What is the net area for the tension member shown in the figure, in case of punched holes ? (Plate 100 × 10 mm² with M20 bolts, two holes along the critical section.)
- A560 mm²
- B1000 mm²
- C520 mm²Correct
- D440 mm²
Solution
Concept
Net area with punched holes
Formula
An = (b − n dh) t, dh = bolt hole diameter (+2 mm for punched holes)
Step-by-step solution
1. M20 bolt: standard hole diameter = 20 + 2 = 22 mm. 2. For punched holes, the diameter is increased by another 2 mm: d = 24 mm. 3. Two holes in the critical section: An = (100 − 2 × 24) × 10 = 520 mm².
Explanation
Punching damages the plate around the hole, so IS 800 takes the effective hole diameter as 2 mm greater than the finished hole diameter. For drilled holes the net area would be (100 − 2 × 22) × 10 = 560 mm².
Why the other options are wrong
560 mm² is the answer for drilled (not punched) holes. 1000 mm² ignores the holes and 440 mm² deducts too large a hole.
Shortcut
Punched hole: bolt dia + 2 + 2 mm.
Common mistake
Using 22 mm (drilled hole).
Correct answer: C) 520 mm²