What is a Factor of Safety?
The factor of safety definition is considered by the following parameters:
- The factor of safety is a structure's absolute strength (structural capability) ratio to the actual applied load. FoS measures the reliability of a particular design.
- A constant required value is imposed by standard specification to which a structure must exceed. This can be referred to as a design factor, design factor of safety, or required factor of safety.
- When the factor of safety is equal to one, it means that the ultimate stress is equal to the working stress; therefore, the body can only support load up to the actual load and no more before failing.
Importance of Factor of Safety
Importance of Factor of Safety to assure the structural designing does not occur any unexpected failure or presence of deformation or defect. The smaller the Factor of Safety, the higher chances there were for the design to be a failure. Resulting in an uneconomical and nonfunctional design.
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Formula For Factor of Safety
The factor of safety is defined as the ratio of ultimate stress of the component material to the working stress. Mathematically, the Factor of safety is the ratio of material strength to allowable stress. The factor of the safety equation depends on the type of material:
Factor of safety = Ultimate strength/Working stress -for brittle material (concrete)
Factor of safety = Yield strength/Working stress -for ductile material (steel)
Example:
FoS for steel = Yield strength (fy)/Working stress (0.87 fy)= 1.15 (approximately)
FoS for concrete = Yield strength (fck)/Working stress (0.67 fck / 1.5)= 3 (approximately)
(∵ 2 times factor of safety is taken for concrete as it has least quality control while preparing the concrete)
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Factors Affecting Factor of Safety (FoS)
The structure's safety depends on two principal design factors, load and material strength, which are not a function of each other. Hence, two different factors, one for load and the other for material strength, are used. Each of the two safety factors contributes partially to safety; they are termed a partial safety factor.
As per Clause 36.4. 2 of IS 456, when assessing the strength and serviceability criteria of the structures or structural members employed in a limit state.
Material | Limit state method | ||
Collapse | Deflection | Cracking | |
Steel | 1.15 | 1.0 | 1.0 |
Concrete | 1.5 | 1.0 | 1.3 |
∴ The factor of safety for steel is lower as compared to concrete. Because concrete is a brittle material and relatively less reliable than steel and steel is manufactured in industry. So, In steel, the quality control is better than in concrete (as it is prepared at the site with different atmospheric conditions).
Examples of Factor of Safety
Here are a few examples of factor of safety of a few elements which can be used while design consideration:
| Elements | Factor of Safety |
| Bolts | 8.5 |
| Structural Members in building services | 2 |
| Automobiles | 3 |
| Structural steel work in buildings | 4.0 to 6.0 |
| Engine components | 6.0 to 8.0 |
| Spring, large heavy-duty | 4.5 |
| Pressure vessels | 3.5 – 6 |
| Aircraft components | 1.5 to 2.5 |
| Turbine components – rotating | 2.0 to 3.0 |
| Pressure Vessel | 3.5 to 4 |
| Brittle Material | 1.0 to 6.0 |
| Boilers | 3.5 to 6 |
| Cast-iron wheels | 20 |
| Turbine components static | 6.0 to 8.0 |
| Impact Forces | 3.0 to 6.0 |
| Heavy duty shafting | 10.0 to 12.0 |
| Lifting equipment – hooks .. | 8.0 to 9.0 |
| Structural steel work in bridges | 5.0 to 7.0 |
| Aircraft and Spacecraft | 1.2 to 3.0 |
| Wire ropes | 8.0 to 9.0 |
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