Permissible stress design (in USA construction more commonly called allowable stress design) is a design philosophy used by civil engineers. The designer ensures that the stresses developed in a structure due to service loads do not exceed the elastic limit.
What is the difference between factored and Unfactored load?
A factored load is a load multiplied by a certain factor designated by codes of practice to drermine the strength of a structural members such as reinforced concrete. Whereas the unfactored loads are used to meet the elastic or linear limit state (below the yield limit) of structural members.
What is allowable strength?
The allowable stress or allowable strength is the maximum stress (tensile, compressive or bending) that is allowed to be applied on a structural material. The allowable stresses are generally defined by building codes, and for steel, and aluminum is a fraction of their yield stress (strength):