A double-U butt weld connects two plates of section 200 mm (depth d) × 20 mm (width b). If the weld is subjected to a moment of 10 000 kN·mm, the stress developed in the weld is:
Try it first: pick an option, then read the explanation.
Correct answer: D. 75 N/mm²
Asked in SSC JE 23/03/2021 (Evening)
Detailed Explanation
A full-penetration butt weld has the same throat section as the plate, so it is checked like a rectangular section in bending.
\[ Z = \frac{b d^2}{6} = \frac{20\times 200^2}{6} = 1.333\times 10^5\ \text{mm}^3 \]
\[ \sigma = \frac{M}{Z} = \frac{10\,000\times 10^3}{1.333\times 10^5} = 75\ \text{N/mm}^2 \]
Why “75 N/mm²” Is Correct
The bending stress in the weld is 75 N/mm².
Why the Other Options Are Wrong
- A. 150 N/mm²: 150 N/mm² uses Z = bd²/12 (half the correct Z).
- B. 50 N/mm²: 50 N/mm² does not follow from M/Z.
- C. 100 N/mm²: 100 N/mm² is not M/Z for this section.
Key Point
Butt weld in bending: σ = 6M/(t·d²) with full-penetration throat.
Key Concepts for Students
- Convert units: 1 kN·mm = 1000 N·mm.
- Butt welds are as strong as the plate when fully penetrated.
- Fillet welds use the throat thickness 0.7 × size.
Practice more: this is question 60 in Bending and Shear Stress MCQs – Part 4. Try the whole set and check every answer instantly.