Factor of safety

See also: safety factor (plasma physics)

Factor of safety (FoS), also known as safety factor (SF), is a term describing the structural capacity of a system beyond the expected loads or actual loads. Essentially, how much stronger the system is than it usually needs to be for an intended load. Safety factors are often calculated using detailed analysis because comprehensive testing is impractical on many projects, such as bridges and buildings, but the structure's ability to carry load must be determined to a reasonable accuracy.

Many systems are purposefully built much stronger than needed for normal usage to allow for emergency situations, unexpected loads, misuse, or degradation.

Definition

There are two distinct uses of the factor of safety: One as a ratio of absolute strength (structural capacity) to actual applied load. This is a measure of the reliability of a particular design. The other use of FoS is a constant value imposed by law, standard, specification, contract or custom to which a structure must conform or exceed.

The first sense (a calculated value) is generally referred to as a factor of safety or, to be explicit, a realized factor of safety, and the second sense (a required value) as a design factor, design factor of safety or required factor of safety, but usage is inconsistent and confusing. The cause of much confusion is that reference books and standards agencies use the term factor of safety differently. Design codes and structural and mechanical engineering textbooks often use the term to mean the fraction of total structural capability over that needed^[1][2][3] (first sense). Many undergraduate Strength of Materials books use "Factor of Safety" as a constant value intended to be a minimum target for design^[4][5][6] (second sense).

factor of safety= ultimate stress/working stress

Calculation

There are several ways to compare the factor of safety for structures. All the different calculations fundamentally measure the same thing: how much extra load beyond what is intended a structure will actually take (or be required to withstand). The difference between the methods is the way in which the values are calculated and compared. Safety factor values can be thought of as a standardized way for comparing strength and reliability between systems.

Design factor and safety factor

The difference between the safety factor and design factor (design safety factor) is as follows: The safety factor is how much the designed part actually will be able to withstand. The design factor is what the item is required to be able to withstand. The design factor is defined for an application (generally provided in advance and often set by regulatory code or policy) and is not an actual calculation, the safety factor is a ratio of maximum strength to intended load for the actual item that was designed.

This may sound similar, but consider this: Say a beam in a structure is required to have a design factor of 3. The engineer chose a beam that will be able to withstand 10 times the load. The design factor is still 3, because it is the requirement that must be met, the beam just happens to exceed the requirement and its safety factor is 10. The safety factor should always meet or exceed the required design factor or the design is not adequate. Meeting the required design factor exactly implies that the design meets the minimum allowable strength. A high safety factor well over the required design factor sometimes implies "overengineering" which can result in excessive weight and/or cost. In colloquial use the term, "required safety factor" is functionally equivalent to the design factor.

For ductile materials (e.g. most metals), it is often required that the factor of safety be checked against both yield and ultimate strengths. The yield calculation will determine the safety factor until the part starts to plastically deform. The ultimate calculation will determine the safety factor until failure. On brittle materials these values are often so close as to be indistinguishable, so is it usually acceptable to only calculate the ultimate safety factor.

The use of a factor of safety does not imply that an item, structure, or design is "safe". Many quality assurance, engineering design, manufacturing, installation, and end-use factors may influence whether or not something is safe in any particular situation.

$\text{Factor of Safety}=\frac{\text{Material Strength}}{\text{Design Load}}$

• Design load being the maximum load the part should ever see in service.

Margin of safety

Many government agencies and industries (such as aerospace) 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. There are two separate definitions for the margin of safety so care is needed to determine which is being used for a given application. One usage of M.S. is as a measure of capacity like FoS. The other usage of M.S. is as a measure of satisfying design requirements (requirement verification). Margin of safety can be conceptualized (along with the reserve factor explained below) to represent how much of the structure's total capacity is held "in reserve" during loading.

M.S. as a measure of structural capacity: This definition of margin of safety commonly seen in textbooks^[7][8] basically says that if the part is loaded to the maximum load it should ever see in service, how many more loads of the same force can it withstand before failing. In effect, this is a measure of excess capacity. If the margin is 0, the part will not take any additional load before it fails, if it is negative the part will fail before reaching its design load in service. If the margin is 1, it can withstand one additional load of equal force to the maximum load it was designed to support (i.e. twice the design load).

$\text{Margin of Safety}=\frac{\text{Failure Load}}{\text{Design Load}}-1$

Margin of Safety = Factor of Safety − 1

M.S. as a measure of requirement verification: Many agencies such as NASA^[9] and AIAA^[10] define the margin of safety including the design factor, in other words, the margin of safety is calculated after applying the design factor. In the case of a margin of 0, the part is at exactly the required strength (the safety factor would equal the design factor). If there is a part with a required design factor of 3 and a margin of 1, the part would have a safety factor of 6 (capable of supporting two loads equal to its design factor of 3, supporting six times the design load before failure). A margin of 0 would mean the part would pass with a safety factor of 3. If the margin is less than 0 in this definition, although the part will not necessarily fail, the design requirement has not been met. A convenience of this usage is that for all applications, a margin of 0 or higher is passing, one does not need to know application details or compare against requirements, just glancing at the margin calculation tells whether the design passes or not.

Design Safety Factor = [Provided as requirement]

$\text{Margin of Safety}=\frac{\text{Failure Load}}{\text{Design Load*Design Safety Factor}}-1$

$\text{Margin of Safety}=\frac{\text{Realized Factor of Safety}}{\text{Design Safety Factor}}-1$

For a successful design, the realized Safety Factor must always equal or exceed the required Safety Factor (Design Factor) so the Margin of Safety is greater than or equal to zero. The Margin of Safety is sometimes, but infrequently, used as a percentage, i.e., a 0.50 M.S is equivalent to a 50% M.S. When a design satisfies this test it is said to have a "positive margin," and, conversely, a “negative margin” when it does not.

Reserve factor

A measure of strength frequently used in Europe is the Reserve Factor (RF). With the strength and applied loads expressed in the same units, the Reserve Factor is defined as:

RF = Proof Strength / Proof Load
RF = Ultimate Strength / Ultimate Load

The applied loads have any factors, including factors of safety applied.

Choosing design factors

Appropriate design factors are based on several considerations, such as the accuracy of predictions on the imposed loads, strength, wear estimates, and the environmental effects to which the product will be exposed in service; the consequences of engineering failure; and the cost of over-engineering the component to achieve that factor of safety. For example, components whose failure could result in substantial financial loss, serious injury, or death may use a safety factor of four or higher (often ten). Non-critical components generally might have a design factor of two. Risk analysis, failure mode and effects analysis, and other tools are commonly used. Design factors for specific applications are often mandated by law, policy, or industry standards.

Buildings commonly use a factor of safety of 2.0 for each structural member. The value for buildings is relatively low because the loads are well understood and most structures are redundant. Pressure vessels use 3.5 to 4.0, automobiles use 3.0, and aircraft and spacecraft use 1.2 to 3.0 depending on the application and materials. Ductile, metallic materials tend to use the lower value while brittle materials use the higher values. The field of aerospace engineering uses generally lower design factors because the costs associated with structural weight are high (e.g. an aircraft with an overall safety factor of 5 would probably be too heavy to get off the ground). This low design factor is why aerospace parts and materials are subject to very stringent quality control and strict preventative maintenance schedules to help ensure reliability. The usually applied Safety Factor is 1.5, but for pressurized fuselage it is 2.0, and for main landing gear structures it is often 1.25.

In aerospace, there are additional requirements. Before and up to Limit Load, the structure may not have failed nor have permanent deformation. In excess of this, plastic deformation is allowed, but failure is not. Reaching the Ultimate Load (usually the Limit Load multiplied by the Safety Factor), the structure is allowed to fail. Civilian aircraft structures are required to meet both Limit Load and Ultimate Load criteria.

For loading that is cyclical, repetitive, or fluctuating, it is important to consider the possibility of metal fatigue when choosing factor of safety. A cyclic load well below a material's yield strength can cause failure if it is repeated enough.

See also

• Limit state design
• Redundancy (total quality management)
• Probabilistic design
• Sacrificial part
• Statistical interference
• Verification and validation

Notes

1. ^ Young, W.: Roark's Formulas for Stress and Strain, 6th edition. McGraw-Hill, 1989.
2. ^ Shigley, J and Mischke, C: Standard Handbook of Machine Design, page 2-15. McGraw-Hill, 1986.
3. ^ ASME BTH-1: Design of Below-the-Hook Lifting Devices, Section 1-5, ASME, 2005.
4. ^ Beer, F and Johnson, R: Mechanics of Materials, second edition. McGraw-Hill,1992.
5. ^ Timoshenko, S: Strength of Materials, Volume 1. Krieger, 1958.
6. ^ Buchanan, G: Mechanics of Materials, Page 55. Holt, Reinhart, and Watson,1988.
7. ^ Burr, A and Cheatham, J: Mechanical Design and Analysis, 2nd edition, section 5.2. Prentice-Hall, 1995.
8. ^ Juvinall, R: Stress, Strain, and Strength, section 14.13, Page 295. McGraw-Hill, 1967.
9. ^ NASA-STD-5001: Structural Design and Test Factors for Spaceflight Hardware, section 3. NASA, 2008.
10. ^ AIAA S-110: Space Systems - Structures, Structural Components, and Structural Assemblies, section 4.2. AIAA, 2005.

Further reading

• Lalanne, C., Specification Development - 2nd Ed., ISTE-Wiley, 2009