IntermediateTopic 51 of 106Engineering Mechanics
Factor of safety
Ratio of failure load to allowable load; accounts for uncertainty.
In depth
FS = failure load / allowable load (or strength / working stress). Chosen from uncertainty in loads, materials, analysis and consequences of failure. Typical values: 1.5–3 for steel structures, higher for brittle materials and life-safety systems.
Key points
- Codes specify load and resistance factors
- Higher FS means heavier designs
- Reliability-based design is the modern approach
ExampleCranes and lifts commonly use factors of 4 to 10.
The factor of safety covers uncertainty in loads, materials and workmanship, so it keeps a design safe when reality differs from calculations.
What is Factor of safety?
Ratio of failure load to allowable load; accounts for uncertainty.
Why is it important to understand factor of safety?
The factor of safety covers uncertainty in loads, materials and workmanship, so it keeps a design safe when reality differs from calculations.
How do I choose a factor of safety?
Depends on the consequences of failure, the confidence in loads and materials, and the applicable code. High-consequence designs use larger values.
Does a large safety factor always mean a better design?
No. An excessive factor adds weight and cost and can hide problems such as fatigue, so the factor should be justified, not simply large.
How is Factor of safety used in practice?
Typical applications include choosing an allowable stress for a crane hook, setting the factor for a lifting sling used by many operators and reviewing the margin on a pressure vessel designed to a code.
What should I learn before Factor of safety?
Study "Thermal stress" first (topic 50 of 106 in Engineering Mechanics). It covers the groundwork this topic builds on.
What are the key points of Factor of safety?
Codes specify load and resistance factors. Higher FS means heavier designs. Reliability-based design is the modern approach.
Can you give an example of Factor of safety?
Cranes and lifts commonly use factors of 4 to 10.
What comes after Factor of safety?
The next topic is "Shear and torsion", which builds directly on this one.
How difficult is Factor of safety?
It is classed as Intermediate in Engineering Mechanics (topic 51 of 106). It is an intermediate topic. Work through the beginner topics in Engineering Mechanics first.