Factor of Safety: Ratio for Safety in Design and Use

Key Takeaways

• Factor of Safety (FoS) is a critical safety measure that determines how much additional load a structure, product, or system can handle beyond its normal operating conditions.

• Industry-accepted safety factors vary as buildings use around 2.0, pressure vessels 3.5 to 4.0, automobiles about 3.0, and aerospace applications 1.2 to 4.0, depending on weight.

• Basic formula differs by material type, with maximum stress used for brittle materials and ultimate stress for ductile ones when calculating FoS.

What is Factor of Safety?

The Factor of Safety (FoS) exists as a safety measure that is designed to make a product, a system, or a structure safe. The higher the number of FoS, the safer the product or structure is. An FoS of 1 indicates that a structure or component will fail immediately when the design load is reached and cannot support any extra load. Structures or components with FoS less than one are not acceptable. If the consequences of failure are severe, such as loss of life or physical injury, a higher FoS will be required either by design or by law.

Simply put, structures or devices must be able to bear more weight than they would under normal use to be safer. FoS indicates by what factor the design is safe.

Typical Overall Factors of Safety

Below is a table of typical FoS of equipment, provided by Engineering ToolBox:

General Recommendations

Factor of Safety Formula

All of the calculations basically measure the same thing—how much additional stress beyond the designed load a structure can withstand. A simple formula for factor of safety is:

• Brittle materials – factor of safety = maximum stress / working or design stress

• Ductile materials –  factor of safety = ultimate stress / working or design stress

Factor of Safety Formula

Many Factor of Safety calculators are available online for calculating FoS values when the maximum stress and allowable stress values are known.

Importance of Safety Factor

Factor of Safety varies according to the situation. Systems are intentionally designed to be way stronger than they need to be for normal settings. This increases the likelihood that they will continue functioning even under extreme conditions such as emergency situations, added loads, overuse, or degradation caused by wear and tear. Moreover, below are more reasons why using FoS in design is important:

• Maintains the structure’s functionality for the future while providing additional safety for current use

• Prevents damage to property, workers, and machines

• Provides protection for unforeseeable risks that may occur while using a product or service

• Reduces the chances that a product may fail

Selection of Factors of Safety

According to A Textbook of Machine Design by R.S.Khurmi and J.K.Gupta, the selection of the appropriate Factor of Safety to be used in the design of any mechanical system is based on a variety of considerations, including the following:

• Material—ductile or brittle; ductile materials use yield strength; brittle materials use ultimate strength.

  • Yield strength—determines the FoS until the beginning of deformation.

  • Ultimate strength—determines the FoS until failure.

• Process of manufacturing

• Type of stress

• General service conditions

• Shape of the parts

Examples

The following are two instances of how the Factor of Safety is used:

Pressure Vessels

Boilers and pressure vessels, as well as nuclear power plant systems, are subject to the American Society of Mechanical Engineers (ASME) International Boiler and Pressure Vessel Code safety guidelines, which control the design, manufacturing, and inspection of boilers and pressure vessels during the construction process. By their very nature, pressure vessels are potentially hazardous. It necessitates the addition of safety factors to protect against failure—uncertainty in design, materials used, manufacture, inspection, and operation.

Personal Fall Arrest Systems (PFAS)

Personal Fall Arrest Systems (PFAS) and other fall protection equipment must be built with a high safety factor. The Occupational Safety and Health Administration (OSHA) standard 1915.159 outlines the criteria for connectors and anchorage to be capable of sustaining a minimum tensile load of 3,000 to 5,000 pounds (22.24 Kn) per employee, and a requirement of a complete personal fall arrest system which maintains a safety factor of at least 2. If the equipment is going to be used in tough circumstances, it may require an even higher safety factor.