Question

Which of the following relation about plastic moment is correct?

(a) Mp = Zp /fy

(b) Mp = Zp + fy

(c) Mp = Zpfy

(d) Mp = Zp – fy

The question was posed to me during an interview.

The origin of the question is Plastic Theory in portion Plastic and Local Buckling Behaviour of Steel of Design of Steel Structures

Answer 1

Correct choice is (c) Mp = Zpfy

The best I can explain: When every fibre of the section has a strain equal to or greater than εy=fy/Es , nominal moment strength is referred as plastic moment and is given by Mp = Zpfy , where Zp = ∫ydA is plastic section modulus and fy = yield stress.