Design elements for specific applications are frequently required by law, policy, or industry standards. For each structural member, a factor of safety of 2.0 is usually used in buildings. Buildings have a low value since the loads are well known and most constructions are redundant. The factor of safety should be higher for non-redundant structures such as bridges, where failure would result in death or serious injury. Aircraft have human occupants who are vulnerable to loss of control of the aircraft if not wearing a parachute. Therefore, aircraft require a factor of safety of at least 7.0 to provide reasonable protection in case of engine failure.
Factors of safety are used in many other areas of engineering to ensure that designs are safe before they are built. For example, when building roads, sidewalks, and similar public works projects, it is important to ensure that the materials used give adequate support for the expected load cases. The factors of safety used to determine how much material to use are called strength-to-weight ratios. They must be calculated so that more material will always be used to prevent failures due to overloads. For example, if a beam is designed to carry 1000 pounds per square foot, then it can be determined that it needs to be at least 6 times stronger than the standard 1-inch-by-1-inch by 8 feet piece of lumber used in buildings to avoid catastrophic failures.
The design factor is defined for an application (usually given ahead of time and frequently determined by regulatory building rules or policy) and is not a calculation. The safety factor is a ratio of maximal strength to intended load for the specific object specified. It represents the maximum level of safety that can be achieved with the product.
Safety factors are usually expressed as a number less than one, and they reduce the rated working force required for a task to avoid injury to workers or damage to property. For example, a safety factor of 0.5 means that if the working force exceeds 50% of the maximum allowable force, then injury will likely occur. Safety factors must be calculated for each application because their values vary depending on the type of work being done. Factors may also vary due to changing conditions such as temperature or humidity.
In general, steel structures have safety factors of 1.5 to 3.0 while concrete structures typically have a safety factor of 2.0 to 4.0. On average, wood buildings have safety factors of 1.5 to 1.8. Structures using timber as a main material element have safety factors lower than those using steel or concrete because they are generally under-designed compared with their corresponding structural elements.
Factors should be selected to provide adequate safety in all use situations.
The design and safety factors The design factor is defined for an application (usually given ahead of time and frequently determined by regulatory building rules or policy) and is not a calculation. For example, the safety factor for wallboard is given by the manufacturer as 10:1. This means that for every 10 pounds of force applied to a 2-by-4 foot section of wallboard, one would expect it to break.
There are two types of safety factors: functional and physical. Functional safety factors are needed when using a product in its intended manner. Physical safety factors are based on actual test results and can vary significantly depending on how the test is performed. For example, if a person were to try to lift a car off of someone who was trapped under it, then the physical safety factor for that product would be very high because it would take much more force than what would be necessary in an ordinary situation to cause an injury.
Safety factors are usually listed on product labels or advertising materials. They provide manufacturers with some protection against lawsuits if someone is injured using their products. A safety factor greater than 1 means that the product can handle more load than what would be considered normal operating conditions. Products with safety factors less than 1 should not be used in situations where they could be expected to fail.
Safety Factor Equation A structure's factor of safety must be larger than one in order for it to be considered safe. A factor of safety of one indicates that the structure's maximum strength or capacity is equal to its calculated design load. This indicates that if any more weight was added to the structure, it would fail. Factors less than one indicate that the structure can withstand a greater load than what is being used now; therefore, additional load could be safely carried.
Factors of safety are usually represented by the letter "f" followed by a number. The f number cannot be more than 100 unless otherwise stated. For example, an f number of 10 means that the structure can safely carry a load ten times its current load. An f number of 100 means that the structure can safely carry a load one hundred times its current load. There are two methods used to determine the factor of safety for a structure: calculation or experience. Calculation is done using structural analysis programs such as ANSYS or SolidWorks and involves determining the design load of the structure and then calculating the strength required of the material. Experience is determined by actually building a structure and observing how it performs under load. If the structure fails when loaded beyond its limit, then more load can be carried.
Factors of safety play an important role in structural engineering because they provide an indication of how strong a structure needs to be.
A partial safety factor is related to the design of the limit state. This design approach is widely employed in current structural engineering design. The design of acceptable stress is a safety consideration. A structural engineering design approach that has been supplanted by limit state design. That is, the designer establishes a limit state for an element based on its load capacity and its environment requirements, and then designs elements to meet or exceed that limit state.
Factors of safety are used to compare the relative risk of different hazards to which workers may be exposed. They indicate how much more likely it is that a worker will suffer harm from an assigned hazard. For example, if two different materials are used in construction, one of which has ten times the lung-irritating strength of the other, the worker would be ten times as likely to suffer damage to lungs due to exposure to the first material. Factors of safety are also used to compare the relative risk of different tasks to be performed by a single worker. If lifting 100 pounds requires using a mechanical lift while lifting 10 pounds does not, then the worker is ten times as likely to suffer injury while lifting the first set of weights.
Partial safety factors are used in determining the appropriate level of protection needed for various activities. For example, if there is a chance that heavy objects may be dropped on the worker, then a protective device such as a fall arrest system should be used when performing work on these objects.