045 Factor of Safety

The design of a system beyond expected loads to offset the effects of unknown variables and prevent failure.

• Increasing the factor of safety of a design is a reliable method of preventing catastrophic failure.

• The size of the factor of safety should correspond to the level of uncertainty in the design parameters. High uncertainty requires a high factor of safety.

• New designs use high factors of safety because uncertainty is high. The focus is on survivability.

• Tried-and-true designs use low factors of safety because more is known. The focus shifts to cost.

• Use factors of safety in proportion to your knowledge of the design parameters and the severity of the consequences of failure.

See Also Design by Committee • Errors • Modularity Structural Forms • Weakest Link

The O-rings of Challenger’s solid rocket boosters had a factor of safety of three—inadequate for low temperature conditions. Catastrophic failure occurred shortly after launch in 1986.