Question

To avoid the limit state of collapse, the safety of RC structures is checked for appropriate combinations of dead load (DL), live load (LL), wind load (WL) and earthquake load (EQ). Which of the following combinations is not considered?

• A. \(1.5\,\mathrm{DL}+1.5\,\mathrm{LL}\)
• B. \(1.5\,\mathrm{DL}+1.5\,\mathrm{WL}+1.5\,\mathrm{EQL}\)
• C. \(0.9\,\mathrm{DL}+1.5\,\mathrm{WL}\)
• D. \(1.2\,\mathrm{DL}+1.2\,\mathrm{LL}+1.2\,\mathrm{WL}\)

Hint:

For limit state of collapse, wind load and earthquake load are generally considered separately, not together with full factored values. Common combinations include: \[ \boxed{ 1.5(DL+LL),\; 1.5(DL+WL),\; 1.2(DL+LL+WL),\; 0.9DL+1.5WL. } \]

Answer 1

The Correct Option is B

Step 1: List the valid load combinations from IS 456:2000.
The code allows combinations like $1.5(DL+LL)$, $1.5(DL+WL)$, $1.2(DL+LL+WL)$, and $0.9DL+1.5WL$, along with their earthquake load counterparts.
Step 2: Check each option against this list.
Option A, $1.5DL+1.5LL$, matches $1.5(DL+LL)$, so it is valid. Option C, $0.9DL+1.5WL$, matches the uplift check, so it is valid. Option D, $1.2DL+1.2LL+1.2WL$, matches $1.2(DL+LL+WL)$, so it is valid too.
Step 3: Look at the remaining option.
Option B combines wind load and earthquake load together, each multiplied by 1.5, in the same load case. Wind and earthquake are alternative lateral loads that are never applied together at full factored value in the same combination, so this one is not considered.
\[ \boxed{1.5DL+1.5WL+1.5EQL} \]