For the Limit State of Collapse, which of the following load combinations is NOT prescribed by IS 456:2000?
- A1.5(DL + LL)
- B1.2(DL + LL + WL)
- C1.5(DL + WL)
- D1.0(DL + LL + WL)Correct
Solution
Concept
Load Combinations for Limit State Design
Formula
N/A
Step-by-step solution
1. Review the prescribed load combinations in IS 456:2000 for the Limit State of Collapse. 2. Recognize that a safety factor of 1.0 in 1.0(DL + LL + WL) is used for the Limit State of Serviceability rather than Collapse. 3. Select Option (d) as the combination NOT prescribed for Limit State of Collapse.
Explanation
Option (d) 1.0(DL + LL + WL) uses a safety factor of 1.0, which is applied for the Limit State of Serviceability (checking deflections and cracking) rather than the Limit State of Collapse. For the Limit State of Collapse, IS 456:2000 applies higher safety factors to ensure structural strength, such as 1.2(DL + LL + WL) or 1.5 for two-load combinations.
Shortcut
To remember Limit State Load Combinations in IS 456:2000, use the rule "Collapse = Factored (> 1.0), Serviceability = Unfactored (1.0)": - Collapse (Strength): Always uses partial safety factors greater than 1.0 (such as 1.5 for two loads or 1.2 for all three loads: DL + LL + WL). - Serviceability (Deflection/Cracking): Always uses a safety factor of 1.0 (e.g., 1.0(DL + LL + WL)).
Common mistake
A common mistake is confusing the load combinations for the Limit State of Collapse with those for the Limit State of Serviceability. Students often incorrectly select option B, assuming 1.2 is too low for a three-load combination, while failing to realize that option D uses unfactored loads (partial safety factor of 1.0) meant specifically for serviceability checks rather than collapse design.
Correct answer: D) 1.0(DL + LL + WL)