Factors of Safety
First published Winter 2019
Factors of Safety
A factor of safety is an engineering principle that involves ensuring a design is much stronger than any anticipated loading involved. One challenge in engineering design is to maximize efficiency of products, machinery, and structures, which involves ensuring the product is not over-designed, as this will cost money as well as increase the weight of the product. Despite this need for optimal efficiency and not over-designing things, without a buffer between the strength of a product and the maximum anticipated load there is bound to be a failure, and potential injury or death.
Figure 1: Examples of Factors of Safety Fails
Why Do Engineers Need a Factor of Safety?
Design of Steel Structures (p. 22-23) discusses the need for an adequate factor of safety due to errors in the classical theory of elasticity, uncertainty of the rigidity of end connections, effects of stiffening elements, variation in mechanical properties of materials, ambient temperature, stress concentrations, residual stress, manufacturing tolerances, secondary stresses, faulty workmanship, and corrosion. This text also explains the minimum required factor of safety is 1.67 based on maximum allowable deviations in material properties. Elementary Mechanics of Deformable Bodies (p. 184) also discusses the need for a factor of safety due to the uncertainties of service conditions, uniformity of material, and design factors. This text states that typical factors of safety range from 2.0 to 2.5. M.F. Spotts in his Design of Machine Elements (p. 94-95) also calls for the use of a factor of safety to accommodate unknown or uncertain conditions such as the magnitude of forces that will actually act on a design, impact loading, corrosion, temperature variation, and variations and nonuniformity of materials. Some shapes are too complex to be evaluated effectively by traditional engineering calculations, making it difficult to obtain an accurate representation of stresses without using computer software like Finite Element Analysis. He also discusses the need to identify consequences of failure of a component. Spotts states on page 95 “When building one general product, the engineer usually does not think in terms of the factor of safety…Although the determination of suitable values for the factor of safety is a matter of great importance, the subject has been much neglected and is in an unsatisfactory state…Many failures are due to circumstances that the designer failed to consider.” Spotts also states, “A failure that involves only a little inconvenience or loss of time might be allowed more frequently than one involving large financial loss or human life.” Bassin, Brodsky, and Wolkoff in their text Statics and Strength of Materials call for a minimum factor of safety of 3 for varying loading conditions, and a factor of safety of 5 for shock loading situations (p. 436).
"Many failures are due to circumstances that the designer failed to consider." M.F. Spotts, Design of Machine Elements
Do Inadequate Factors of Safety Cause Failures?
YES!!! This happens often, because the forces that will act to reduce the strength of products and structures are many and some are always acting. All products, structures, and machinery have finite life spans, and manufacturers for years have been integrating this as part of their designs, some manufacturers choosing to design obsolescence into their designs so users will have to buy the same product over and over. This is most often seen on popular brand name products, that have already earned a reputation so that people will continue to purchase the product even despite greatly reduced quality. For products or machines that have the potential to cause harm, there is a need to prevent design inadequacies, and to ensure that if a failure does occur, it fails in “safe” or zero-energy state, which is the definition of the term “failsafe”.
When Does this Happen / What is the Relevance to Product Liability Cases?
The author has seen product, machinery, and structural failures due to a lack of an adequate factor of safety combined with not considering strength-reducing factors or identifying and mitigating all product hazards in a variety of accidents. These include ladder accidents, climbing gear failures, parts of structures not designed by engineers such as balconies, stairs, and handrails, machinery guards that break or do not fail safe, machinery guards intended to guard by distance that do not consider overreaching or people with longer reaches or smaller hands than normal, bolts, screws, welds, and other fasteners, machine wired or programmed wrong without checks and fail-safes, aging webbing, unintended loading of safety equipment, roll cages, auto roof structures, not accommodating the human factor, choosing safety limit switches that can be defeated, pressure cookers, misassembly of products, wood working machinery, failure to lock out or tag out all relevant machinery hazards, unforeseen situations with overhead doors, cross loading of carabiners, and failure to design structures to code including wind and snow loads.
The single biggest causal factor the author has seen with inadequate factors of safety is that not every building, bridge, balcony, ladder, safety gear, or product are designed by true engineers. An engineer by definition is a licensed professional engineer, which requires a minimum of a Bachelor of Science from an accredited school, years of experience, and passing two examinations which test for competency. Many people who tout the title of “engineer” have no formal education in engineering principles, much less the necessary experience and knowledge to become a professional engineer. While these individuals can have vast experience and knowledge, the lack of formal education can result in defective designs due to lack of awareness of concepts such as factor of safety.
When true engineers fail to incorporate an adequate factor of safety it is often due to mistakes in quantifying loads that will be acting on a product or structure, or failing to account for factors that either increase the load or decrease the strength including dynamic loading, corrosion, fatigue, or other factors found on the first page of this Clues. There is always the potential for succumbing to pressure from clients for cost savings or weight savings that can result in engineers pushing the limits of safe design resulting in using a lower factor of safety that will not always be adequate.
How We Can Help
At MASE, we can determine if an accident involves an inadequate factor of safety, or other design, manufacturing, or material defect. We offer full service mechanical engineering expert witness services. Call us at (855) 627-6273 / email info@mase.pro