Wind Load Calculation As Per IS 875 (Part 3)

Introduction

Wind load calculation is a critical step in designing steel buildings and industrial sheds, as it determines the forces acting on purlins, columns, bracings, and cladding systems, ensuring structural safety, serviceability, and cost efficiency.

In India, the procedure for evaluating wind loads is outlined in IS 875 (Part 3): 2015 – Design Loads (Other than Earthquake) for Buildings and Structures: Wind Loads. Similarly, in Bhutan, the same standard is adopted for wind load assessment. This code provides a comprehensive methodology for determining the design wind speed, design wind pressure, and the corresponding pressures on different building surfaces.

This article provides an explanatory, step-by-step guide to wind-load calculation using IS 875 (Part 3), applied to a 40 m × 20 m low-rise steel industrial building located in Thimphu (Zone V, 47 m/s basic wind speed).

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1. Understanding Wind Load as per IS 875 (Part 3)

Wind exerts pressure on a structure through its dynamic action on exposed surfaces. The magnitude of this pressure depends on wind velocity, topography, terrain roughness, and the geometry of the structure.

According to Clause 5.3 of IS 875 (Part 3), the design wind speed (\(V_z\)) at any height z above ground is given by:

\(V_z=V_b\times k_1\times k_2\times k_3\times k_4\)

where:

• \(V_b\)= Basic wind speed (from the wind map in Fig. 1 of IS 875 Part 3)
• k₁ = Risk coefficient (probability factor)
• k₂ = Terrain, height, and structure size factor
• k₃ = Topography factor
• k₄ = Importance factor for cyclonic regions (introduced in 2015 revision)

The corresponding design wind pressure \((P_z\)) at that height is:

\(P_z = 0.6 \times V_z^2\)

(Clause 5.4.1)

This pressure acts normal to the building surfaces and is modified by external and internal pressure coefficients to determine the net pressure on walls and roofs.

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2. Key Parameters Affecting Wind Load

2.1 Basic Wind Speed \((V_b\))

The basic wind speed is defined as the peak gust velocity averaged over a short duration of 3 seconds at a height of 10 m in open terrain, corresponding to a 50-year return period. is the reference wind speed provided in the wind map of IS 875 (Part 3:2015).

For Thimphu, Bhutan, we adopt an equivalent basic wind speed of 47 m/s, comparable to Zone V in the IS 875 wind map.

2.2 Risk Coefficient (k₁)

This factor accounts for the probability of exceedance of the basic wind speed during the structure’s design life (Table 1, IS 875 Part 3).

From the table of IS875, for an industrial shed (50-year life), k₁ = 1.0.

Also, Read: Bolted Connection: Comprehensive Guide. 101

2.3 Terrain, Height, and Structure Size Factor (k₂)

The factor k₂ accounts for the variation of wind speed with terrain roughness, height, and the effective size of the structure.
The terrain categories (Clause 6.3.1) are:

Terrain Category	Description
Category 1	Exposed open terrain with few obstructions (airfields, coastal areas)
Category 2	Open terrain with scattered obstructions (typical for industrial areas)
Category 3	Suburban areas, numerous closely spaced buildings
Category 4	Dense urban areas with tall buildings

For a place like Thimphu, Terrain Category 3 is appropriate.

Our building has a height of 10 m. From Table 2 of IS 875 (Part 3) for Class B (medium-sized) structures and Category 3 terrain:

\(k_2 = 0.91 ; \text{at 10 m height}\)

2.4 Topography Factor (k₃)

Topography affects local wind acceleration due to hills, ridges, or escarpments. For gentle terrain with slope < 3°, k₃ = 1.0 (Clause 6.4.2).
Thimphu is generally in the valley, so k₃ = 1.0 is reasonable.

2.5 Importance Factor (k₄)

For cyclonic regions or structures requiring special safety, k₄ (Clause 6.5) modifies the design wind speed.

• For general buildings outside cyclone regions, k₄ = 1.0.
  Thus, k₄ = 1.0 for our case.

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3. Step-by-Step Calculation Example

Given Data

• Location: Thimphu (Zone V)
• Basic wind speed, \(V_{b}\) = 47 m/s
• Dimensions: 40 m × 20 m, height 10 m
• Terrain Category 3, Class B structure
• Roof type: double-slope pitched roof (10° inclination)
• Site topography: plain (k₃ = 1.0)

Step 1: Determine Design Wind Speed

\(V_z = V_b \times k_1 \times k_2 \times k_3 \times k_4\)
\(V_z = 47 \times 1.0 \times 0.97 \times 1.0 \times 1.0 = 45.59 , \text{m/s}\)

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Step 2: Determine Design Wind Pressure

\(P_z = 0.6 \times V_z^2 = 0.6 \times 45.59^2 = 0.6 \times 2078.45 = 1247 , \text{N/m}^2\)
\( P_z=1.25kN/m^2 \) …This is the base pressure acting on surfaces normal to the wind direction at 10 m height.

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Step 3: External Pressure Coefficients (Cpe)

The external pressure coefficient depends on the building’s shape, aspect ratio, and roof slope (Table 5 & 6 of IS 875 Part 3).

For a rectangular building (L/B = 2, height = 10 m) with a 15° pitched roof:

• Wind normal to ridge:
  • Wall
    • Windward wall: Cpe = +0.7
    • Leeward wall: Cpe = −0.5
    • Side walls: Cpe = −0.7
  • Roof surfaces (15° slope):
    • Windward = −0.2
    • Leeward = −0.3

Also, Read: Comprehensive Guide to Tension Members in Steel Structure 101

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Step 4: Internal Pressure Coefficient (Cpi)

For buildings with openings (Clause 7.2.1):

Opening Ratio	Cpi
< 5 %	± 0.2
5–20 %	± 0.5
> 20 %	± 0.7

Industrial buildings typically have large doors and vents → assume 5 to 20 % openings, hence Cpi = ± 0.5.

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Step 5: Net Pressure on Surfaces

\(P_{net} = (C_{pe} – C_{pi}) \times P_z\)

We compute for both suction and pressure cases.

(a) Windward Wall

\(P_{net} = (+0.7 – (+0.5)) \times 1.5 = 0.2 \times 1.5 = 0.30,\text{kN/m}^2\)
\(P_{net} = (+0.7 – (−0.5)) \times 1.5 = 1.2 \times 1.5 = 1.8,\text{kN/m}^2\)

Hence, net pressure range: 0.3 – 1.8 kN/m².

(b) Leeward Wall

\(P_{net} = (−0.5 – (+0.5)) \times 1.5 = −1.0 \times 1.5 = −1.5 \text{kN/m}^2\)
\(P_{net} = (−0.5 – (−0.5)) \times 1.5 = 0 \text{kN/m}^2\)

Thus, design for 1.5 kN/m² suction.

(c) Side Walls

\(P_{net} = (−0.7 – (+0.5)) \times 1.5 = −1.8 \times 1.5 = −2.7 \text{kN/m}^2\)
\(P_{net} = (−0.7 – (−0.5)) \times 1.5 = −0.2 \times 1.5 = −0.3 \text{kN/m}^2\)

Adopt 2.7 kN/m² suction as the design case.

(d) Roof (Windward Slope)

\(P_{net} = (−0.2 – (+0.5)) \times 1.5 = −1.05 \text{kN/m}^2\)
\(P_{net} = (−0.2 – (−0.5)) \times 1.5 = +0.45 \text{kN/m}^2\)

Hence, the roof experiences both uplift (1.05 kN/m²) and downward (0.45 kN/m²) pressures.

(e) Roof (Leeward Slope)

\(P_{net} = (−0.3 – (+0.5)) \times 1.5 = −1.2 \text{kN/m}^2\)
\(P_{net} = (−0.3 – (−0.5)) \times 1.5 = +0.3 \text{kN/m}^2\)
Adopt uplift 1.2 kN/m² as critical.

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Step 6: Distribution of Wind Load

Wind pressure acts as:

• Lateral load on walls is transferred through purlins and wall girts to columns and bracings.
• Uplift on roof surfaces, resisted by purlins, roof sheeting, and connection bolts.

For a roof span of 20 m, windward uplift can reach 1.05 kN/m² × 20 m × 1 m = 21 kN/m per metre width**, which is significant compared with self-weight and live load.

Also, Read: Types of Loads on Building

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4. Application to Structural Elements

4.1 Roof Sheeting and Purlins

Design purlins for uplift pressure using the critical net suction from Step 5. Ensure proper anchorage to rafters with sufficient bolt or clip capacity.

4.2 Rafters and Main Frames

Wind loads act as lateral forces on the rigid frames. Frame analysis must include:

• Horizontal thrust on eave level,
• Uplift on roof members,
• Pressure on windward side walls.

4.3 Columns and Bracings

Columns transfer wind shear to the foundation through longitudinal and cross bracings. For the 40 m building, consider at least one bay of bracing per 20 m in each direction for stability.

4.4 Foundations

Base plates and anchor bolts must resist combined vertical uplift and horizontal shear. Adequate embedment and foundation mass are essential to prevent overturning.

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5. Wind Load Combinations (IS 875 Part 5)

For limit-state design as per IS 800:2007 or IS 456:2000, typical load combinations involving wind are:

• 1.5 (DL + WL)
• 1.2 (DL + LL + WL)
• 0.9 DL ± 1.5 WL

where DL = Dead Load, LL = Live Load, WL = Wind Load.

For industrial roofs with significant uplift, the last combination (0.9 DL ± 1.5 WL) often governs connection and anchorage design.

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6. Key Design Considerations for Industrial Buildings

1. Openings: Large doors and vents increase internal pressure. Always assess both positive and negative internal pressures.
2. Orientation: The building orientation relative to prevailing wind direction affects effective area.
3. Bracing Layout: Adequate wind bracing in roof and side walls is essential for load transfer.
4. Cladding Fixing: Ensure adequate screw spacing and sealant performance under suction.
5. Serviceability: Check lateral drift and cladding deflection limits to prevent leakage or fatigue.
6. Dynamic Effects: For low-rise buildings (< 20 m), gust effects are usually covered in the design wind speed; however, tall or flexible structures require dynamic analysis (Clause 8.3).

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7. Common Mistakes in Wind Load Estimation

1. Incorrect Terrain Category: Choosing a smoother terrain (Category 1) for an urban or industrial site overestimates wind loads.
2. Ignoring k₄ Factor: In coastal or cyclonic regions, neglecting k₄ > 1 can lead to unsafe design.
3. Neglecting Uplift: Roof sheeting and purlin design often fail due to overlooked uplift pressures.
4. Assuming Zero Internal Pressure: Even closed sheds have openings; internal suction or pressure must be considered.
5. No Combination Check: Failing to verify both +Cpi and –Cpi combinations can miss the worst case.

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8. Practical Tips for Engineers and Students

• Always read Clause 5–8 of IS 875 (Part 3) carefully—each coefficient and assumption has significant effect.
• Prepare a spreadsheet or software template to calculate \(V_{z}\) and \(P_{z}\) for various heights.
• Cross-check results with wind tunnel or CFD studies for complex buildings.
• Keep in mind local wind records—mountainous regions like Thimphu may experience channelled winds not reflected in general maps.
• Ensure connection detailing (bolts, welds) are verified for uplift and reversal of forces.

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9. Summary and Conclusion

This article demonstrated how to determine wind loads on a 40 m × 20 m × 10 m steel industrial building in Thimphu, using IS 875 (Part 3): 2015.

Key results:

• Basic Wind Speed \((V_{b}\)): 47 m/s
• Design Wind Speed \((V_{z}\)): 47 m/s
• Design Wind Pressure \((P_{z}\)): 1.5 kN/m²
• Net Uplift on Roof: ≈ 1.2 kN/m²
• Net Pressure on Walls: 0.3 – 2.7 kN/m²

These pressures form the basis for designing cladding, purlins, frames, and anchorage systems.

Understanding the factors k₁, k₂, k₃, and k₄, and the interaction between external and internal pressures, is crucial to safe, economical design. By correctly applying IS 875 (Part 3), structural engineers can ensure that industrial buildings remain stable and serviceable under severe wind conditions.

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FAQs:

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References:

1. IS 875 (Part 3): 2015 – Design Loads (Other than Earthquake) for Buildings and Structures, Bureau of Indian Standards, New Delhi.
2. IS 800: 2007 – General Construction in Steel – Code of Practice.
3. IS 875 (Part 5): 2015 – Load Combinations for Design of Buildings and Structures.