Clause 7.4 provides force coefficients (Cf) for calculating wind drag on buildings and structures of various cross-sections. The force coefficient depends on the shape of the structure (rectangular, circular, polygonal, etc.), its aspect ratio (height/breadth), and the Reynolds number (for circular sections). Key tables include: Table 23 for rectangular clad buildings, Table 24 for other cross-section shapes (circular, octagonal, etc.), Table 25 for unclad buildings and open frameworks, and Table 26 for single frames.
Key Requirements
•Cf for rectangular clad buildings shall be obtained from Table 23 based on height/breadth and breadth/depth ratios
•Cf for circular cross-sections depends on Reynolds number and surface roughness — Table 24
•For lattice/open frame structures, Cf depends on solidity ratio (φ) — Table 25
•Shielding effects for multiple parallel frames shall be applied using Table 26
•Force coefficients apply to the overall structure for stability and overturning checks
Reference Tables
Table 23 — Force Coefficients Cf for Rectangular Clad Buildings (Clause 7.4.2)
h/b
a/b = 1
a/b = 2
a/b = 4
a/b ≥ 10
≤ 0.5
0.9
0.9
0.9
0.9
1
1
1.05
1.1
1.2
2
1.15
1.2
1.25
1.35
4
1.3
1.35
1.4
1.5
≥ 10
1.4
1.45
1.5
1.6
≥ 20
1.5
1.55
1.6
1.7
h = height of building, b = breadth (dimension perpendicular to wind), a = depth (dimension parallel to wind). For intermediate values, linear interpolation may be used.
Force Coefficients for Circular Cross-Sections (Clause 7.4.3, Table 24)
Surface
D×Vz (m²/s)
Cf
Rough or with projections
All values
0.7
Smooth
< 6
1.2
Smooth
≥ 6
0.6
Very smooth (polished)
< 6
1.2
Very smooth (polished)
≥ 6
0.5
D = diameter of circular section. The product D×Vz determines the Reynolds number regime. Post-critical (D×Vz ≥ 6) gives much lower Cf due to turbulent boundary layer reattachment.
Force Coefficients for Open/Lattice Structures (Table 25)
Solidity Ratio φ
Cf (Flat-sided)
Cf (Circular)
0.1
1.9
1.2
0.2
1.8
1.2
0.3
1.7
1.2
0.4
1.7
1.1
0.5
1.6
1.1
0.8
1.5
1
1
2
1.2
φ = solidity ratio = projected solid area / total enclosed area. For completely clad (φ = 1.0), Cf returns to the clad building value.
Practical Notes
✓For a typical 15 m tall residential building (h/b ≈ 1–2, a/b ≈ 1): Cf ≈ 1.0–1.15 from Table 23. This is the most commonly used range in Indian practice.
✓Circular chimneys and tanks in the post-critical Reynolds number range (D×Vz ≥ 6) benefit from much lower Cf ≈ 0.6 compared to Cf ≈ 1.2 in the sub-critical range.
✓For telecom towers and lattice structures, the solidity ratio governs Cf. Typical lattice towers have φ ≈ 0.2–0.3, giving Cf ≈ 1.7–1.8 on the solid projected area.
✓Shielding between parallel frames (e.g., multiple portal frames in a truss bridge) reduces the total force — shielding factors from Table 26 can reduce loads by 20–50% on the leeward frames.
Common Mistakes
⚠Confusing a/b and h/b ratios when reading Table 23 — a is the depth (parallel to wind), b is the breadth (perpendicular to wind), h is height.
⚠Using sub-critical Cf = 1.2 for large diameter chimneys operating in the post-critical regime (D×Vz ≥ 6) — this over-estimates the force by 100%.
⚠Not accounting for wind from multiple directions — a rectangular building has different Cf depending on whether wind hits the wide or narrow face.
⚠Applying force coefficients from Table 23 to individual cladding panels — these are for overall building force, not local pressures.