23-08-2012, 11:26 AM
Factor of safety (FoS)
[attachment=32478]
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. It is important to keep track of which of the two definitions is being used. The cause of much confusion is that various reference books and standards agencies use the factor of safety definitions and terms differently. Design codes and structural and mechanical engineering textbooks often use "Factor of Safety" 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 as a minimum target for design[4][5][6] (second sense).
This may sound similar, but consider this: Say a beam in a structure is required to have a safety factor of 3. The engineer chose a beam that will be able to withstand 10 times the load. The required safety 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 realized safety factor should always meet or exceed the required safety factor or the design is not adequate. Meeting the required safety 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.
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.
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.
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 (first "sense" from above). The design factor is what the item is required to be able to withstand (second "sense"). 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.
[attachment=32478]
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. It is important to keep track of which of the two definitions is being used. The cause of much confusion is that various reference books and standards agencies use the factor of safety definitions and terms differently. Design codes and structural and mechanical engineering textbooks often use "Factor of Safety" 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 as a minimum target for design[4][5][6] (second sense).
This may sound similar, but consider this: Say a beam in a structure is required to have a safety factor of 3. The engineer chose a beam that will be able to withstand 10 times the load. The required safety 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 realized safety factor should always meet or exceed the required safety factor or the design is not adequate. Meeting the required safety 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.
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.
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.
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 (first "sense" from above). The design factor is what the item is required to be able to withstand (second "sense"). 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.