Question

The check for design capacity of channel/I-section purlin is given by

(a) Mdz ≤ 1.2Zeyfy/γm0, Mdy ≤ 2.4Zezfy/γm0

(b) Mdz ≤ Zezfy/γm0, Mdy ≤ 1.2Zeyfy/γm0

(c) Mdz ≤ γfZeyfy/γm0, Mdy ≤ 1.2Zezfy/γm0

(d) Mdz ≤ 1.2Zezfy/γm0, Mdy ≤ γfZeyfy/γm0

This question was posed to me in an online quiz.

My enquiry is from Purlins in chapter Design of Beams of Design of Steel Structures

Answer 1

Correct option is (d) Mdz ≤ 1.2Zezfy/γm0, Mdy ≤ γfZeyfy/γm0

Easy explanation: The check for design capacity of channel/I-section purlin is given by Mdz ≤ 1.2Zezfy/γm0 , Mdy ≤ γyZeyfy/γm0 , where Mdz and Mdy are design moment capacity about Z and Y axes, respectively, Zez and Zey  are elastic section modulus about Z and Y axes, respectively and fy is yield stress of steel. Since in y-direction, the shape factor Zp/Ze will be greater than 1.2, γf is used instead of 1.2. If 1.2 is used the onset of yielding under unfactored loads cannot be prevented.