The equation for allowable stress design is modified as under:
Load effects = Material resistive forces / Factor of safety
If factor of safety is applied by reducing the material strengths and service loads are considered as such without any increase, the design method is called allowable stress design or working stress design method.
This method is conservative and give safe results. The structure can be assumed to act perfectly within elastic range. Elastic analysis for loads and elastic behavior of material become studied in this procedure.
To separate it from the load and resistance factors (phi), this factor of safety is named as safety factor. There is a deviation between actual strength and nominal strength, actual loads and nominal loads. Safety factor is the factor that accounts for such deviation.
- Allowable strength
Allowable strength is defined as the nominal strength (Rn) divides by the safety factor.
- Required ASD strength (Ra)
It is the load effect obtained from the service loads without any additional factor.
The design equation for ASD becomes:
Ra is less than or equal to Rn/Safety factor
This method is included in the specifications as an alternate method of design. Allowable stress design method is replaced by LRFD for some structures where behavior is understood before collapse. For some important structures like atomic reactors and pre-stressed concrete structures, this method is still use by Engineers.
Reference : Steel structures by Zahid Ahmad siddiqi and Muhammad ashraf