Limit State Method MCQ with answer and solution

Concept

Load Combinations for Limit State of Serviceability

Formula

DL + 0.8(LL + WL)

Step-by-step solution

Identify the loads: Dead Load (DL), Live Load (LL), and Wind Load (WL). Under the limit state of serviceability, when all three loads act simultaneously, apply a partial safety factor of 0.8 to the variable loads (LL and WL), resulting in DL + 0.8(LL + WL). Therefore, simply adding them as DL + LL + WL without the 0.8 factor is incorrect.

Explanation

Under the limit state of serviceability in IS 456, when Dead Load (DL), Live Load (LL), and Wind Load (WL) are considered together, it is extremely unlikely that both maximum live load and maximum wind load occur simultaneously. Therefore, the code applies a reduced factor of 0.8 to both variable loads, making the correct combination DL + 0.8(LL + WL) rather than simply adding them directly as DL + LL + WL.

Shortcut

Remember: For Serviceability, factors are 1.0 (unfactored) when 2 loads act (DL + LL or DL + WL), but reduced to 0.8 for variable loads when all 3 act (DL + 0.8 LL + 0.8 WL). Plain "DL + LL + WL" without 0.8 is never used.

Common mistake

Students often confuse the load combinations for the limit state of collapse with those for the limit state of serviceability, assuming that all partial safety factors must be equal to 1.0 without applying the 0.8 reduction factor when all three loads (DL + LL + WL) act simultaneously.