Loads on Roof Trusses
Complete Guide to Dead Load, Live Load, Snow Load and Wind Load in Roof Truss Design
Civil Engineer | Geotechnical Engineer
Introduction
Roof trusses are one of the most important structural components used in industrial buildings, warehouses, auditoriums, residential structures, stadiums and commercial facilities. A roof truss transfers all roof loads safely to the supporting columns or walls.
For safe and economical structural design, engineers must properly calculate all loads acting on the roof truss. Incorrect estimation of loads may lead to excessive deflection, instability or even structural failure.
The major loads considered in roof truss design are:
- Dead Load
- Live Load (Imposed Load)
- Snow Load
- Wind Load
1. Dead Load on Roof Trusses
Dead load refers to the permanent static load acting on the roof structure throughout its life. These loads remain constant and do not change with time.
Components of Dead Load
- Weight of roof covering materials
- Weight of purlins
- Self-weight of truss members
- Weight of bolts, gusset plates and connections
- Ceiling load (if provided)
- Electrical and mechanical fixtures
Typical Values
The self-weight of roof truss generally varies between:
Importance of Dead Load
Dead load is always present on the structure and forms the basis for all structural calculations. Underestimating dead load can cause excessive stresses and unsafe design.
2. Live Load (Imposed Load)
Live load represents temporary loads acting on the roof due to maintenance activities, workers, movable equipment and occasional construction loads.
For Roof Slopes ≤ 10°
| Condition | Live Load |
|---|---|
| Access Provided | 1500 N/m² |
| Access Not Provided | 750 N/m² |
For Roof Slopes > 10°
For sloping roofs, live load decreases with increase in roof slope because steeper roofs are less likely to accumulate maintenance loads or workers.
Live Load = 750 − 20(θ − 10) N/m²
where θ = roof slope in degrees
However, the minimum live load should not be less than:
Load on Supporting Members
For trusses, beams and girders supporting the roof members, the imposed load is generally taken as:
3. Snow Load
Snow load is important in cold regions where snow accumulation occurs on roofs. Heavy snow deposition may cause excessive loading and roof collapse if not considered properly.
Snow Load Formula
Effect of Roof Slope
When roof slope exceeds 50°, snow usually slides off naturally and therefore snow load may be neglected in design.
4. Wind Load on Roof Trusses
Wind load is one of the most critical loads in roof truss design, especially for industrial buildings and large-span structures.
Wind creates both positive pressure and suction effects on roofs. Improper consideration of wind load may lead to uplift failure or instability.
Design Wind Pressure
The design wind pressure is calculated using:
Where:
- Pz = Design wind pressure
- Vz = Design wind velocity at height z
Design Wind Velocity
Factors Affecting Wind Velocity
- k₁ = Probability or risk coefficient
- k₂ = Terrain, height and structure size factor
- k₃ = Topography factor
- Vb = Basic wind speed at 10 m height
Internal Wind Pressure
Internal pressure depends upon the permeability and openings present in the building.
| Type of Building | Internal Pressure |
|---|---|
| Low permeability (No openings) | 0 |
| Normal permeability (up to 5% openings) | ±0.2 P |
| Medium openings (5% – 20%) | ±0.5 P |
| Large openings (>20%) | ±0.7 P |
5. Load Combinations
Structural design is performed using different combinations of loads to ensure maximum safety under all possible conditions.
Common Load Combinations
- Dead Load + Live Load
- Dead Load + Wind Load
- Dead Load + Snow Load
- Dead Load + Live Load + Wind Load
- Dead Load + Snow Load + Wind Load
6. Practical Design Considerations
- Proper drainage should be provided to avoid water accumulation.
- Wind uplift must be carefully checked in coastal and cyclone-prone regions.
- Snow load should be considered in hilly and cold regions.
- Connection design is equally important as member design.
- Deflection limits must satisfy serviceability criteria.
- Use relevant IS codes for accurate calculations.
Conclusion
Roof trusses must be designed carefully considering all possible loads such as dead load, live load, snow load and wind load. Each load affects the structural behavior differently and proper load combinations are essential for safe and economical design.
Wind load is generally the governing factor for long-span industrial roof trusses, whereas snow load becomes important in cold regions. Accurate estimation of these loads ensures structural stability, durability and long-term performance.
References
- IS 875 (Part 1): Code of Practice for Design Loads – Dead Loads
- IS 875 (Part 2): Imposed Loads
- IS 875 (Part 3): Wind Loads
- IS 875 (Part 4): Snow Loads
- Design of Steel Structures – N. Subramanian
- Limit State Design of Steel Structures – S.K. Duggal
Disclaimer
This article is intended for educational and informational purposes only. Actual structural design should always be carried out by qualified engineers using relevant codes, standards and project-specific conditions.
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