Which one of the following statements related to IS 875 (Part 3) – 1987 is not correct ?
- AThe design wind velocity depends on terrain, height and structure size.
- BThe design wind depends on topography.
- CThe design wind pressure is 0·6 × (design wind velocity)².
- DUp to a height of 50 m, the wind pressure is considered to act uniformly.Correct
Solution
Concept
Design wind speed and pressure
Formula
Vz = Vb k₁ k₂ k₃ k₄; pz = 0.6 Vz²
Step-by-step solution
1. Design wind speed Vz = Vb k₁ k₂ k₃ k₄ depends on risk (k₁), terrain, height and structure size (k₂), topography (k₃) and importance (k₄) → (a), (b) correct. 2. Design wind pressure pz = 0.6 Vz² (N/m², V in m/s) → (c) correct. 3. Wind speed (and pressure) increases with height through k₂, so the pressure is not uniform up to 50 m. Uniform pressure is assumed only for low buildings (height up to about 10 m).
Explanation
The terrain–height factor k₂ changes with height, so the wind pressure varies with height. A uniform-pressure assumption applies only for low structures (up to 10 m), not up to 50 m.
Why the other options are wrong
Statements (a), (b) and (c) are correct expressions of IS 875 Part 3.
Shortcut
pz = 0.6 Vz². Pressure varies with height except for buildings below 10 m.
Common mistake
Thinking that wind pressure is uniform for all buildings below 50 m.
Correct answer: D) Up to a height of 50 m, the wind pressure is considered to act uniformly.