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Whenever we come across designing and engineering any component that potentially carries a load; be it a bridge, foundation, or even a desk with a LOT of books on it ;P, we come across the term Factor of Safety of FOS. What exactly is FOS? Why does it matter? What happens if you neglect this FOS? Let's get to know about it!
Introduction to FOS
The factor of safety is defined as the ratio of ultimate stress of the component material to the working stress. It denotes the additional strength of the component than the required strength to carry that load. It tells us how much stronger a system is or needs to be for an intended load. It is also called Safety Factor.
The factor of safety is the crux of all structures and safety equipment design that the engineer needs to be aware of. In the planning phase of all structures and safety equipment, engineers determine the required overload from any object to remain safe in the event of an emergency. From this, they come with the factor which is known as the Factor of Safety.
How to Determine FOS
There are several ways to compare the factor of safety for structures, but fundamentally measure the same thing: how much extra load a structure will actually take or be required to withstand. Let us look at the most widely used formula and understand how to calculate FOS.
Mathematically factor of safety can be expressed as:
Factor of safety=Ultimate Load (Strength)/Allowable Load (Stress)
From the above equation, the allowable stress is always less than the ultimate failure stress. Hence, the factor of safety is always greater than 1. The ultimate stress for brittle material is considered as ultimate tensile strength and for ductile material is considered as yield strength. Also, as the equation for the factor of safety is the ratio of two stress or load values, it is dimensionless. The difference between FOS and 1 is known as the Margin of safety.
It can be summarized as M.S. = FOS - 1
Many government agencies and industries require the use of a Margin of Safety (MoS or M.S.) to describe the ratio of the strength of the structure to the requirements. M.S. as a measure of structural capability. In short :
1. If the margin is 0, the part will not take any additional load before it fails
2. If the margin is less than 0, the part will fail before reaching its design load in service
3. If the margin is 1, it can withstand twice the load.
Now let us discuss the term Design Factor that is often used along with the term FOS or safety factor and understand the difference.
By definition, a design factor or design safety factor is a margin applied to a load or structural strength to account for uncertainty as to the load, the structural properties, or both. A bit confused? XO Lemme help with an example.
Imagine you need to design a structure that has to withstand some load in the future. You think of the instance when it will be "used" for the first time i.e. withstands the load for the first time. Then you would think of all the future instances where it has to be loaded. Now would you consider Safety Factor or Design Factor? Which term is to be used when? Lemme explain. The difference between the safety factor and design factor is as follows: The safety factor, or yield stress, is how much the designed part actually will be able to withstand (first "use" from above). The design factor, or working stress, is what the item is required to be able to withstand (second "use"). So I hope you got the gist of it :)
Factors Affecting FOS
Some of the major factors that can affect FOS are:
The properties of the material used
The changes in its intrinsic properties over the time
Materials strength
Type of loading
Why is FOS Important?
When designing and engineering a product that interacts with humans or any living being, safety and reducing all risks matters most over anything else. 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 may occur in those things.
Hence, one must check the safety factor of a product or structure that is employed in the community. Here, the FOS becomes a critical need, and is very important that the personnel handling and working with these should understand the same.
Summary
The factor of safety is defined as the ratio of ultimate stress of the component material to the working stress. It is also called Safety Factor
The factor of safety=Ultimate Load (Strength)/Allowable Load (Stress)
The difference between FOS and 1 is known as the Margin of safety.
Design factor means the ratio of the ultimate failure strength of a member to the actual working stress.
A factor of safety increases the safety of people and reduces the risk of failure of a product.
So that's all from my side regarding Factor of Safety. Hope you have now understood what FOS is and why it is important while designing any products. Till next time with more interesting and new subjects to talk about, stay safe and take care. Ciao!