Questions: Factor of Safety

Question 1

A cable has a breaking strength of 6,000 N and is designed to carry a maximum load of 2,000 N. What is its factor of safety?

• A. 0.33
• B. 2
• C. 3
• D. 6000

Answer: (C)

Factor of safety = strength / load = 6,000 / 2,000 = 3. The cable is three times as strong as the maximum expected load.

Question 2

An aircraft wing uses a lower factor of safety than an elevator cable because aircraft engineers are less concerned about safety.

• T. True
• F. False

Answer: False

Aircraft engineers are deeply concerned about safety. They use lower factors of safety because extra weight in aircraft severely impacts performance and fuel efficiency. They compensate by using more precise analysis, higher-quality materials, rigorous inspection schedules, and extensive testing to reduce uncertainty.

Question 3

Why do building codes specify minimum factors of safety rather than letting each engineer choose?

Model answer: Building codes set minimum factors of safety to ensure consistent public safety. Without mandated minimums, competitive pressure might lead some engineers or builders to cut margins to reduce costs, potentially creating dangerous structures. Codes represent collective engineering judgment about acceptable risk.

Building codes are informed by decades of engineering experience, failure investigations, and probabilistic analysis. They set the minimum acceptable safety margin for structures that the public uses and trusts. Individual engineers may choose higher factors than the code requires, but never lower.