Determination of Wind Loads as per IS 875 (Part 3) Design wind speed – Vz = Vbk 1k 2k 3 Vb = Basic wind speed k 1 = Probability factor or risk coefficient k 2 = Terrain and height factor k 3 = Topography factor
Basic wind speed
IS 875 (Part 3) gives the basic wind speeds having a return period of 50 years and at a height of 10 m above ground level. Entire country is divided into six wind zones.
• Basic wind speeds in m/s (Based on 50yr return period) • For some important cities, basic wind speed is given in Appendix A of the code
Probability factor / Risk Coefficient (k 1)
Basic wind speed is based on a 50yr return period. There is always a probability (howsoever small) that basic wind speed may be exceeded in a storm of exceptional violence; the greater the number of years over which there will be exposure to wind, the greater is the probability. The factor k 1 is based on statistical concepts, which take account of the degree of reliability required, and period of time during which there will be exposure to wind i.e. life of the structure. IS 875 gives values of k 1 for different classes of buildings.
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For some important structures (nuclear power plants, satellite communication towers etc.) code gives a formula to calculate the value of k 1.
Terrain and Height Factor (k 2)
Four terrain categories have been considered by the code depending on the surroundings of structure.
Category 1 : Exposed open terrain with few or no obstructions – Avg. height of surrounding objects is 1.5 m. Eg. Open sea coasts, flat treeless plains.
Category 2 : Open terrain with well scattered obstructions with height b/w 1.5 – 10 m. Eg. includes airfields, open parklands etc.
Category 3 : Terrain with numerous closely spaced obstructions having the size of building-structures up to 10 m Eg. Towns and industrial areas, full or partially developed
Category 4 : Terrain with numerous large high closely spaced obstructions.
• Buildings have been divided into 3 classes – – Class A : Structures having maximum dimension (greatest horizontal or vertical dimension) less than 20 m. – Class B : Maximum dimension b/w 20 – 50 m. – Class C : Maximum dimension greater than 50 m.
• IS 875 gives the values of k 2 at different heights for the above four categories and different classes of buildings.
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Wind profile does not develop fully at the start of the terrain. Height of development increases with the upward distance or fetch distance.
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For structures of height greater than the developed height velocity profile can be determined from the method described in Appendix B of the code.
Topography Factor (k 3)
Vb does not take into account the local topography features such as hills,valleys etc.
Topography features affect the wind speeds.
Accelerated near the summits and decelerated in the valleys
• Value of k3 level ground or when upwind slope is less than 30 is equal to 1.0 • Otherwise k3 = 1 + Cs where
C = 1.2 Z / L for θ = 3o – 17o = 0.36 for θ > 17o
Z = height of the crest or hill L = length of the upward slope θ = upwind slope of ground • s is the factor obtained from figures.
Design Wind Pressure
Design wind pressure –
pd = 0.6 Vz2 where
pd = design wind pressure Vz = design wind speed
Note : In cyclone prone regions, design wind speed is increased by a certain factor to account for drastic increase in wind speed during cyclone.
Wind Pressure on Roofs
Pressure acts normal to the element
F = (Cpe - Cpi ) A pd where F = net wind force on the element A = surface area of the element pd = design wind pressure Cpe = external pressure coefficient Cpi = internal pressure coefficient
External Pressure Coefficients
Y = h or 0.15 W whichever is less
Internal Pressure Coefficients • Depends upon permeability of the building and the direction of wind. Type of building
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Cpi
Low permeability (less than 5% openings)
0.2
Medium permeability (5 – 20% openings)
0.5
Large permeability (openings > 20%)
0.7
Different coefficients for buildings with large openings on one side.
Internal Pressure coefficients for buildings with large openings on one side and top closed.
An industrial shed of 32 m×16 m is to be built for a manufacturing unit. The frames are spaced at 4 m centers and the ceiling height is 8 m measured at the roof truss bottom from the shop floor. The shed is located in a fully developed industrial area. Analyse and design the building considering various load combinations (DL+ LL+ WL).
Sectional Elevation
Side Elevation
Elevation at Centre
Roof Bracing in Top Chord
Purlins
Top Chord Level (Roof Plan)
Eaves Level (Roof Plan)
Basic Design Data Roof span
16 m
Bay width
4m
Column height
8m
Total roof dead load on plan (due to CGI sheeting, insulation & lighting, purlins)
0.5 kPa
Total roof imposed load on plan
0.75 kPa
Roof slope with horizontal
3o
Required Work
Calculate the wind load acting on the roof (as per IS 875: Part 3). Ignore the frictional drag and dynamic effects due to wind. Also calculate total dead and live load as per IS 875 (pt 1 and 2). These loads are transferred to the truss via purlins (i.e. a concentrated load will be transferred on the truss at the purlin points.) Model and analyze the truss in SAP 2000 with the loads calculated in the part (a) and determine the forces in the members of the truss for applicable load cases as per IS 800 with suitable load factors. Analyze the 2D truss only. Determine the member which is in maximum tension and size the section for economical design. You can use either a double angle or a pipe section.
Project Report
The report will be graded for its technical accuracy and presentation, which include the following:
Lightest section where appropriate Correct numerical calculations Appropriate solution procedure and Appropriate documentation of work.
