A factor of safety is the load carrying capacity of a system beyond what the system actually supports. Bridges, buildings, safety equipment and fall protection all start with a factor of safety.
Simply put, the safety factor is how much stronger a system is than required. The factor of safety is the backbone of all structures and safety equipment and originates with engineers. In the planning phase of all structures and safety equipment, engineers determine required overload from any object to remain safe in the event of an emergency.
For reliability, structures are typically built stronger than necessary. This is in case a structure experiences a heavier-than-expected load. This is a factor of safety. Ultimately, the amount of stress and overload a structure can handle comes down to the material used to build it.
Determining the factor of safety:
Engineers perform strength tests to determine how much weight a material can handle. Certain materials are more ductile than others, meaning they deform to pressure before breaking more so than others, like brittle materials. Brittle materials simply break once they meet the maximum force.
Ductile materials use the yield strength to determine the safety factor. Brittle materials use the ultimate strength.
Yield strength: Determines the safety factor until the start of deformation.
Ultimate strength: Determines the safety factor until failure.
Ductile materials often test the factor of safety against yield and ultimate strengths while brittle materials usually only calculate the ultimate safety factor since the yield and ultimate values are often so close.
The necessary factor of safety of any structure determines the materials used. If a structure has a high required factor of safety, then engineers use a ductile material to build it. The realized factor of safety determines the amount of material used.
Required (design factor): A constant value imposed by law or standard. A structure is required to meet this value at minimum.
Realized (safety factor): A calculated ratio of structural capacity to actual applied load. A measure of the reliability of a design.
Why is the safety factor important?
A factor of safety increases the safety of people and reduces the risk of failure of a product. When it comes to safety equipment and fall protection, the factor of safety is extremely important. If a structure fails there is a risk of injury and death as well as a company’s financial loss. The safety factor is higher when there is a possibility that a failure will result in these things.
An understanding of factors of safety will help those in the construction industry better comprehend OSHA standards such as 1910 Section 1C Design for System Components.
Anchorages to which personal fall arrest equipment is attached shall be capable of supporting at least 5,000 pounds per employee attached, or shall be designed, installed and used as part of a complete personal fall arrest system which maintains a safety factor of at least two, under the supervision of a Qualified Person.
Workers should understand the maximum load that any equipment can handle and know how to calculate for it based on safety factors. Although a factor of safety exists for safety in the event a structure experiences a larger load than expected, workers should not try their luck with the limits of safety equipment.
Keep in mind:
Safety factors do not imply that a system is safe and free from accidents. Parts to a whole may all have the same factory of safety, but that does not give the system as a whole the same FoS. Likewise, stress to one part of whole can easily change the stress distribution to the whole itself. Remember, a factor of safety is a good tool to determine how to properly install and use equipment, but many other factors go into determining safety.