.MECHANICS AND STRUCTURES fundamental

 Safety Factor

In applying Equation 10-1, a safety factor or factor of safety is often introduced. A factor of safety makes an allowance for many unknowns related to materials, assembly, or use. Unknowns may be inaccurate estimates of real loads or differences between actual materials and those tested in laboratories. They may be changes in area resulting from corrosion, wear, manufacturing, assembly, or use. They may be irregularities or nonhomogeneity in materials. The unknowns may include suddenly applied, dynamic loads. Technically, a safety factor (SF) refers to the ratio of a failure-producing load to the maximum safe stress a material may carry. The maximum safe stress is often called the allowable stress. Failure may not be by rupture or fracture. A failure could be a change in area or properties of the material that affect the load-carrying capacity and its safety. For structural steel, the allowable stress is derived at the yield point in a stress-strain (load per unit area-unit elongation or deformation) diagram from laboratory tests. For other materials, the allowable stress is based on the ultimate strength from similar tests. Refer to references on strength of materials for more details about test methods and stress-strain diagrams. There are many ways to determine a safety factor SF. A common way is

Safety factors are often based on experience with the material in question and many of its

properties and applications. Safety factors should include analysis of risk and potential

consequences of failure. Different safety factors may be appropriate for different applica-

tions and use conditions for the same material. Usually, the safety factor will be higher

for materials with less homogeneity. Safety factors are higher for sudden, dynamic loads.

Designs that anticipate reductions in a cross-sectional area of a component through wear

or some other change in properties may incorporate higher safety factors. Some safety

factors are specified in regulations and standards.

In safety engineering, one must be very careful in using data from tables dealing

with strength of materials. Some tables include a factor of safety, whereas others do not.

Using strength data in error from a table for which a factor of safety is not included poses

a significant risk. The factor of safety incorporated in a table also must be applied care-

fully to ensure that the assumed safety factor is suitable for the actual use conditions.

When load and strength tables are intended for field use, they should incorporate

appropriate safety factors. Field personnel who have to perform computations and complex

interpretations of data tables are more likely to make errors as the number of steps in using

a table increases. Field tables should reflect decision tasks and situations expected.