Question

Which of the following relation is correct for I- section of width b and depth d subjected to axial force N together with moment M?

(a) (N/Np) – (1/1.18)(M/Mp) ≤ 1, when N/Np > 0.15

(b) (N/Np) – (1/1.18)(M/Mp) ≤ 1, when N/Np < 0.15

(c) (N/Np) + (1/1.18)(M/Mp) ≤ 1, when N/Np < 0.15

(d) (N/Np) + (1/1.18)(M/Mp) ≤ 1, when N/Np > 0.15

The question was asked during an online exam.

This intriguing question originated from Plastic Design of Portal Frames, Effect of Axial and Shear force on Plastic Moment Capacity in section Plastic and Local Buckling Behaviour of Steel of Design of Steel Structures

Answer 1

Correct answer is (d) (N/Np) + (1/1.18)(M/Mp) ≤ 1, when N/Np > 0.15

To elaborate: For I- section subjected to axial force N together with moment M,

(N/Np) + (1/1.18)(M/Mp) ≤ 1, when N/Np > 0.15

M = Mp, when N/Np < 0.15 .