Question

According to IS : 875 Part 3, design wind speed is obtained by multiplying the basic wind speed by factors k₁, k₂, and k₃, where k₃ is 
1. terrain height factor
2. structure size factor
3. topography factor
4. risk coefficient 

Answer 1

Correct Answer - Option 3 : topography factor

As per IS 875, CL: 5.3, the design wind speed is calculated as:

V_z = V_b K₁ K₂ K₃ K₄

Wherein

V_b is the basic wind speed

K₁ is probability factor or risk coefficient which increases with increase in design life of structure. For instance, for all general buildings having design life 50 year risk coefficient is 1 for basic wind speed of 33 m/s and for same basic wind speed risk coefficient for important structures such as hospitals is 1.05 having design life of 100 years.  

K₂ factor is the height and terrain multiplier. As per IS 875, CL: 5.3.2, for a given height of terrain, K₂ decreases, if the category of structure changes from Category 1 to Category 4.

K₃ is topography factor. As per IS 875, CL: 5.3.3.1, it is in the range of 1.0 to 1.36 for slopes greater than 30° and below 30° it is equal to one.

K₄ is importance factor for cyclonic region.