Identify the INCORRECT statement with respect to specifications on the partial safety factors for material strength, and are used in limit state design of RC structures:
- AA higher partial safety factor has been assigned to concrete compared with reinforcing steel under the ultimate limit state.
- BA partial safety factor '1' is used for concrete under serviceability limit state
- CA partial safety factor '1.15' is used for steel under serviceability limit stateCorrect
- DA partial safety factor '1.5' is used for concrete under ultimate limit state
Solution
Concept
Partial safety factor γm is 1.5 for concrete and 1.15 for steel at ultimate limit state. It is 1.0 for both at serviceability limit state.
Formula
Design strength = fk / γm
Step-by-step solution
1. At ultimate limit state, γm for concrete = 1.5 and γm for steel = 1.15. 2. At serviceability limit state, γm = 1.0 for both concrete and steel. 3. A: 1.5 > 1.15, so concrete has the higher factor. A is correct. 4. B: Concrete at serviceability uses 1.0. B is correct. 5. C: Steel at serviceability uses 1.0, not 1.15. C is incorrect. 6. D: Concrete at ultimate limit state uses 1.5. D is correct. 7. So the answer is C.
Explanation
IS 456 gives γm = 1.5 for concrete and 1.15 for steel at the ultimate limit state. For the serviceability limit state, γm = 1.0 for both materials. So 1.15 for steel at serviceability is wrong.
Why the other options are wrong
A: Correct, concrete (1.5) has a higher factor than steel (1.15). B: Correct, concrete uses 1.0 at serviceability. D: Correct, concrete uses 1.5 at ultimate limit state.
Shortcut
Serviceability: γm = 1.0 for all materials. 1.15 is used only for steel at the ultimate limit state.
Common mistake
Using the ultimate limit state values (1.5 and 1.15) for the serviceability limit state as well.
Correct answer: C) A partial safety factor '1.15' is used for steel under serviceability limit state