Design of Steel Structures Questions and Answers – Purlins

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This set of Design of Steel Structures Multiple Choice Questions & Answers (MCQs) focuses on “Purlins”.

1. What are purlins?
a) beams provided in foundation
b) beams provided above openings
c) beams provided over trusses to support roofing
d) beams provided on plinth level

Explanation: Purlins are beams provided over trusses to support sloping roof system between adjacent trusses. Channels, angle sections, and old formed Z-sections are widely used as purlins.

2. Theoretically, purlins are generally placed at
a) only at panel points
b) only at edges
c) only at mid span
d) only at corners of roof

Explanation: Theoretically, it is desirable to place purlins only at panel points. They are placed at panel points to avoid bending in the top chords of roof trusses. For large trusses, it is more economical to space purlins at closer intervals.

3. Purlin section is subjected to
a) not subjected to bending or twisting
b) twisting only
c) symmetrical bending
d) unsymmetrical bending

Explanation: The wind force is assumed to act normal to roof truss and gravity load pass through centre of gravity of purlin section. Hence, the purlin section is subjected to twisting in addition to bending. Such bending is called unsymmetrical bending.
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4. If purlins are assumed to be simply supported, the moments will be
a) wl2/10
b) wl/8
c) wl/10
d) wl2/8

Explanation: Purlins can be designed simple, continuous or cantilever beams. If purlins are assumed to be simply supported, the moments will be wl2/8. If they are assumed to be continuous, the moments will be slightly less and taken as wl2/10. IS 800 recommends the purlins to be designed as continuous beams.

5. While erecting channel section purlins, it is desirable that they are erected over rafter with their flange
a) facing down slope
b) facing up slope
c) does not depend whether up slope or down slope
d) flanges are placed randomly

Explanation: While erecting angle, channel or I- section purlins, it is desirable that they are erected over rafter with their flange facing up slope. In this position, the twisting moment does not cause any instability. The twisting moment will cause instability if the purlins are kept in such a way that the flanges face the downward slope.

6. Sag rods are provided at
a) one-third points between roof trusses
b) end of span
c) two-third points between roof trusses
d) are never provided

Explanation: Purlin sections have tendency to sag in the direction of sloping roof . So, sag rods are provided midway or at one-third points between roof trusses to take up the sag.

7. 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

Explanation: 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.

8. When one sag rod is used, the moment about web axis
a) reduces by 50%
b) increases by 50%
c) increases by 75%
d) reduces by 75%

Explanation: If sag rods are not used, the maximum moment about web axis would be wl2/8. When one sag rod is used, the moments are reduced by 75% and when two sag rods are used at one-third points, the moments are reduced by 91%.

9. The maximum bending moment for design of channel/I-section purlin is calculated by
a) Wl/10, where W= concentrated load
b) Wl/8, where W= concentrated load
c) W/10, where W= concentrated load
d) W/8, where W= concentrated load

Explanation: The gravity load, P1 and load due to wind component, H1 are computed. The loads are multiplied by load factors. Thus, P = γfP1, H = γfH1 . The maximum bending moment are calculated as Mz = Pl/10 and My = Hl/10, where P= factored load along z-axis, H = factored load along y-axis, l= span of purlin (c/c distance between adjacent trusses).

10. The required section modulus of the channel/I-section purlin can be determined by
a) Zpz = Myγm0/fy + (b/d)(Mzγm0/fy)
b) Zpz = Mzγm0/fy + (b/d)(Myγm0/fy)
c) Zpz = Mzγm0/fy + 2.5(b/d)(Myγm0/fy)
d) Zpz = Myγm0/fy + 2.5(b/d)(Mzγm0/fy)

Explanation: The required section modulus of the purlin section can be determined by Zpz = Mzγm0/fy + 2.5(b/d)(Myγm0/fy ), where γm0 is partial safety factor for material = 1.1, d is depth of trial section, b is the breadth of the trial section, Mz and My are factored bending moments about Z and Y axes, respectively, and fy is yield stress of steel. Since the above equation involves b and d of a section, trial section must be used and from the above equation , it is checked whether chosen section is adequate or not.

11. The design capacity of channel/I-section purlin is given by
a) M = Zp/fy
b) M = Zpγm0fy
c) M = Zpγm0/fy
d) M = γm0/fy

Explanation: The design capacity of channel/I-section purlin is given by Mdz = Zpzγm0/fy and Mdy = Zpm0/fy , Mdz and Mdy are design moment capacity about Z and Y axes, respectively, Zpz and Zpy are plastic section modulus about Z and Y axes, respectively and fy is yield stress of steel. For safety, design moment capacity should be always greater than or equal to factored bending moments.

12. The check for design capacity of channel/I-section purlin is given by
a) Mdz ≤ 1.2Zeyfym0, Mdy ≤ 2.4Zezfym0
b) Mdz ≤ Zezfym0, Mdy ≤ 1.2Zeyfym0
c) Mdz ≤ γfZeyfym0, Mdy ≤ 1.2Zezfym0
d) Mdz ≤ 1.2Zezfym0, Mdy ≤ γfZeyfym0

Explanation: The check for design capacity of channel/I-section purlin is given by Mdz ≤ 1.2Zezfym0 , Mdy ≤ γyZeyfym0 , 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.

13. Which of the following relation is correct for design of channel/I-section purlin?
a) (Mz/Mdz) + (My/Mdy) ≥ 1
b) (Mz/Mdz) + (My/Mdy) ≤ 1
c) (Mdz/Mz) + (My/Mdy) ≤ 1
d) (Mdz/Mz) + (Mdy/My) ≥ 1

Explanation: The local capacity of the section is checked by interaction equation. It is given by (Mz/Mdz) + (My/Mdy) ≤ 1 , where Mdz and Mdy are design moment capacity about Z and Y axes, respectively, and Mz and My are factored bending moments about Z and Y axes, respectively.

14. For which of the following slope of roof truss, angle section purlin can be used?
a) 25˚
b) 50˚
c) 75˚
d) 60˚

Explanation: Angle sections are unsymmetrical about both the axes. Angle sections can be used as purlin section. provided slope of the roof truss is less than 30˚.

15. The modulus of section required for angle section purlin is given by
a) Z = M/(0.66xfy)
b) Z = M/(1.33×0.66xfy)
c) Z = M/(1.33×0.66xfy)
d) Z = M/(1.33xfy)

Explanation: The modulus of section required for angle section purlin is given by Z = M/(1.33×0.66xfy), M = maximum bending moment = wl2/10, w = unfactored uniformly distributed load, l = span of purlin, fy is yield stress. The gravity and wind loads are determined to calculate bending moment and both loads are assumed to be normal to roof truss.

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