Q.9 As per Indian Standard code (IS-456), what should be the minimum clear cover provided during the design of a water tank during moderate exposure conditions?
- A75 mm
- B10 mm
- C30 mmCorrect
- D45 mm
Solution
Concept
IS 456 Table 16 sets the nominal cover by exposure condition. For moderate exposure it is 30 mm.
Step-by-step solution
1. Water tank surfaces are always in contact with water, so the exposure class is set by the question as moderate. 2. Check IS 456:2000 Table 16 for nominal cover to meet durability. 3. Mild = 20 mm, Moderate = 30 mm, Severe = 45 mm, Very severe = 50 mm, Extreme = 75 mm. 4. For moderate exposure, cover = 30 mm.
Explanation
IS 456 gives nominal cover based on exposure condition. For moderate exposure the cover is 30 mm. So the answer is 30 mm.
Why the other options are wrong
A: 75 mm is for extreme exposure (or footings in contact with soil, as general guidance). B: 10 mm is too small and is not given for any exposure class. D: 45 mm is for severe exposure in IS 456 (IS 3370 also asks 45 mm for liquid retaining structures, but the question asks IS 456 moderate).
Shortcut
Remember the cover series 20-30-45-50-75 for mild-moderate-severe-very severe-extreme.
Common mistake
Students often choose 45 mm because IS 3370 recommends 45 mm cover for water tanks.
Correct answer: C) 30 mm