LRFD for Geotech – Note 2

5.0  Comparing Factor of Safety and Reliability Index

The factors of safety commonly used in ASD range from 1.5 to 3.0.  Considering that the reliability indices shown in Figure 2.1 or Figure 4.1 appear to be in the same range as values of commonly used factors of safety, it is often tempting to compare the two directly.  This temptation should be avoided because the two values are based on very different assumptions.  For example, the concept of factor of safety is subjective while the concept of reliability index is based on the theory of probability where the data are statistically determined.  Thus, any similarity between values of FS and β is purely coincidental.