DESIGN STRESS

http://www.ce.cmu.edu/~garrett/courses/12-100/LECTURES/strength-of-materials.pdf
Design stress is the maximum stress an engineer expexts under all loading. To address uncertainty,
1.allowable stress, make allowable design stress be less than yield stress and by a factor of safety (for example allowable stress = .66 x yield stress)
or
2.multiply all loads by a FOS and ensure design stress is less than yield.