Table of Contents
What Is FoS?
Factor of safety (FoS) is the ratio of a structure's strength to the maximum expected stress. A FoS of 2.0 means the structure can handle twice the expected load before failure. Engineers use safety factors to account for uncertainties in material properties, loading conditions, manufacturing quality, and environmental degradation over the service life.
Too low a FoS risks catastrophic failure; too high wastes material, increases weight, and adds cost. Selecting the appropriate FoS requires balancing safety requirements against economic and performance constraints for the specific application.
Formula
Typical Values
| Application | FoS | Reason |
|---|---|---|
| Aircraft | 1.5 | Weight critical |
| Bridges | 2-3 | Public safety |
| Buildings | 2-4 | Seismic/wind |
| Pressure vessels | 3.5-4 | Catastrophic risk |
| Elevators | 8-12 | Life safety |
FAQ
Why are aircraft FoS so low?
Every kilogram of extra structure reduces payload and increases fuel burn. Aerospace uses precise analysis, rigorous testing, and mandatory inspections to ensure safety with FoS of 1.5. Loads are well-characterized through extensive flight testing and simulation.
What is margin of safety?
MoS = FoS - 1. NASA commonly uses MoS. Positive means adequate; negative means failure. MoS of 0.5 means 50% more strength than needed.
Should I use ultimate or yield?
Both. Yield FoS (1.5-2) prevents permanent deformation. Ultimate FoS (2-4) prevents fracture. Many codes require satisfying both simultaneously.