IS 875 PART - 3 WIND LOAD ON BUILDINGS AND STRUCTURE

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• This Code provides information on wind effects for buildings and structures, and their components. Structures such as chimneys, cooling towers, transmission line towers and bridges are outside the scope of this Code. There are Indian Standards dealing with chimneys and cooling towers separately. Information on bridges (only static forces) is given in IRS and IRC Specifications. For aerodynamics of bridges, specialist literature may be consulted. With substantial work being done worldwide in the area of wind engineering, there is growing body of new information. The user of this Code is advised to consult specialist literature for the design of large or important projects involving various types of structures.
• Wind is not a steady phenomena due to natural turbulence and gustiness present in it. However, when averaged over a sufficiently long time duration (from a few minutes to an hour), a mean component of wind speed can be defined which would produce a static force on a structure. Superimposed on the mean/static component is the time varying component having multiple frequencies spread over a wide band.
• Wind causes a random time-dependent load, which can be seen as a mean plus a fluctuating component. Strictly speaking all structures will experience dynamic oscillations due to the fluctuating component (gustiness) of wind. In short rigid structures these oscillations are insignificant, and therefore can be satisfactorily treated as having an equivalent static pressure. This is the approach taken by most Codes and Standards, as is also the case in this Standard. A structure may be deemed to be short and rigid if its natural time period is less than one second. The more flexible systems such as tall buildings undergo a dynamic response to the gustiness of wind. Methods for computing the dynamic effect of wind on buildings have been introduced in this Standard.
• Apart from tall buildings there are several other structural forms (though outside the scope of this Standard) such as tall latticed towers, chimneys, guyed masts that need to be examined for aerodynamic effects.
• 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 locations where the strongest winds and icing may occur simultaneously, loads on structural members, cables and ropes shall be calculated by assuming an ice covering based on climatic and local experience.
• The construction period of a structure is much smaller than its expected life. Therefore, 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. In snowfall areas where icing occurs, wind loads have to beassessed accordingly. Elements such as cables and ropes can undergo a dynamic response in such cases and have to be examined accordingly
• In the design of special structures, such as chimneys, overhead transmission line towers, etc., 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. 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)- 1985 Code of practice for design, installation and maintenance of overhead power lines: Part 1 Lines up to and including 11 kV, 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, unusual locations, and abnormal environmental conditions that have not been covered in this Code. Special investigations are necessary in such cases to establish wind loads and their effects. Wind tunnel studies may also be required in such situations.
• NOTE: 2 – In the case of tall structures with unsymmetrical geometry, the designs ought to be checked for torsional effects due to wind pressure.

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