Partial Safety Factors in Steel Structures
Partial safety factors are one of the most important concepts in Limit State Design (LSD) of steel structures. These factors are introduced to account for uncertainties in loads, material strengths, workmanship, fabrication quality, and analysis assumptions. The use of partial safety factors ensures that structures remain safe, serviceable, and economical throughout their design life.
According to IS 800:2007, partial safety factors are broadly classified into:
- Partial Safety Factors for Loads (Load Factors)
- Partial Safety Factors for Strength or Resistance
1. Partial Safety Factors for Loads (Load Factors)
Load factors are used to increase the characteristic loads to account for possible variations and uncertainties during the life of the structure.
Load Combinations as per IS 800:2007
| Combination | Limit State of Strength | Limit State of Serviceability | |||||||
|---|---|---|---|---|---|---|---|---|---|
| DL | LL Leading | LL Accompanying | WL / EL | AL | DL | LL Leading | LL Accompanying | WL / EL | |
| DL + LL + CL | 1.5 | 1.5 | 1.05 | — | — | 1.0 | 1.0 | 1.0 | — |
| DL + LL + CL + WL/EL | 1.2 | 1.2 | 1.05 / 0.53 | 1.2 / 0.6 | — | 1.0 | 0.8 | 0.8 | 0.8 |
| DL + WL/EL | 1.5 (0.9)* | — | — | 1.5 | — | 1.0 | — | — | 1.0 |
| DL + ER | 1.2 (0.9)* | 1.2 | — | — | — | — | — | — | — |
| DL + LL + AL | 1.0 | 0.35 | 0.35 | — | 1.0 | — | — | — | — |
Abbreviations Used
- DL = Dead Load
- LL = Imposed Load (Live Load)
- CL = Crane Load
- SL = Snow Load
- WL = Wind Load
- EL = Earthquake Load
- AL = Accidental Load
- ER = Erection Load
* The value 0.9 is considered when stability against overturning or stress reversal is critical.
Explanation of Important Load Combinations
a) DL + LL + CL
This combination is commonly used in industrial buildings where crane loads act along with dead and live loads.
b) DL + LL + WL/EL
This combination considers the effect of wind or earthquake forces together with dead and live loads. It is critical for high-rise buildings, towers, bridges, and industrial structures.
c) DL + WL/EL
This combination is important for uplift and overturning checks where live loads may not be present.
d) DL + ER
This load combination is mainly used during construction and erection stages of steel structures.
e) DL + LL + AL
This combination considers accidental loads such as blast loads, vehicle impact, or accidental explosions.
2. Partial Safety Factors for Strength or Resistance
Resistance factors are used to reduce material strength considering uncertainties in fabrication, workmanship, material properties, and construction quality.
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| Sl. No. | Definition | Partial Safety Factor |
|---|---|---|
| 1 | Resistance governed by yielding (γm0) | 1.10 |
| 2 | Resistance governed by buckling (γm0) | 1.10 |
| 3 | Resistance governed by ultimate stress (γm1) | 1.25 |
Partial Safety Factors for Connections
| Connection Type | Shop Fabrication | Field Fabrication |
|---|---|---|
| Bolts – Friction Type (γmf) | 1.25 | 1.25 |
| Bolts – Bearing Type (γmb) | 1.25 | 1.25 |
| Rivets (γmr) | 1.25 | 1.25 |
| Welds (γmw) | 1.25 | 1.50 |
Importance of Partial Safety Factors
- Provide adequate safety against failure
- Consider uncertainties in loading conditions
- Account for material variability
- Ensure reliability of structures
- Improve durability and service life
- Help achieve economical structural design
Why Different Factors are Used?
Different safety factors are used because uncertainties vary for different loads and resistance conditions. For example:
- Live loads are more uncertain than dead loads.
- Field welds may have lower quality control compared to shop welds.
- Earthquake and wind loads are probabilistic in nature.
- Buckling failures are more sensitive to imperfections.
Advantages of Limit State Design Using Partial Safety Factors
- Provides balanced safety and economy
- Ensures rational design approach
- Reduces chances of structural collapse
- Allows efficient utilization of materials
- Suitable for modern structural analysis methods
Conclusion
Partial safety factors form the foundation of modern Limit State Design philosophy. They account for uncertainties in loads, materials, fabrication, and structural behavior to ensure safe and reliable structures. Understanding these factors is essential for every civil and structural engineer involved in steel design.
References
- IS 800:2007 – General Construction in Steel
- IS 875 – Code of Practice for Design Loads
- IS 1893 – Criteria for Earthquake Resistant Design of Structures
- Limit State Design of Steel Structures by S.K. Duggal
- Design of Steel Structures by N. Subramanian
Disclaimer
This article is intended for educational purposes only. Engineers should refer to the latest Indian Standards, project specifications, and professional engineering practices before applying any design values in actual projects.
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