According to IS 456:2000, in the limit state design, the strain in the extreme compressive fibre of concrete in an over-reinforced section will be
- ALess than 0.0035Correct
- BExactly 0.0035
- C0.0035 × 1.15
- D0.0035 × 1.5
Solution
Concept
Under limit state design as per IS 456:2000, the maximum strain in the extreme compressive fibre in bending is assumed to be 0.0035. In an over-reinforced section, concrete reaches this ultimate strain before the tension steel yields.
Step-by-step solution
1. According to IS 456:2000 (Clause 38.1), the maximum strain in the extreme compressive fibre in concrete in bending is assumed to be 0.0035. 2. In a balanced or over-reinforced section, failure is initiated by the crushing of concrete, meaning the extreme fiber strain reaches 0.0035. 3. Therefore, the strain in the extreme compressive fibre of concrete is 0.0035.
Why the other options are wrong
A: Concrete strain at failure is 0.0035, not less. C & D: These values include partial safety factors incorrectly applied to the strain limit.
Shortcut
Remember that the maximum concrete strain at failure in LSM is always fixed at 0.0035 regardless of whether the section is under-reinforced, balanced, or over-reinforced.
Common mistake
Confusing concrete strain with steel strain. In an over-reinforced section, the *steel* strain is less than its yield strain at failure, but the *concrete* strain reaches its ultimate limit of 0.0035.
Correct answer: A) Less than 0.0035