Limit State Method MCQ with answer and solution

Concept

Partial Safety Factors for Materials

Formula

No, there is no mathematical formula. These partial safety factors (1.15 for steel and 1.5 for concrete) are standard empirical values specified by design codes (such as IS 456) based on material quality control and reliability.

Step-by-step solution

1. Recall standard partial safety factor values per IS 456 for Limit State of Collapse. 2. For steel, partial safety factor γm = 1.15. 3. For concrete, partial safety factor γm = 1.5. 4. Select Option D (1.15, 1.5).

Explanation

In the limit state method of design, partial safety factors account for potential variations in material strength. A factor of 1.15 is used for steel because steel manufacturing is strictly controlled, resulting in consistent quality. A higher factor of 1.5 is used for concrete due to greater variability in mixing, placing, and curing on-site. Therefore, option D (1.15 for steel and 1.5 for concrete) is correct.

Shortcut

Concrete is made on site (more quality variation) so it gets a higher safety factor (1.5). Steel is factory-made (strictly controlled) so it gets a lower safety factor (1.15). Just remember: Concrete > Steel (1.5 > 1.15).

Common mistake

Students often confuse or swap the partial safety factors for steel and concrete (selecting 1.5 for steel and 1.15 for concrete) or confuse limit state of collapse factors with working stress method factors.