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IS 875-3 (1987)

Wind Load Case according to IS 875 (Part 3) - 1987 is implemented. Refer to the following document for further information: IS 875-3 (1987): Code of Practice for Design Loads (Other than Earthquake) for Buildings and Structures, Part 3: Wind Loads [CED 37: Structural Safety]
In the design document, the following methods are included for wind loads generation:
  1. Pressure Coefficient Method (Section 6.2)
  2. Force Coefficient Method (Section 6.3)
  3. Gust Factor Method (Section 8)
Only method 3 "Pressure Coefficient Method (Section 6.2)" is implemented in the program.

Basic Wind Speed (Vb): Section 5.2

Basic wind speed is given in Figure 1 (or Appendix A) for different locations. It is defined in terms of "m/s". Basic wind speed is provided directly by engineer.

Design Wind Speed (Vz): Section 5.3

Design wind speed (m/s) is defined as follows:

Vz = k1k2k3Vb

where
k1
=
Probability factor (risk coefficient, Section 5.3.2)
k2
=
Terrain, height and structure size factor (Section 5.3.2, also see Table 2)
k3
=
Topography factor (Section 5.3.3)

Probability Factor (k1): Section 5.3.1

Table 1 provides a method to calculate k1. This is not implemented. Instead, engineer is required to calculate this factor and enter it into the program directly.

Terrain, Height, and Structure Size Factor (k2): Section 5.3.2

Table 2 is implemented in the program. Two choices are offered:
  • Use k2 provided by engineer (in the case, k2 is assumed to be constant and not varying with height)
  • Calculate k2 value according to Table 2 (based on Terrain Category and Terrain Class type). In the case, k2 is varying with height of structure.

Topography Factor (k3): Section 5.3.3

Appendix C provides a method to calculate k3. This is not implemented in the program. The engineer is required to provide this factor.

Note: k3 may vary with height (according to Appendix C).In the current implementation, it is considered as constant for whole structure. Also, it is limited to k3 ≥ 1.0.

Design Wind Pressure (Vb): Section 5.4

It is defined as follows:

P z = 0.60 V z 2

Note that Pz and Vz are expressed in terms of "N/m2" and "m/s," respectively.

Calculation of Wind Load According to Pressure Coefficient Method (Section 6.2)

Wind load applied on a surface (perpendicularly) is calculated as follows:

Fz=CpPzA

where
Cp
=
(external) pressure coefficient
A
=
surface area

External Pressure Coefficient Factor (Cp)

Table 4 is implemented in the program. Two choices are offered:
  • Use the Cp value provided by the engineer – In this case, Cp is assumed to be constant and not varying with height. You are required to enter two values, one for windward and one for leeward surfaces.
  • Calculate the Cp value as determined according to Table 4. – In this case, the value may change over the height of the building, depending on the building depth and width at floor levels.

Implementation Details

  • Current implementation is for building type structures. It is assumed that structure as a whole is a closed (sealed) and therefore, internal pressures cancel out each other. Wind pressure calculations for buildings with openings are not included.
  • Calculated wind pressures are for whole building. Local pressures effects are not considered.
  • Wind pressures and loads are calculated according to Section 6.2 (Pressure Coefficient Method). Other two methods (Force Coefficient Method of Section 6.3 and Gust Factor Method of Section 8) are not implemented.
  • Dynamic effects mentioned in Section 7 are not considered.
  • Pressure on surfaces (walls) are assumed to be linearly varying over height of structure. Pressure on parapets are assumed to be constant.
  • Table 4 tabulates external pressure coefficients (Cp) for rectangular buildings.
    • Drag coefficients (those on the side of structure parallel to wind direction) are not considered in the current implementation. Hence, only wind pressures perpendicular wind direction are calculated (i.e., pressure on windward and leeward surfaces)
    • Table 4 does not provide information for l w > 4.0 (also, for the case h w > 6.0 , coefficients are only provided for l w > 3 2 , l w = 1 , and l w = 2 ). The following values from the Table 4 are used in the implementation:
      Table 1. Wind direction: X (θ = 0)
      Height Ratio Length Ratio Windward Leeward
      h w ≤ 0.50 l w ≤ 1.50 0.70 -0.20
      l w ≤ 1.50 0.70 -0.25
      h w > 1.50 l w ≤ 1.50 0.70 -0.25
      l w ≤ 1.50 0.70 -0.30
      h w ≤ 6.0 l w ≤ 1.50 0.80 -0.25
      l w ≤ 1.50 0.70 -0.40
      h w > 6.0 l w ≤ 1.0 0.95 -1.25
      l w ≤ 1.50 0.95 -1.85
      l w ≤ 1.50 0.85 -0.75
      Table 2. Wind direction: Y (θ = 90)
      Height Ratio Length Ratio Windward Leeward
      h w ≤ 0.50 l w ≤ 1.50 0.70 -0.20
      l w ≤ 1.50 0.70 -0.10
      h w > 1.50 l w ≤ 1.50 0.70 -0.25
      l w ≤ 1.50 0.70 -0.10
      h w ≤ 6.0 l w ≤ 1.50 0.80 -0.25
      l w ≤ 1.50 0.80 -0.10
      h w > 6.0 l w ≤ 1.0 0.95 -1.25
      l w ≤ 1.50 0.90 -0.85
      l w ≤ 1.50 0.85 -0.75
      where
      l
      		=
      greater horizontal dimension of a building
      w
      		=
      lesser horizontal dimension of a building
      h
      		=
      height of structure above mean ground level
Parent topic: Story Forces: Wind