for such shapes. and are contained in the literature (Krishna.2.5– Pressure coefficients on overhangs
from roofs .75 if the overhang slopes upwards.3. more as a result of case specific studies. which are perhaps the most common amongst curved roofs.3. domical. The pressure coefficients for the underside surface of the overhanging portion shall be taken as follows and shall be taken as positive if the overhanging portion is on the windward side: a) 1. Considering that these roof shapes are not a common occurrence.2.3.00 if the overhang is horizontal. For overhanging portions on sides other than the windward side. these are not covered in this Code or in other International Codes. b) 1.
IITK-GSDMA-Wind02-V5. and c) 0. These combined with pressures (or suctions) on the top surface often create a severe design condition.0
6. Most part of the roof exterior is subjected to suction. 1989). singly curved concave.The pressure coefficients on the top-overhanging portion of a roof shall be taken to be the same as that of the nearest top portion of the non-overhanging portion of the roof. the average pressure coefficients on adjoining walls may be used.
2 C1 +0.8 -0.8 -0.2 0.
Central Half (C) Leeward Quarter (–0.6] C
–0.3 +0.4 +0.7 -0.4 0.8 -0.3 +0.7 for the full width of the roof over a length of l/2 from the gable ends and –0.4 0.6
l L +0.3 0.0
.7H Wind +0.1 –0.5 C -0.9 -1.7
NOTE – When the wind is blowing normal to gable ends.1 -1.6
IITK-GSDMA-Wind02-V5. Cpe may be taken as equal to –0.Code & Commentary IS 875 (Part 3)
External pressure coefficients for curved roofs.6 +0.8 -0.2.7H C1 H
l (a) Roof springing from ground level C H/l 0.1 +0.4) Windward Quarter (C2)
H/L > 0.4 Wind 0.6 C2 H 0.7 C2 -0.4 +0. [Clause 6.5 for the remaining portion.6 a/l > 0.3.0 -1.
For details of roof pressure distribution see Fig. a value commonly achieved in practice.
Slender cylinders.5 where h/D is less than 0.8 where h/D is not less than 0. While Table 14 gives the overall force coefficients.
IITK-GSDMA-Wind02-V5. placed on ground or elevated. The roof may be flat. Where there is a free flow of air around both ends. 5.0
. the value of Cpi shall be taken into account.2. D the diameter. Cpi should be taken as zero for an R. In the calculation of the resulting pressure on the periphery of the cylinder.000. and ν the kinematic viscosity of air.C.785 D2 (pi – Cpe pd) where pi is the pressure inside the tank caused by the stored fluid vapours.2.7 –
Wind effects on cylindrical structures are influenced by the Reynold’s Number. These are dealt with later in Sections 8 and 9. These may be used for wind blowing normal to the axis of cylinders having axis normal to the ground plane (that is. h is height of a vertical cylinder or length of a horizontal cylinder.1 D from the center of the roof on the windward side. This may be due to the vapour of the liquid stored. as the roof is made monolithic with the walls and the opening in roof is always kept closed.
C6.8 –
The clause specifies forces on roofs over a cylindrical structure. The values given in the Code are for Re greater than 10.3. and values of Cpi are specified for open ended cylinders. horizontal tanks) provided that the clearance between the tank and the ground is not less than the diameter D of the cylinder. 5. The resultant of P for roofs lies at 0.2. These are given for different proportions of a cylinder. Cpi shall be taken as follows: a) -0.Code & Commentary IS 875 (Part 3)
6.2.8– Roofs and bottoms of cylindrical
structures The external pressure coefficients for roofs and bottoms of cylindrical elevated structures shall be as given in Table 14. water tank. Re given by VD/ν.3.7 – Cylindrical structures
For the purpose of calculating the wind pressure distribution around a cylindrical structure of circular cross-section.3. In addition to the external pressures/forces.3.3. h is to be taken as half the length when calculating h/D ratio.3. and b) –0. detailed pressure distribution over a conical roof is given in Fig.
6. The total resultant load (P) acting on the roof of the structure is given by the following formula: P = 0.0
IITK-GSDMA-Wind04-V3. where V is the velocity of wind. chimneys and silos) and cylinders having their axis parallel to the ground plane (that is. sloping or domical.
C6.C. such as those with h/D greater than 5 may experience aerodynamic effects in the along-wind as well as across-wind direction. the value of external pressure coefficients given in Table 13 may be used provided that the Reynolds number is greater than 10. or due to wind where there is a degree of permeability to allow entry to the wind.000. For open-ended cylinders. internal pressure may also occur on the roof of a container.
1 -0.8 -0.6 -0.2.9 -0.5 -0.Code & Commentary IS 875 (Part 3)
External pressure distribution coefficients around cylindrical structures [Clause 6.7 -0.7 -2.8 -1.0 0.0
IITK-GSDMA-Wind04-V3.0 0.6 -1. 7] D
Position of Periphery θ (degrees) 0 15 30 45 60 75 90 105 120 135 150 165 180 Pressure Coefficients.5 -2.4 -0.6 -0.3.2 -0.2 -1.7 -1.1 -0.9 -0.5 h/D = 1 1.2 -2. Cpe h/D = 25 1.9 -1.7 -0.8 0.7 -0.1 -0.0
.6 h/D = 7 1.2 -1.8 0.4 -0.0 0.5 -0.6 -1.4
IITK-GSDMA-Wind02-V5.5 -0.9 -2.7 -1.6 -0.8 0.
5 1.Code & Commentary IS 875 (Part 3)
External pressure coefficients for roofs and bottoms of cylindrical buildings [Clause 6.3.00 1.75 Bottom -0.0
IITK-GSDMA-Wind04-V3.75 -0.00 Roof -0. Cpe Roof shape / Bottom of Elevated Structure a.65 -1.00 -1.1 D
Coefficient of External Pressure.75 -0.00 2.0
IITK-GSDMA-Wind02-V5.50 d Roof -0.7 -0. 8] P
P e = 0.25 1.1 D Cpe Cpe e = 0.00 (Z/H)-1 1.8 -0. b and c H/D 0.
See Table 20) Figure 5 : External pressure coefficient on the upper conical roof surface of a circular structure standing on ground.8]
IITK-GSDMA-Wind02-V5.2D < h < 3.2.Code & Commentary IS 875 (Part 3) 1.0 PLAN Cpe = -0.5 1.2 (α < 11.0
IITK-GSDMA-Wind04-V3.2D for 0.15h + 0.0
.D tan α < 0.5 a Cpe = -1.5o) D SECTION AA h α
45o Wind A 0.2a Cpe = -1.5 a 0.3.0 0.2 < (h/D) < 2
(For Force Coefficient Corresponding to Shell Portion.5
a = 0.[Clause 6.5D for 2< (h/D)<3 = 0.
0.9 a e
b Cpe = 2(h1/h2)-2.5 3.2 Direction 1 From the Diagram Cpe = -0.3.5
Cpe = (h1/h2)-1. (h1/h2) > 1.3.2.6
.2 See 6.8
1.3.0.2.0
Values of Cpe 6.0.0 3.1.9] 0.0
IITK-GSDMA-Wind04-V3.5.13
.0.0 2.7.1.6
. Cpe for combined roofs.4 Direction 2
IITK-GSDMA-Wind02-V5.8 2.2 0.5 -0. (h1/h2) ≤ 1. [Clause 6.0 1.Code & Commentary IS 875 (Part 3)
External pressure coefficients.1.7 Cpe = +0.1 Portion a b c and d e 6.1.0.6
.7 See 6.4
0.5 1.2.7
.4 (h1/h2 )-0.
+0.Code & Commentary IS 875 (Part 3)
Table 16: External pressure coefficients.5 h2 h1
b1 > b2 Portion Cpe a -0.8 b -0.6 b +0. –[Clause 6.5
IITK-GSDMA-Wind04-V3.7
IITK-GSDMA-Wind02-V5.10 ]
Wind -0. Cpe for roofs with a skylight.6 -0.6 -0.3.2.0
The frame will experience the integrated effect of these different force components.3.The pressure coefficients on the roof (top and bottom) and rear wall of a typical grandstand roof.13–
Frames – When taking wind loads on frames of clad buildings it is reasonable to assume that the pressures or suctions over the entire structure will not be fully correlated. namely the walls or the roof slopes are given in the Code. is given in Table 17.0.5 for assigning pressure coefficients on roof slopes and on overhangs respectively.9 – Combined roofs . For analysing a frame.2. both open and closed.The average external pressure coefficients for roofs with skylight are shown in Table 16.10 – Roofs with skylight .Code & Commentary IS 875 (Part 3)
6.12–
Spheres – The external pressure coefficients for spheres shall be as given in Table 18. caused by wind which is characterized by randomness. These values. Forces on account of internal pressures will also be included. as explained earlier.8 and 1. reactions from these different parts will be accounted for.3.2.3.
C6. which is open on three sides. The factor varies between 0. The pressure coefficients are valid for a particular ratio of dimensions as specified in Table 17 but may be used for deviations up to 20 percent. the forces obtained in the frame may be reduced as per values of Kc given in Table 19.11 –
6.11 Grandstands .
C6.13 –
Pressure coefficients on different faces of a structure. In general.2.3.The average
C6.3.12 –
The clause gives pressure coefficients and the corresponding figures/tables are self explanatory.2.2.2.
C6. causing positive pressure under the roof and negative pressure on the roof.9 –
This clause deals with a situation often found in practice – that of a pitched roof with a porch (or a car park).2.0
.3. the maximum wind load occurs when the wind is blowing into the open front of the stand.2. Thus the forces from different components are going to be only partly correlated.2.
C6.2.3. A reduction factor on the computed responses of the frame is thus being permitted. Two principal wind directions are covered for the building with varying geometrical proportions.3.3.3.3. The clause takes recourse to the use of clauses 6.3. Therefore when taking the combined effect of wind loads on the frame.
6.2 and 6.2.2. are obtained by averaging values of measured pressures in different parts of the structure.0
2) AC EG θ 0.6 +0.6
F +0.9 +0.3
K -0.0
.5 -0.3 -1.6
B +0.0
IITK-GSDMA-Wind04-V3.4 h B
Front and Back of Wall J +0.0 -0.0 -1.9 -0.5 -0.1 -0.9 +0.9
L +0.7 -0.9
A -1.9 -1.4 -1.3.2.5 -0.Code & Commentary IS 875 (Part 3)
Pressure coefficients at top and bottom of roof of grandstands open on three sides (roof angle up to 5o) [Clause 6.4 -1.9 -0.4 +0.8 : 1 : 2.7 -0.4 -0.4 +0.9 +0.0 -0.0 -0.9 +0.3
E -0.0 -0.6 +0.3
M -0.7 -0.1 -0.1 -0.7 -0.5 -0.3
IITK-GSDMA-Wind02-V5.3
G -0.11] (h:b:l = 0.6
D +0.9 +0.6
H +0.9 +0.8 -1.3
C -1.0 -0.8 -1.
1 -1.0 +0.2 for DVz ≥ 7 where D is the diameter of the sphere
IITK-GSDMA-Wind02-V5.5 for DVz < 7 = 0.2 +0.5 -0.7 -1.0 -0.2 -1.0
.9 +0.6 -0.12]
Position of Periphery.3 +0. θ in Degrees 0 15 30 45 60 75 90 105 120 135 150 165 180
+1.1 -0.4
Cf = 0.Code & Commentary IS 875 (Part 3)
External pressure distribution coefficients around spherical structures [Clause 6.2.1 +0.3.0
NOTE: The action combination factors less than 1.3.8
Combination factors for wind pressure contributed from two or more building surfaces to effects on major structural elements [Clause 6.0
IITK-GSDMA-Wind04-V3.2. Pressures from windward and leeward walls in combination with positive or negative roof pressures
0.0 can be justified because wind pressures are highly fluctuating and do not occur simultaneously on all building surfaces.
In other words.2. When multiplied by the area over which the pressure is acting. For rectangular clad buildings. as relevant.3. The value of Cf′ has the following values: Cf′ = 0. Such a force can be obtained on a clad building. the total wind load should be calculated for each wind direction.1 –
The flow of wind around/over a structure. F = Cf Ae pd where F is the force acting in a direction specified in the respective tables and Cf is the force coefficient for the building.
6. and If h > b. F′. cladding. as in this clause. NOTE 3 –In tapered buildings/structures. This results in a frictional force in the direction of wind. in a direction normal to it. the distribution of pressure (and hence the local pressure coefficient) is essentially required for designing the fasteners. there is friction between the surface of the structure and the wind flowing over it.
Cf′ = 0. in the direction of wind is given by the following formulae: If h ≤ b. and when multiplied by the effective frontal area Ae of the building or structure and by design wind pressure pd. and then summing up the forces over these small areas.3 An obstruction to the flow of wind by an object results in creating a pressure on the surface of the object. causes surface pressures.3.0
IITK-GSDMA-Wind04-V3. IITK-GSDMA-Wind02-V5. and its support system.02 for surfaces with corrugations or ribs across the wind direction. or the structure as a whole is obtained as an integration of the term ‘pressure × area’. gives the total wind load on that particular building or structure. while that in a direction perpendicular to it is called ‘lift’.0
. a force results. In deducing the force coefficient the direction of the force has to be specified.1 – Frictional Drag:
A force due to frictional drag shall be taken into account in addition to those loads specified in 6. F′ = Cf′ (d – 4b) bpd + Cf′ (d – 4b) 2 hpd The first term in each case gives the drag on the roof and the second on the walls. Whereas the use of the force coefficients as given will only help in determining the overall force system on the structure and its foundation in order to design the framework or to compute stability. this addition is necessary only where the ratio d/h or d/b is greater than 4.01 for smooth surfaces without corrugations or ribs across the wind direction. or an unclad building or its components and can be expressed in terms of a force coefficient.
NOTE 1 –The value of the force coefficient differs for the wind acting on different faces of a building or structure. a force over an element of a structure. the surface area of the building/structure may be sub-divided so that specified pressures are taken over appropriate areas. NOTE 2 – If surface design pressure varies with height. Since the pressure over a surface varies. F′ = Cf′ (d .Code & Commentary IS 875 (Part 3)
6. as mentioned already.4h) bpd + Cf′ (d . A force taken to act in the direction of the wind is called ‘drag’.
C6.3 – Force Coefficients
The value of force coefficients apply to a building or structure as a whole. the force coefficients shall be applied after subdividing the building/structure into suitable number of strips and the load on each strip calculated individually. In order to determine the critical load. In addition. This clause specifies the frictional drag for a rectangular clad building.4h) 2 hpd. taking the area of each strip as Ae. The frictional drag force. the computation of force over an area can be done by dividing the surface into small tributary areas.
for rounded edges. The wind load on appurtenances and supports for hoardings shall be accounted for separately by using the appropriate net pressure coefficients.
For other buildings. both with sharp edges as well as rounded corners.3. the design shall also be checked for net pressure normal to the surface varying linearly from a maximum of 1. For cases where the edges are sharp.
6. To allow for oblique winds.Code & Commentary IS 875 (Part 3)
Cf′ = 0.
6. However. in the values of pressure coefficients and force coefficients. noticing the lack of sensitivity to the height/breadth ratio. Reynolds number has a marked effect.2 – Free standing walls and hoardings
Force coefficients for free standing walls and hoardings shall be as given in Table 21 . However. 6 and for other clad buildings of uniform section (without projections.
IITK-GSDMA-Wind02-V5. the effect of frictional drag has been indicated where necessary. This has been accounted for in Table 20 by specifying the applicable range of Vzb.3. the Reynolds number has only a limited influence on the wind pressures or forces.0
IITK-GSDMA-Wind04-V3.2.1 – 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.3. Table-20 from the existing Code has been consolidated.04 for surfaces with ribs across the wind direction.0
. except where otherwise shown) shall be as given in Table 20. Allowance shall be made for shielding effects of one element on another. or for shapes which are circular or near circular.2 – Force Coefficients for Clad Buildings
C6.2 –
Clad rectangular buildings with different proportions are covered in this clause in addition to buildings of a variety of other shapes.
6. or nearly so.7 Cf at the windward edge to 0.44 Cf at the leeward edge.2.3. The force values are also a function of the aspect ratio.
C 6. held in free space. wind would escape at the two ends of the member. 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.3. i. For example. and give the forces normal and transverse. the ratio l/b shall be taken as infinity for the purpose of determining K. Normal force. respectively.3 – Force Coefficients for Unclad Buildings and Frameworks
This section applies to frameworks of unclad buildings and to frameworks of buildings while temporarily unclad. then the ratio of l/b shall be doubled for the purpose of determining K. Table 24 provides the coefficients for flat–sided members. with reduction factor becoming 1. The angle of wind incidence can also affect the coefficients. if a member is connected into plates at the ends.
(c) Circular sections – Force coefficients for members of circular section shall be as given in Table 20. and ii.2. Thus there is a reduction in the overall wind force acting on the member. in which Reynolds number will not have an influence.3. for which Reynolds number will have a marked influence. Reynolds number effect and shielding amongst members.
IITK-GSDMA-Wind02-V5. Table 23 gives the required values of K.0
IITK-GSDMA-Wind04-V3.3. the end conditions play a role. aspect ratio. to the reference plane as shown in Table 24.The force coefficients for solid circular shapes mounted on a surface shall be as given in Table 22 . They are denoted by Cfn and Cft. Transverse force. The shorter the member the greater is this reduction.3 –
Force coefficients in this section are given for skeletal frameworks or individual elements.
6. Table 25 likewise gives the values for wires and cables.1 – Individual members (a) The force coefficients (Table 24) refer to members of infinite length. the design wind speed (Vz) and the surface roughness. For members of finite length.1 –
For a member of finite length. which imply considerations of their shape. When both ends of a member are so obstructed.
6. Fn = Cfn pdK l b Ft = Cft pdK l b
C6.Code & Commentary IS 875 (Part 3)
6. In reckoning the length of the member. its length is to be treated as infinite.3. Where any member abuts onto a plate or wall in such a way that free flow of air around that end of the member is prevented. (d) Force coefficients for wires and cables shall be as given in Table 25 according to the diameter (D).3 – Solid circular shapes mounted on
a surface .0. Table 23 gives the reduction factors.3.0
. (b) Flat-sided members – Force coefficients for wind normal to the longitudinal axis of flatsided structural members shall be as given in Table 24.3. Wind forces will also be influenced by the surface roughness.3. The force coefficients are given for two mutually perpendicular directions relative to a reference axis on the structural member. The following special cases must be noted while estimating K.
.2.5 1.5 1.0 1.0 2.3.5 3.0 F = Cf pd bh 3 1 h F 20 10 5 Plan Wind b a F
6A Values of Cf versus a/b for h/b ≥ 1 1.5 a/b 2.4 1.0 1.5 0 Elevation 0.Code & Commentary IS 875 (Part 3)
IITK-GSDMA-Wind02-V5.2 Cf 1.5 1.5 3.8 1/4 1/2 1/2 0 0.0 h/b = ∞ 2.0 1.5 a/b 2.0 0.5 Cf 2.0 1/4 h/b = 1/4 h/b = 1/2
Force coefficients for rectangular clad buildings in uniform flow [Clause 6.0 2.0
0.1 1.2 1.0
≥ 10 0.7
≥8 0.2 1.4 0.0
≥4 0.6
≥ 10 0.2 0.6
<6 ≥6 ≥6 0.5 0.Code & Commentary IS 875 (Part 3)
Force coefficients Cf for clad buildings of uniform section (acting in the direction of wind) [Clause 6.5
0.7 0.9 1.3.9 1.6 0.5
2 FOR VzD ≥ 7
Reduction factor K for individual members.80 10 0.00 1. [Clause 6. subcritical flow Circular cylinder.95
1. [Clause 6.82 20 0.0
.98 50 0.87 0.4
0.68 0.1 (a)] 2 0.2
1.74 0.80 5 0.00
l /b or l /D Circular cylinder.Code & Commentary IS 875 (Part 3)
Force coefficients for solid shapes mounted on a surface.3.5 FOR VzD < 7 0.66
0.3.3.90
0.90 40 0.0
IITK-GSDMA-Wind04-V3.81
IITK-GSDMA-Wind02-V5.82 0.4
1.99 100 0.87
0.98 1.62 0.00 ∞ 1.69
0. supercritical flow (DVz≥ 6 m2/s) Flat plate perpendicular to wind (DVz ≥ 6 m2/s)
0.1 -1.5 +1.1 +0.75 +0.05 +1.6 +1.0 +1.9 +1.5 -0.8 -2.7 Cft 0 +0.Code & Commentary IS 875 (Part 3)
Force coefficients (Cf) for individual structural members of infinite length [Clause 6.5 -1.05 +1.1 b
Cfn +1.5 0
Cft 0 +1.75 -0.6 +1.55 +2.4 +1.6 +2.0
Cft +0.5
Cft 0 +0.0 -1.9 -2.7 -0.85 +1.0
IITK-GSDMA-Wind04-V3.45 b
0.4 ±2.55 0
Cft 0 +1.85 0 -1.75 +0.0 +0.5 b
Cfn +2.8 +2.43 b
Cfn +1.0 -1.8 0
Cft 0 +0.6 -1.8 +1.1 Cfn +2.8 +2.1 -1.3.3.1
Cfn +2.6
0.1 +0.48 b
Cfn +1.05 0 Ft Cfn +2.1 +0.2 -1.4 0
Cft 0 +0.6 +0.95 -0.1(b)]
0.95 +0.9
Cfn +1.8 -1.8 +1.4
Cfn +1.6 +0.
IITK-GSDMA-Wind02-V5.1 b
Fn 0o b Fn 0o 1.9
Cfn +2.15 +2.1 +0. in relation to effective frontal area Ae.9 +2.3 -1.85 +0. the force coefficient Cf is given in relation to the dimension b and not.0 +1.1
Cfn +1.0
NOTE: In this table.1 b Cft 0 -0.75
Cft +0.0 +1.2 0
Cft 0 +1.7 +1. as in other cases.95 +0.1b
0.75 -1.6 +0.4 Ft b
Cft +1.1 -0.
2 0.3.3.
shall be as given in Table 26 according to the type of the member. the diameter (D).1 and 0. or DVz greater than 6 m2/s
C 6. [Clause 6.2 – Single frames .
IITK-GSDMA-Wind02-V5. The solidity ratio implies the ratio of net exposed area of the frame members divided by the gross area bound by these members.3. Where a frame consists of both flat sided members and members of circular cross-section.1(d)] Force Coefficients.Force coefficients
for a single frame having either: (a) all flat sided members.3.0
.2 0.3. C6.9 ---
Thick Stranded Cables 1. See Fig. or (b) all circular members in which all the members of the frame have either: i) ii) DVz less than 6 m2/s.4. Cf for
Smooth Surface --1.Code & Commentary IS 875 (Part 3)
Force coefficients for wires and cables (l/d =100).0
IITK-GSDMA-Wind04-V3.3.1 ---
DVz < 0.3.5
Moderately Smooth Wire (Galvanized or Painted) --1. The solidity ratio of the frame also affects the value of Cf. the latter being influenced by the flow regime.2 0.3.3. C6
d Fig.7
Fine Stranded Cables 1. the design wind speed (Vz) and the solidity ratio (φ).6 m2/s DVz < 6 m /s DVz ≥ 6 m /s
Force coefficients for single frames are given for flat sided members or members of circular section.6 m2/s DVz ≥ 0.3 1. For lattice frames see 6. the coefficients for these respective shapes may be taken from Table 26.
For Circular Members Flat-sided members 1.0 Supercritical flow (DVz ≥ 6 m2/s) 0.3.7 0.3 0.1 1.7 1.7 1. Cf sub = force coefficient for subcritical circular members as given in Table 26.0
IITK-GSDMA-Wind04-V3.4 0.0
.3 – Multiple frame buildings .6 2.4 2.5 0.3.2 1.1 0.2 1.
where Cf super = force coefficient for the supercritical circular members as given in Table 26.75 1.6 1. Acirc sub = effective area of subcritical circular members.8 0.3 –
During the construction of a clad building.0
0.5 2. Cf flat = force coefficient for the flat sided members as given in Table 26.9 1.Code & Commentary IS 875 (Part 3)
Force coefficients for single frames [Clause 6.00
NOTE: Linear interpolation between the values is permitted.2] Force Coefficients.8 0.0 Sub-critical flow (DVz < 6 m2/s) 1.2 0.3. a situation will often occur when the framework will still be unclad (This may occur for a structure
IITK-GSDMA-Wind02-V5.3.2 1.3.8 1.3.8 1. and
6.This section applies to structures having two or more parallel frames where the windward frames may have a shielding effect upon the
C6.1 1.8 0. Aflat = effective area of flat-sided members. Asub = Acirc sub + A flat.
0 0. The windward frame and any unshielded parts of other frames shall be calculated in accordance with 6.3. beam or girder measured at right angles to the direction of the wind.2 0.8 0.0 1.7 0.0 1. centre to centre of the frames.0
NOTE: Linear interpolation between values is permitted.
Where there are more than two frames of similar geometry and spacing.9 0.5 0. as envisaged in this clause.6 0.3 0.0 1. If there are multiple frames.2.5 1.0 1.7 1.0 0.8 0.3.4 0.0 1. The loads on the various frames shall be added to obtain total load on the structure.3 0. one frame may shield the other.0 1.0 1. or parts thereof.8 0.3. is clearly explained in the Clause. For such unclad frames.0 0. 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.0 1.8 > 8.6 0.0 1. force coefficients can be taken as in 6. For triangular framed structures or
IITK-GSDMA-Wind02-V5.0 1.3.0 1.0 1. The manner of accounting for this shielding.6 2.9 0.3 1.6 0. the wind load on the third and subsequent frames should be taken as equal to that on the second frame.0 1.Code & Commentary IS 875 (Part 3)
frames to leeward side.6 4.
or a part of it even permanently).2.9 0.0 1.3.0 1.0 1.7 0.0
IITK-GSDMA-Wind04-V3.9 0. the types of members comprising the frame and the spacing ratio of the frames. and its extent.0 1.0 1.0
. and as will occur commonly.0 0.3.5 0.0
Shielding factor η for multiple frames [Clause 6. β 0 0.0 1. The values of the shielding factors are given in Table 27.0 1. a) The frame spacing ratio is equal to the distance.3] Frame Spacing Ratio <0.
Table 27: Effective Solidity Ratio.1 0.0 1. beams or girders divided by the least overall dimension of the frame.
3. (a).4 0.3. The frontal area exposed to the wind as well as the solidity ratio therefore goes on changing.25 0.0
IITK-GSDMA-Wind04-V3.3.3.4 – Lattice towers a) Force coefficient for lattice towers of square or equilateral triangle section with flatsided members for wind blowing against any face shall be as given in Table 28.3 0.2 0. β:
β = φ for flat-sided members.5 0. Such towers often taper from the base towards the top.35
Effective solidity Ratio.1 0.3.40 0. β
0.3(b)]
Solidity Ratio.0
.10 0. For square based towers. (b) and (c) deal with towers with flat sided members.4 –
The clause provides for force coefficients for square as well as triangular based latticed towers with flat-sided or circular sections used for the members. 7 for members of circular crosssections. Former is critical for the design of bracings while the latter for tower legs.Code & Commentary IS 875 (Part 3)
rectangular framed structures diagonal to the wind.20 0.6 0.30 0. φ
Figure 7: Effective solidity ratio.7 0.3. the spacing ratio should be calculated from the mean distance between the frames in the direction of the wind. while (d) and (e) address towers with circular members. It may thus become necessary to divide the tower into several smaller parts along the height and compute forces on each part separately.15 0. it is pertinent to distinguish between ‘wind onto face’ or ‘onto corner’. β for round section members [Clause 6.05
β is to be obtained from Fig.
IITK-GSDMA-Wind02-V5. b) Effective solidity ratio.
2 times the load for the wind blowing against a face.4 2.3.0
.3 0.4(a)]
Force Coefficient for Square Towers 3.2 1.3. shall be taken as 1.Code & Commentary IS 875 (Part 3)
Table 28: 6. may be as given in Table 29.9 1.1 0.1 Equilateral Triangular Towers 3.1 Supercritical flow (DVz ≥ 6 m2/s) Onto face 1.6
IITK-GSDMA-Wind02-V5.3 2. all in the same flow regime.3.1 1.9 Onto corner 2. 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.0
IITK-GSDMA-Wind04-V3.2 1.2 0.5 2.3.3 1. which occurs when the wind blows into a corner.1 0.3 2.7 2.4 (d)] Solidity Ratio of Front Face Force Coefficient for Sub-critical flow (DVz < 6 m2/s) Onto face 0.8 3.2 1. Force coefficients for lattice towers of equilateral-triangle section with circular members all in the same flow regime may be as given in Table 30.3 2. Force coefficients for lattice towers of square section with circular members.4 0.8 2.5
0.05 0.2 2.3 Onto corner 1.3 1.
Table 29: Overall force coefficient for square towers composed of rounded members [Clause 6.1 2.5 b)
For square lattice towers with flat-sided members the maximum load.
9 1. etc.3.1 1.4 1.9 1.9 1.6
Overall force coefficient for equilateral-triangular towers composed of rounded members [Clause 6.0
IITK-GSDMA-Wind04-V3.1 0.8 1.5 1.Code & Commentary IS 875 (Part 3) 0.7 1.
IITK-GSDMA-Wind02-V5. lights.6 1. conduits. such as ladders.4 1.2
Supercritical Flow (DVz ≥ 6 m2/s) 0.6 1.7 1.3.8 0.5 – Tower Appurtenances -The wind loading on tower appurtenances.4 (e)]
Force Coefficient for Subcritical Flow (DVz < 6 m /s) 1.1 1.4 1.0
.5 1.4 0.5
6.3.3.6 1. Allowance may be made for shielding effect from other elements.3 0.5 1.6 1.4 0. elevators.3 0.1 1.2 0. shall be calculated using appropriate net pressure coefficients for these elements.8 1.05 0.
1 . take IF as unity) in such cases or take specialist advice.2 – Roof of Low-rise Buildings
Maximum increase in wind force on the roof due to interference from similar buildings in case of closely spaced low-rise buildings may be upto 25% for distance (x) between the buildings of 5 times the dimension (b) of the interfering building normal to the direction of wind. which has made generalization and codification of interference effect difficult. large size eddies are formed and released at more or less regular intervals. As a result. layout of interfering and interfered structures defined by their relative distances and orientations which could be innumerable). there could be some shielding effect when the two buildings are too close (x < 0. the fluctuating speed component of the wind gets significantly enhanced on account of the high turbulence so generated. To account for the effect of interference. wind angle of attack and Strouhal Number/Reduced velocity. specialist literature be consulted or a wind tunnel study carried out.. In tall structures. While the mean speed may reduce (shielding effect). some guidance can be provided for the purpose of preliminary design. The modifications in the flow affect the small height (rigid) structures differently than the flexible tall structures. though there can also be some shielding effect between two very closely spaced buildings/structures.
7. stand-alone condition). The designer is adviced that for assigning values of IF for final design particularly for tall buildings.0
IITK-GSDMA-Wind04-V3. though much research has been done on the subject and a good amount of literature is available on the same.
C7.Code & Commentary IS 875 (Part 3)
CODE 7.2 –
As explained above. the given values of IF are a kind of median values and are meant only for preliminary design estimates. Interference is governed by a large number of geometrical parameters (shape and size of various interfering structures vis-à-vis those of the interfered structure. and flow parameters (wind direction. This non-dimensional term is called Interference Factor (IF). the oncoming wind characteristics may get substantially modified due to formation of a highly turbulent wake zone on the downstream side of the interfering structure(s). Since the values of IF can vary considerably based on building geometry and location.5 b) to each other. which can only be ascertained by detailed wind tunnel/CFD studies. Besides. the wind pressures usually get enhanced. besides Reynolds number in case of rounded bodies). The actual phenomenon is too complex to justify generalization of the wind forces/pressures produced due to interference. However.1 When a structure is surrounded by other structures of similar size. The
IITK-GSDMA-Wind02-V5. and. narrow but unstable shear zones are created at the wake boundaries. a wind interference factor (IF) has been introduced as a multiplying factor to be applied to the design wind pressure/force. – INTERFERENCE EFFECTS
7.e. It is usual to express the effect of interference in terms of the ratio of the modified pressure/force due to interference and the wind pressure/force without any interference (i.0
. The mean pressures over the short rigid structures may often reduce (shielding) while the fluctuating ones may get enhanced. Interference effects can be more significant for tall buildings.e. Such situations may arise in rowhousings or group/mass housing schemes.General
Wind interference is caused by modification in the wind characteristics produced by the obstruction caused by an object or a structure in the path of the wind. If such wind strikes another structure.. the fluctuating along-wind response (dynamic component) and the across-wind response quite commonly get enhanced. whereas the mean along-wind response may often reduce. It would be conservative to neglect shielding (i.
25 IF = 1.0 to 1.00 IF = 1. Use of interference factors will be of greater relevance for buildings in terrain categories 1 and 2.0
IITK-GSDMA-Wind04-V3.10 IF = 1.25 considering worst wind direction.2]
IITK-GSDMA-Wind02-V5. linear interpolation may be used.
Code recommends IF values in the range of 1. Figure C7 shows how the value of IF varies for a typical case.00
Figure 8 : Interference factor (IF) for roof of low buildings [Clause 7. For intermediate spacings.5 b 2b ≤ x ≤ 5b x = 10 b x = 20b IF = 1.Code & Commentary IS 875 (Part 3)
Interference effect beyond 20b may be considered to be negligible (Figure 8).
x x ≤ 0.0
C 7 : Typical Interference factor contours for design pressure coefficients over the roof of a Gabled building (TB) as a building of same size is placed at different positions in plan (Kwatra 2000)
Zone IF Z1 1.25 Z3 1. 9 gives various zones of interference. There is a decrease in the interference effect as the height of the interfering structure(s) becomes smaller than the affected structure/building. Fig.0 sec) would respond to wind dynamically not only in the direction of wind (along-wind or drag) but also normal to the flow direction (across-wind or lift). The values may vary widely depending upon the interf-se location of the buildings / structures involved. Interfering structures sometime bring the vortex shedding frequency close to the natural frequency of the structure while in other situation they may push it away from the natural frequency.3 Tall flexible buildings (Time period of vibration. Interference could increase the dynamic response substantially in either directions.0
.60 Z2 1.15 Z4 1. Interference effects are primarily due to modifications in the incident and wake flow characteristics. The turbulence characteristics (like eddy sizes. The complex phenomenon of interference due to several upstream or downstream structures has to be studied through the wind tunnel studies as generalization is difficult if at all possible.) of he wind approaching the building face and then releasing from the sides get affected.07
C7. The values of IF provided in the Code are for guidance only and for assigning values of IF for final design specialist literature be consulted or wind tunnel model study carried out. The wind interference factor (IF) for preliminary estimate of the wind forces may be assumed as follows for interference caused by a tall building of same or greater height. T > 1.
Figure 9 : Interference zones for tall rectangular buildings of same or greater height [Clause 7.0
IITK-GSDMA-Wind04-V3.3 – Tall Buildings
Based on studies on tall rectangular buildings. etc.3]
IITK-GSDMA-Wind02-V5. becoming insignificant for height of the interfering structure(s) smaller than one-third the height of the building under interference. linear interpolation may be used between one–third and full height.
The interference effect due to buildings of height less than one-third of the height of the building under consideration may be considered to be negligible while for interference from a building of intermediate height.Code & Commentary IS 875 (Part 3)
7. generally to different extent as one might expect.
This force due to regular shedding of the eddies was first observed by Von Karman. if necessary. However.1 –
Section 8 of the Code contains methods of evaluating the dynamic effects of wind on flexible structures that can oscillate in the wind. Furthermore. NOTE 2 – If preliminary studies indicate that windinduced oscillations are likely to be significant.Code & Commentary IS 875 (Part 3)
CODE 8. .0
.09H d
H = total height of the main structure of the building in meters. (b) Resonant vibrations of a structure. In general. The frequencies away from the natural frequencies of vibration of a structure (about ± 20% on either sides) have relatively very small dynamic effect and the
IITK-GSDMA-Wind02-V5. The dynamic response factor for rigid structures is reported to be less than 0.
0. a flexible structure would tend to oscillate due to shedding of the eddies alternately from either sides of the structure at regular intervals.90. The fluctuating wind pressures are random in nature and have a wide range of frequencies. T may be determined as follows for multi-storied buildings: (a) For moment resistant frames without bracings or shear walls for resisting the lateral loads T = 0.0. all grouped into a non-dimensional parameter called Strouhal Number. or b) Buildings and closed structures with natural frequency in the first mode less than 1.0
IITK-GSDMA-Wind04-V3. flexible structures also respond in the across-wind direction on account of vortex shedding. the dynamic response factor Cdyn may be taken as 0. the following guidelines may be used for examining the problems of windinduced oscillations: a) Buildings and closed structures with a height to minimum lateral dimension ratio of more than about 5. The dynamic part of the wind pressures would set up oscillations in a flexible structure.1 .
NOTE 1 – The fundamental time period (T) may either be established by experimental observations on similar buildings or calculated by any rational method of analysis. thus imposing a dynamic force that has a major component in a direction normal to that of the wind (lift) and only a small component along the direction of wind (drag). For buildings and closed structures with natural frequency in the first mode more than 1 Hz.85 (ASCE 7-02).
(c) Vortex shedding forces acting mainly in a direction normal to the direction of wind causing across-wind as well as torsional response. Oscillations will thus be caused in the along-wind direction. In the cross-wind direction. and d = maximum base dimension of building in meters in a direction parallel to the applied wind force. The wind on earth’s surface is turbulent in nature that gives rise to randomly varying wind pressures about a certain value associated with the mean wind speed.General
Flexible slender structures and structural elements shall be investigated to ascertain the importance of wind induced oscillations or excitations along and across the direction of wind.0 Hz. and (b) For all others
C 8.1 n where n = number of storeys including basement storeys.DYNAMIC EFFECTS
8. shape and wind speed.0 second. The dynamic response induced by the wind can be attributed to the following actions of wind: (a) Non-correlation of the fluctuating alongwind pressures over the height and width of a structure. which may be defined as one having the fundamental time period of vibration more than 1. Any building or structure which satisfies either of the above two criteria shall be examined for dynamic effects of wind. Structures which are relatively stiff are not dynamically sensitive to wind. The frequency of eddy shedding is dependent on structure size. investigations should be pursued with the aid of analytical methods or. a somewhat conservative value is recommended in this revised draft. In the absence of such data. by means of wind tunnel tests on models. The IS:875(Part 3) – 1987 Code does not lay down any specific procedure for determining the design wind force related to the cross-wind motion.
force. polygons. The dynamic along-wind response of a structure comprises of a non-resonant component and a resonant component. NOTE 6 – The designer shall also be aware of the following three forms of wind induced motion which are characterized by increasing amplitude of oscillation with the increase of wind speed. etc. torsional flutter. such as triangular. At the same time. taken individually. by the following expression:
where xpk. Long span suspension bridge decks or any member of a structure with large values of d/t ( where d is the length of the member and t is its dimension parallel to wind stream) are prone to low speed flutter. are close to each other (ratio being typically less than 2.5 and 4. needing calculation models that are based on spectra generated from wind-tunnel studies.
b) Flutter . It is characterized by the progressively increasing amplitude of transverse vibration with increase of wind speed. The latter can be determined by applying the theory of distribution of random variables and expressed in terms of standard deviation. Flutter can set in at wind speeds much less than those required for exciting the individual modes of motion. NOTE 5 – The eddies shed from an upstream body may intensify motion in the direction of the wind. Perhaps the most common form is oscillatory motion due to combined bending and torsion. the contribution from higher modes of vibration being rarely significant. c) Ovalling – Thin walled structures with open ends at one or both ends such as oil storage tanks and natural draught cooling towers. and T-sections. Wind tunnel testing is required to determine critical flutter speeds and the likely structural response. These cross-wind motions may become critical in the design of tall buildings/structures. Although oscillatory motion in each degree of freedom may be damped. Thus the total along-wind response (deflection. Other types of flutter are single degree of freedom stall flutter. a) Galloping – Galloping is transverse oscillations of some structures due to the development of aerodynamic forces which are in phase with the motion. as well as angles. The across-wind response. also called as the root mean square (rms). instability can set in due to energy transfer from one mode of oscillation to another. The crosssections which are particularly prone to this type of excitation include the following: i) All structures with non-circular crosssections. negative aerodynamic damping or due to a combination of these effects. ⎯x and σ are the peak. etc. has zero mean and involves a different mechanism of excitation (vortex shedding) and is more structure specific.0. vortex shedding). This is particularly true of tall buildings and towers. and may also affect crosswind motion. so that the higher frequency modes of a structure would be subjected to lower excitation forces. Such energy transfer takes place when the natural frequencies of modes. cables and cables with ice
associated wind pressures are almost static in nature while those in the narrow bands around the natural frequencies of vibration of the structure produce a large response that is essentially dynamic and limited only by damping in the system. on which is superimposed a static component due to the mean wind speed. ii) Twisted encrustations. on the other hand. The excitation depends on gust energy available at the resonant frequency. unsteady wake flow (for example. in which the ratio of the diameter of minimum lateral dimension to the
IITK-GSDMA-Wind02-V5. the lower frequency components of the wind speed and pressures have the greatest energy.0
NOTE 3 – Cross-wind motions may be due to lateral gustiness of the wind.) is obtained as the sum of ‘mean’ value and a ‘peak’ value. Thus. generally the major dynamic response of a flexible structure due to wind is confined only to the fundamental mode of vibration of the structure. crosses.0
IITK-GSDMA-Wind04-V3.Flutter is unstable oscillatory motion of a structure due to coupling between aerodynamic force and elastic deformation of the structure. NOTE 4 – Motions in the direction of wind (known also as buffeting) are caused by fluctuating wind force associated with gusts. value. and the structure is seen to execute sustained or divergent oscillations with a type of motion which is a combination of the individual modes of vibration.0). square. mean and standard deviation respectively of the variable x and g is called the peak factor having a value between 3.
Pirner. O. Strouhal number values for shapes often encountered are also given. Fishcer and J. Vz = design wind speed. Kolousek. Amsterdam.
Sr = Strouhal number. if the required information is not available either in the references of Note 8 or other literature. Scanlan.1 –
Expression for the frequency of vortex shedding by a structure / member has been given.. v) Wind Engineering – A Handbook for Structural Engineers by Henry Liu.0
IITK-GSDMA-Wind04-V3. New Jersey. 1991
vi) Wind Effects on Civil Engineering Structures (Part 2) by V. It is to be noted that wind induced oscillations may occur at wind speeds lower than the static design wind speed for the location. Van Nostrand Reinhold Co.3) NOTE 9 – In assessing wind loads due to such dynamic phenomenon as galloping.1 – Slender Structures
For a structure. Naprstek. Co.Motion due to Vortex Shedding
8.17724. Elsevier Science Pub. vii) Appropriate Indian Standards (See 1. John Wiley and Sons. Supplement to the National Building Code of Canada. 1980. Pergamon Press. NOTE 7 – Buildings and structures that may be subjected to serious wind excited oscillations require careful investigations. are prone to ovalling oscillations. Ottawa.Code & Commentary IS 875 (Part 3)
wall thickness is of the order of 100 or more. M.H. NRCC.1.
iv) Flow Induced Vibration by Robert D. and b = breadth of a structure or structural member normal to the wind direction as well as the axis of the structure/member.2. New York. including experiments on models in wind tunnels. Explanatory notes 1 to 4 give more information on the phenomenon and its effect. 1984. New York.0
IITK-GSDMA-Wind02-V5. 1990. Prentice Hall. NOTE 8 – Analytical methods for the response of dynamic structures to wind loading can be found in the following publications: i) ii) Wind Effects on Structures by E. flutter and ovalling. These oscillations are characterized by periodic radial deformation of the hollow structure. 1978.2 . Wind forces on structures by Peter Sachs. the eddy shedding frequency η shall be determined by the following formula: where
C 8. specialist advise shall be sought. No. 1996. Simiu and R. National Research Council of Canada. Blevins. 1980.
25 for bVz greater than 7. b) Rectangular Structures – For structures of rectangular crosssection: Sr = 0.0
IITK-GSDMA-Wind04-V3. Vortex shedding need not be considered if the ratio of length to maximum transverse dimension is less than 2.20 for bVz not greater than 7.1 (a) and (b) are valid for infinitely long cylindrical structures.
IITK-GSDMA-Wind02-V5.1.15 for all values of bVz. The value of S decreases slowly as the ratio of length to maximum transverse width decreases. if the structure is only three times higher than its width. NOTE 3 – Intensification of the effects of periodic vortex shedding has been reported in cases where two or more similar structures are located in close proximity. at less than 20 b apart. NOTE 4 – The formulae given in 8. and = 0.0
. In such cases.Code & Commentary IS 875 (Part 3)
a) Circular Structures – For structures circular in cross-section: Sr = 0. NOTE 2 – Unlined welded steel chimneys/stacks and similar structures are prone to excitations by vortex shedding. the reduction being up to about half the value.0.2. further analysis should be carried out on the basis of references given in Note 8 of 8.
NOTE 1 – Significant cross wind motions may be produced by vortex shedding if the natural frequency of the structure or structural element is equal to the frequency of the vortex shedding within the range of expected wind speeds. where b is the dimension of the structure normal to the wind. for example.
shear forces. For calculation of base bending moment. – Dynamic Response Factor
9. This approach is based on the stochastic response of an elastic structure acted upon by turbulent wind producing random pressures. The resonant response is insignificant for rigid structures (T < 1. deflections and acceleration at the top of the structure.0
. Ih is defined as the average level of fluctuations in the wind speed as a ratio of the mean wind speed.1 – Along Wind Load
For calculation of action effects (bending moments. The definition of dynamic response factor Cdyn has changed from that in the earlier Code (1987 edition) which was applied to the wind loading due to hourly mean wind speed. 10). The first term accounts for the quasi-static dynamic response below the natural frequency of vibration of the structure while the second term depends on the gust energy and aerodynamic admittance at the natural frequency of vibration as well as on the damping in the system. member forces) at a height s on the structure.1 –
To obtain the along-wind response of a flexible structure (time period > 1. the background factor Bs may be small resulting in reduced wind forces obtained from dynamic analysis as compared to the static analysis.
C 9. For flexible structures. one for the low frequency wind speed variations called the nonresonant or ‘background’ effects. the wind pressures on the structure at a height z shall be multiplied by dynamic response factor Cdyn.
IITK-GSDMA-Wind04-V3. This factor is dependent on both z and s and s < z < h (Fig.
Turbulence intensity. the design wind pressure pz has to be multiplied by the dynamic response factor Cdyn. and the other for resonance effects. against the 3-sec gust speed being used now. The structure is considered to vibrate in its fundamental mode of vibration. The dynamic response factor.0 sec). S. The equation for Cdyn contains two terms. Cdyn includes the effect of non-correlation of the peak pressures by defining a size reduction factor.0 Sec). It also accounts for the resonant and the nonresonant effects of the random wind forces.Code & Commentary IS 875 (Part 3)
CODE 9. a single value of Cdyn shall be used with s taken as zero.
= Dynamic response factor (= total load/ mean load). = wind pressure at height z obtained as 0. = force coefficient for the building.1]
Along-wind load on a structure on a strip area (Ae) at any height (z) is given by: Fz = Cf Ae pz Cdyn where Fz = along-wind equivalent static load on the structure at any height z corresponding to strip area Ae. caused by low frequency wind speed variations. which shall be taken as 3.Code & Commentary IS 875 (Part 3)
Figure 10 : Notation for heights [Clause 9.0
IITK-GSDMA-Wind04-V3. = effective frontal (strip) area considered for the structure at height z.5
= turbulence intensity.6 V z (N/m2). obtained from Table 31 by setting z equal to h = peak factor for the upwind velocity fluctuations. given as follows:
IITK-GSDMA-Wind02-V5. which is a measure of the slowly varying background component of the fluctuating response. and is given by:
2 ⎡ 2 H g SE ⎤ 1 + 2I h ⎢g v Bs + s R ⎥ β ⎦ ⎣ = (1 + 2gv Ih ) 0.5 = background factor.
h at the top of a tall x building in the along-wind direction shall be obtained from the following expression.0
= (π/4) times the spectrum of turbulence in the approaching wind stream. .1.1 –
Acceleration is assumed to be produced only by the resonant component of the response. hence only the second term under the square-root has been considered. IITK-GSDMA-Wind02-V5.1. given as follows:
= ratio of structural damping to critical damping of a structure. Also only the first mode response is IITK-GSDMA-Wind04-V3. as given in Table 32.0
The peak acceleration..25 = first mode natural frequency of vibration of a structure in the alongwind direction in Hertz = average breadth of the structure between heights 0 and h = reduced frequency = f0Lh [1+(gvIh)]/ Vh. = design wind speed at height h
9.1 – Acceleration
C9. = average breadth of the structure between heights s and h = measure of the integral turbulence length scale at height h = 100 (h/10)0.Code & Commentary IS 875 (Part 3)
β 0.248 0.233 0.162 0.171 0.140 0.128 0.210 0.183 0.152 0.
Table 31 : Height (z) m 10 15 20 30 40 50 75 100 150 200 250 300 400 500 Terrain category 1 0.150 0.058
Turbulence intensity (Iz) [Clause 9.1] Damping Coefficient.133 0.074 0.342 0.092 0.203 0.188 0.098 0.068 0.117 0.342 0.195 0.108 0.085 0.225 0. Thus the acceleration would also vary linearly with height
where m0 = average mass per unit height (kg/m).082 0.0
IITK-GSDMA-Wind04-V3.095 0.183 0.107 0.215 0.139 0..131 0.196 0.176 0.157 0.285 0.156 0.010 0.
assumed to dominate and it is assumed that the mode shape is linear.080 0. The acceleration at any height may be obtained by considering it to vary linearly with height.020 0.176 0.108 0.239 0.121 0.342 0.166 0.173 0.305 0.147 0.151 0.1] Terrain category 2 0.129 0.Code & Commentary IS 875 (Part 3)
.118 0.140 0.020
IITK-GSDMA-Wind02-V5.270 0.098 Terrain category 4 0.0
Damping coefficient [Clause 9.074 Terrain category 3 0.155 0.
This effect is important for canopy and similar roofs and bridge decks particularly with fast rate of change of the lift and moment coefficients with respect to the angle of attack.0
. All these excitations are also affected by turbulence in the wind. such as galloping. The latter is spread over a band of frequencies.0
IITK-GSDMA-Wind04-V3. flutter. – CROSS-WIND RESPONSE
10. Computation of these forces need wind tunnel studies and / or CFD analysis and are outside the scope of this Code. The magnitude of the across-wind force (also called as lift) and the pitching moment thus produced would depend not only on the turbulence level but also the mean wind speed and the angle of attack (angle of wind incidence in elevation).Code & Commentary IS 875 (Part 3)
CODE 10.1 Forces causing cross-wind response of tall structures are of three kinds. (a) Incident turbulence : Turbulence In the wind gives rise to fluctuations in wind speeds and directions which in turn produce forces varying with time. etc. Resonance would result when the frequency of eddy shedding matches the natural frequency of vibration of the structure. In case of tall structures the wind speed as well as turbulence vary with the height of structure. Calculation of cross– wind response is not required for lattice towers.
(c) Wake Excitation : It is the most common type of across-wind excitation and is caused by shedding of the vortices by a structure at regular intervals alternately from its two opposite sides. (b) Cross-wind displacement : Mechanisms that may get activated under the cross-wind displacements are of different nature. The periodicity of eddy shedding is defined by Strouhal Number that depends on the shape of cross-section of the structure.1 – General
This Section gives method for determining equivalent static wind force and base overturning moment in the cross-wind direction for tall enclosed buildings and towers of rectangular cross-section. including rotations. They are named differently depending upon the type of excitation. lock-in. The excitations under all these mechanisms are dependent on displacement and their derivatives. They occur only in very flexible structures with damping that is a fraction of 1% of the critical damping. For this reason wake excitation includes also the response due to non-resonant frequencies. This would give rise to large amplitudes of vibration which are limited only by the damping present in the system. The analysis of these structures is also beyond the scope of this Code. This Code describes the methods of computing the cross-wind response at resonant wind speeds due to wake excitation
3 for a tower decreasing in stiffness with height.0
IITK-GSDMA-Wind04-V3.3 for buildings having square and rectangular plan shapes. gv.2.
10.2.1– Equivalent static wind force
The equivalent cross–wind static force per unit height (We) as a function of z in Newton per meter height.24 k where k = mode shape power exponent for the fundamental mode of vibration = 1. Ih and β have been defined in 9.
where Km = mode shape correction factor for cross–wind acceleration = 0.0 for building with central core and moment resisting façade = 2.1.
C10.1 It represents a lateral load varying with height and is proportional to the mode shape of the fundamental mode of vibration of the structure. The forcing function for the across-wind excitation is based on studies conducted by Saunders.2.76 + 0. Melbourne and others on specific cases of width-to-length ratios and heightto-width ratios of the buildings and is expressed in a non-dimensional spectral form.Code & Commentary IS 875 (Part 3)
.2 The excitation of the building/structure is considered in its natural mode of vibration.2.2– Cross–wind overturning moment
C10. shall be as follows: We (z) = 0.
⎡ 0. 5 g R b ⎢ h ⎜ ⎟K m 2 ⎥ β ⎣ (1 + g v I h ) ⎦ ⎝ k + 2 ⎠
IITK-GSDMA-Wind02-V5. The force spectrum coefficient Cfs for some typical cases is given in 10.6(Vz )2 ⎤ 2 ⎛ 3 ⎞ πCfs M 0 = 0. 5 g R ⎜ ⎟ 2 d ⎠ (1 + g v I h ) ⎝ ⎛z⎞ ⎜ ⎟ ⎝h⎠
C10. defined by the power exponent k.5 for a uniform cantilever = 0.2.5 for a slender framed structure (moment resistant) = 1.6 [Vh]2 d Cdyn where d = Lateral dimension of the structure parallel to the wind stream.2 It is obtained from the equivalent static wind force given in the preceding clause. and
C dyn Km ⎛b⎞ = 1. or with a large mass at the top Cfs = cross–wind force spectrum coefficient generalized for a linear mode shape (Figures 11–14).
IITK-GSDMA-Wind04-V3. cross–wind base
C10.5 -2.3 Since the spectra given in the Code are based on linear mode shape.
Turbulence intensity at 2h/3 of 0.1.2.5 -4.2.0
.0 -3.12 Turbulence intensity at 2h/3 of 0.2.20
-2.0 -1.5 -3.2.
10.3 Cross–wind force spectrum coefficient (Cfs)
Values of the cross–wind force spectrum coefficient (Cfs) generalized for a linear mode shape shall be calculated from the reduced velocity (Vn) given in Figures 11–14. Thus a correction factor Km has been incorporated in the expression for Cdyn given in Section 10. these need a correction to be applied for the non-linear mode shapes defined by mode shape power exponent (k) values other than unity.Code & Commentary IS 875 (Part 3)
correction factor for overturning moment.0
Figure 11: Cross–wind force spectrum coefficient for a 3:1:1 square section [Clause 10.
IITK-GSDMA-Wind04-V3.0 -1.0 -2.2.5 -4.0 -3.12 Turbulence intensity at 2h/3 of 0.12
-3.0 -1.5 -4.5 -3.3]
-1.20 Turbulence intensity at 2h/3 of 0.5 Log 10 (Cfs) -2.5
Figure 13: Crosswind force spectrum coefficient for a 6:2:1 rectangular section [Clause 10.0
Figure 12: Cross–wind force spectrum coefficient for a 6:1:1 square section [Clause 10.0
Turbulence intensity at 2h/3 of 0.0 -2.2.20
-3.3]
IITK-GSDMA-Wind02-V5.0 Turbulence intensity at 2h/3 of 0.Code & Commentary IS 875 (Part 3)
IITK-GSDMA-Wind02-V5. yh at the top of a tall building is given by the following expression
⎤ 0. The variation of displacement and hence the acceleration is assumed to be linear along the height. assuming acceleration at the base to be zero.4– Peak Acceleration Across-wind .20
-1. mo has been assumed to be constant. The mass per unit height.5 -2.0
.5 -4.1. accelerations are obtained from the expression for the cross-wind force given in Section 10.2.0
Figure 14: Cross–wind force spectrum coefficient for a 6:1:2 rectangular section [Clause 10.5 -3.90 bg R ⎡ V h yh = ⎢ ⎥ Km m0 ⎣1 + g v I h ⎦
.2..2.4 In the cross-wind direction.2.
where m0 = mass per unit height.12 Turbulence intensity at 2h/3 of 0.
The peak across-wind acceleration.3]
10. The acceleration at any level may be obtained by linear interpolation.
C10..0 -3.
.2) Basic Wind Speed at 10m Height for some Important Cities/Towns
15 a and 15 b. and (b) Above height hx. The speed at height hx.
NOTE: Examples of determination of speed profiles in the vicinity of a change in terrain category are shown in Fig. the speed profile over the rougher terrain shall be determined as follows:
(a) Below height hx.
B–2 High To Low Number B–2. as determined in relation to the rougher (more distant) terrain. the speeds shall be determined in relation to the rougher (nearby) terrain.1 Terrain changes involving more than one category shall be treated in similar fashion to that described in B-1 and B-2. and (b) Below height hx. the speeds shall be determined in accordance with the rougher (more distant) terrain. That determined in accordance with the less rough (nearby) terrain. the speeds shall be determined in relation to the less rough (more distant) terrain. the speed shall be taken as the lesser of the following: i. 15 c.Code & Commentary IS 875 (Part 3)
[Clause 5. the speed profile shall be determined as follows:
(a) Above height hx.4 (b)(ii)] Changes in Terrain Categories
B–1 Low To High Number B–1.2.
NOTE: Example involving three terrain categories is shown in Fig.0
IITK-GSDMA-Wind04-V3. and ii.0
.1 In case of transition from a low category number (corresponding to a low terrain roughness) to a high category number (corresponding to a rougher terrain).
B–3.3.1 In case of transition from a more rough to a less rough terrain.
. h4 = height for category 4
x2 = fetch. h2 = height for category 2
IITK-GSDMA-Wind04-V3. h4 = height for category 4 x1 = fetch.
3. In such cases.
L Z/0. it is often not possible to decide whether the local topography to the site is significant in terms of wind flow. NOTE: 2 – In an undulating terrain. the average value of the terrain upwind of the site for a distance of 5 km should be taken as the base level to assess the height Z. and the upwind slope θ.
If the zone downwind from the crest of the feature is relatively flat (θ < 3o) for a distance exceeding Le. in evaluating k3 between a three dimensional hill and a two dimension ridge.5 Le upwind and 2. Z = effective height of the feature. The influence of the topographic feature is considered to extend 1. Examples of typical features are given in Figure 16. 15).5 Le downwind of the summit of crest of the feature where Le is the effective horizontal length of the hill depending on slope as indicated below (see Fig.0
C-1. of the feature.
NOTE:1–No difference is made.3
L = actual length of the upwind slope in the wind direction.3.0
. then the feature should be treated as an escarpment. then the feature should be treated as a hill or ridge.Code & Commentary IS 875 (Part 3)
[Clause 5. If not. and
θ = upwind slope in the wind direction.
x from the summit or crest. k3
The topography factor k3 is given by the following: k3 = 1 + C . Topography Factor.2 (z/L) 0. H on the structure above the mean local ground level. and the distance. should be determined from:
C-2. Values of s from Fig.36
and s is a factor derived in accordance with C-2. there will be large regions of reduced accelerations or even shelter and it is not possible to give general design rules to cater for these circumstances.
Region 1.1 The factor.
NOTE: Where the downwind slope of a hill or ridge is greater than 3o. 18 for hills and ridges. and (b) Fig.0
IITK-GSDMA-Wind04-V3. s. s where C has the following values:
1. relative to the effective length. 17 for cliffs and escarpments.1 appropriate to the reference height.5 Le Downwind slope
IITK-GSDMA-Wind02-V5. Le. 18 may be used as upper bound values.5 Le Wind Average ground level Z Upwind slope θ L
Topographical H 2.0
IITK-GSDMA-Wind04-V3.5 -1.0
2.5 0 2.0 1.5
0.0 -0.6 08
0.5 1.4 0.5 0 -1.0
IITK-GSDMA-Wind02-V5.4 0.5
0.5 1.0 1.0 0.0
1.Code & Commentary IS 875 (Part 3)
C. 22. “A full scale study of wind loads on agricultural canopy structures and proposal for design”. pp. B. Vol. “Recent development in the codification of wind loads on low–rise structures”.D. “The prediction of mean wind speeds above simple 2D hill shape”. “Wind loads on circular storage bins. Journal of Wind Engineering and Industrial Aerodynamics. Melbourne. 46–47. ASCE.J. 1996. and Melbourne.
24. pp. 5. P. 3. Vol.D. “Interference Excitation of Twin Tall Buildings”. Engrg. 369–379.D. “Parameters of risk coefficients for structures located in cyclone prone regions”. Vol. pp 1 to 27. May 2000.. Vol. China. P. and Moran. “The improved performance of hip roofs in extreme wind – A case study”.D. Saunders. 10. The designer’s guide to wind loading of building structures – Part 1”. 1990..
15.. and Kwok. Gumley. “Buffeting effects of upwind buildings”. 2. Cambridge University Press. Roorkee. Journal of Wind Engineering & industrial Aerodynamics.D. University of Roorkee (Now Indian Institute of Technology Roorkee). “Mean and fluctuating internal pressures induced by wind”. D.H. pp 259–220. 41-44. Gupta.
9. Journal of the Structural Division. 1984. Preprints.A. Robertson. pp. Proceeding of Second Asia Pacific Conference on Wind Engineering (APCWE-II). N.W. December 1985. P. 1979. “A parametric study of extreme pressures for the static design of canopy structures”. and Kwok.. Civil Engineering Department.H. Journal of Wind Engineering and Industrial Aerodynamics. J. P.P. S. 5th International Conference on Wind Engineering (Fort Collins).. 1979.. Vol. “A wind tunnel study of the mean pressure forces acting on large groups of low rise buildings”. 6. N.
14.. W. “Wind tunnel studies on aerodynamic interference in tall rectangular buildings”. P.. pp 43–56. 1994.D. Naveen. Journal of Wind Engineering and Industrial Aerodynamics. pp 167–205. Fort Collins. 11. “Wind loads on curved roofs”. P.A. 20.. Proc. Proc. and Melbourne.. Hussain.D. Davenport. Pergammon Press. Effect of grouping”. 18. J.. 15. Macdonald. “Wind pressures on low rise hip roof buildings”. Roorkee. Struct. Ph. T. W.
IITK-GSDMA-Wind02-V5. London. “Tall rectangular building response to cross–wind excitation”.. 118(2) 429–446. 21. 145–153.0
IITK-GSDMA-Wind04-V3. 11–34. 1985. 16. K. silos and tanks. W. February 2000. pp. 6. India.0
. ASCE. 1975. pp 471–476. Shakeel. K. Sachs. p. 1985. 21. “Wind loads on buildings with sawtooth roofs”. 77–95. and Stathopoulos. 1992.. Journal of Wind Engineering and Industrial Aerodynamics. Civil Engineering Department. and Harikrishna. iii–xvi. A. University of Roorkee (Now Indian Institute of Technology Roorkee). University of Roorkee (Now Indian Institute of Technology Roorkee).S.H. pp. Bayar. Proc. Beijing. R. A. Butterworths.C. and Holmes.D. “Cross–wind response of structures to wind action”. 1993.
Ahmad. Krishna. 435–450. “Wind forces in engineering”. 112.. 13.. Vol. Vol. 34. S.W.. Journal of Wind Engineering and Industrial Aerodynamics.
21. 1983.A. pp. Thesis.1717-1726. 1986. ASCE.J. “Computation of wind flow over topography”. and Holmes.G. London. Paterson. Holmes. “Gust loading factors”.. II. C. J. Saunders. 1980.. pp 207–225. 323–338. J. 1989. 23. Ph. 417–430.. P. 1978. Bailey. Hoxey. Cook. and Lee.. Pergamon Pres. 4th International Conference on Wind Effects on Buildings and Structures... Abhay. 4. Holmes. Vol. April 2002. Cambridge University Press. 16. Ph. 4th International Conference on Wind Engineering Effects on Buildings and Structures. 1967.. 7. 5th International Conference on Wind Engineering. 1985..
17. Vol. “Drag coefficients of latticed towers”. Meecham.. J. Asia–Pacific Symposium on Wind Engineering...C.S.
12. J. pp. Bowen.P. “Experimental studies and ANN modelling of wind loads on low buildings”. Proc.. Vol. 93. Holmes. Thesis. “Wind loads on freestanding walls in turbulent boundary layers”. Letchford. 51. Vol.S..E. September 1975. Kwatra. Thesis.. U.. 8. Saathoff.. D.Code & Commentary IS 875 (Part 3)
1. Arunachalam. of the National Conference on Wind Engineering.W. Journal of Wind Engineering and Industrial Aerodynamics.A. J. D. Lakshmanan.J. Journal of Wind Engineering and Industrial Aerodynamics. Journal of Wind Engineering and Industrial Aerodynamics. Civil Engineering Department. J. A. Journal of Structural Engineering. M.
of Second Asia Pacific International Conference on Wind Engineering 123–130.. 25–27. Robert H. and Scanlan. Structural Engineering Research Centre. 1985. University of Roorkee (Now Indian Institute of Technology Roorkee). “Risk analysis of cyclonic wind data”. 605 Third Avenue. China.M.. B. The views and opinions expressed herein are those of the authors and not necessarily those of the GSDMA or the World Bank... of National Seminar on Tall Reinforced Concrete Chimneys. 1991. and Venkateswaralu. Mahmood. G. T. 28. J. Arumugam. “Code provisions for wind pressures on low buildings with mono slope roofs”.
IITK-GSDMA-Wind02-V5. T. New Delhi. Godbole of VNIT Nagpur were the reviewers of the document. April 1985. 1989.0
IITK-GSDMA-Wind04-V3. A.R. and Industrial Aerodynamics. 1996 Stathopoulos..
29. Aust. Stathopoulos. 13. of Asia Pacific Symposium on Wind Engineering.. “Aerodynamic interference in tall buildings”. S. Gupta of VNIT Nagpur also contributed through review comments. Ashiwini Kumar. IIT Kanpur. P. December 1990. – third edition. B. Lakshmanan. E. Prof. Gandhinagar through World Bank finances.D.
This work has been supported through a project entitled Review of Building Codes and Preparation of Commentary and Handbooks awarded to IIT Kanpur by the Gujarat State Disaster Management Authority (GSDMA).. N. Venkateswaralu.N. Simiu. New York. Wind Engrg. Proc. 30. Prof. Thesis. Proc.. Prof. pp. Vickery. I. Civil Engineering Transactions. December 5–7.E. and Saathoff. 273–284. 38. “Wind pressures on roofs of various geometries”..
Shanmugasundaram. M. Beijing. and Mohammadian. B. 1–9. Roorkee.. L. and Arunachalam. J. “Wind effects on structure – Fundamentals and applications to design”.
26. P. Civil Engineering Department. Ph. Yahyai.0
. Dr. Chennai and Prof. 31.. Annamalai. 1971. Vol. Book published by John Willey & Sons Inc.
27..Code & Commentary IS 875 (Part 3)
25. “Probabilistics models for cyclonic wind speeds in India”.J. “On the reliability of gust loading factors”.
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