As per IS 1893 for DL + LL + EQ combination, the factors are
- A1.5 DL + 1.5 LL + 1.5 EQ
- B1.2 DL + 1.2 LL + 1.2 EQCorrect
- C1.2 DL + 0.5 LL + 1.0 EQ
- D1.5 DL + 1.0 LL + 1.0 EQ
Solution
Concept
Load factors for earthquake collapse limit state
Formula
1.2 × (DL + LL ± EQ)
Step-by-step solution
1. In limit state design for structural safety against collapse, appropriate load combinations and partial safety factors are applied. 2. When considering simultaneous dead load (DL), live load (LL), and earthquake load (EQ), the probability of maximum simultaneous occurrence is accounted for. 3. IS 1893 and IS 456 Table 18 prescribe the design combination factor as 1.2(DL + LL + EQ). 4. Thus, the factored load combination is 1.2 DL + 1.2 LL + 1.2 EQ. Answer: B
Explanation
Under IS 1893 (Part 1) and IS 456 (Table 18), the partial safety factor for the limit state of collapse under the concurrent action of dead load, live load, and earthquake load is 1.2 for all three actions: 1.2 DL + 1.2 LL + 1.2 EQ.
Why the other options are wrong
A) 1.5 factor applies only to two-load combinations (1.5 DL + 1.5 LL or 1.5 DL + 1.5 EQ). C) 0.5 LL factor applies in seismic weight calculation, not limit state collapse load combination. D) 1.5 DL + 1.0 LL + 1.0 EQ is not an Indian standard load combination.
Shortcut
3-load combination (DL + LL + EQ) = 1.2 factor across all three loads.
Common mistake
Using 1.5 DL + 1.5 LL, which applies only when earthquake load is not present.
Correct answer: B) 1.2 DL + 1.2 LL + 1.2 EQ