Clause 6.3.2 defines the terrain roughness and height factor (k2) which accounts for the variation of wind speed with height and the roughness of the surrounding terrain. Four terrain categories are defined: Category 1 (exposed open terrain with few obstructions — sea coast, flat plains), Category 2 (open terrain with scattered obstructions 1.5–10 m — the reference category), Category 3 (terrain with numerous closely spaced obstructions — suburbs, industrial areas), and Category 4 (terrain with numerous large high closely spaced obstructions — city centres). Table 2 gives k2 values for each category at various heights.
Key Requirements
•Terrain shall be classified into one of four categories based on surface roughness
•k2 shall be obtained from Table 2 for the appropriate terrain category and height
•The terrain category used shall be that prevailing in the upwind direction for a fetch length appropriate to the height of the structure
•For structures on the boundary of two terrain categories, the more severe category shall be used
•For heights between the tabulated values, linear interpolation is permitted
Reference Tables
Table 2 — k2 Values for Terrain Categories 1–4 at Various Heights (Clause 6.3.2)
Height (m)
Category 1
Category 2
Category 3
Category 4
Up to 10
1.05
1
0.91
0.8
15
1.09
1.05
0.97
0.8
20
1.12
1.07
1.01
0.8
30
1.15
1.12
1.06
0.88
50
1.2
1.17
1.12
0.98
100
1.28
1.24
1.2
1.1
150
1.32
1.28
1.24
1.16
200
1.34
1.32
1.28
1.22
250
1.36
1.34
1.32
1.24
300
1.38
1.36
1.34
1.28
350
1.39
1.37
1.36
1.3
400
1.4
1.38
1.37
1.32
450
1.41
1.39
1.38
1.34
500
1.42
1.4
1.39
1.36
Values are for Class A structures (max dimension < 20 m, height < 25 m). For Class B (20–50 m) and Class C (> 50 m), adjusted values apply. Category 2 at 10 m = 1.00 is the reference condition matching Vb.
Terrain Category Descriptions (Clause 6.3.2.1)
Category
Description
Examples
1
Exposed open terrain with few or no obstructions, average height of objects < 1.5 m
Open sea coast, flat treeless plains, airfields
2
Open terrain with well-scattered obstructions having heights 1.5–10 m
Farmland with few trees, open suburbs with low-rise
3
Terrain with numerous closely spaced obstructions 10 m height, area coverage ≥ 1/3
Suburbs, industrial areas, well-wooded areas
4
Terrain with numerous large high closely spaced obstructions
City centres with tall buildings, well-developed industrial complexes
Practical Notes
✓Most urban construction sites in Indian cities are Category 3 (suburban) or Category 4 (dense urban). Using Category 2 for an urban site is unconservative for low-rise and over-conservative for high-rise.
✓k2 at 10 m in Category 4 is only 0.80 — buildings in dense city centres experience significantly lower wind speeds at low heights compared to open terrain.
✓For a 30 m tall building in Category 3, k2 = 1.06 versus k2 = 1.12 in Category 2 — the terrain shielding effect is worth about 6% in wind speed (12% in pressure).
✓The fetch length (distance of uniform terrain upwind) required increases with building height. For a 50 m tall structure, approximately 2 km of consistent terrain is needed.
Common Mistakes
⚠Classifying a suburban residential area as Category 2 instead of Category 3 — suburbs with closely spaced houses are Category 3.
⚠Using Category 4 for a building at the edge of a city — the upwind terrain determines the category, not just the building location.
⚠Not interpolating k2 between tabulated heights — the code permits linear interpolation.
⚠Ignoring structure class (A, B, C) when selecting k2 — the tabulated values differ slightly by class.