Question

Which of the following is not true about sag rods?

(a) sag rods are provided at midway or at one-third points between roof trusses

(b) these rods reduce the moment  Myy

(c) these rods increase the moment  Myy

(d) these rods result in smaller purlin sections

I have been asked this question by my college professor while I was bunking the class.

Question is taken from Purlins topic in section Design of Beams of Design of Steel Structures

Answer 1

The correct choice is (c) these rods increase the moment  Myy

To explain I would say: Sag rods are provided midway or at one-third points between roof trusses to take up the sag in the direction of sloping roof by purlins. These rods provide lateral support with resprct to y-axis bending. Consequently, moment Myy is reduced and thereby result in smaller purlin section. they are useful in keeping the purlins in proper alignment during erection until roofing is installed and connected to purlins.