As per limit state of collapse, the partial safety factors for concrete and steel are 1.5 and 1.15 respectively, because
- Athe concrete is hetrogenous while steel is homogenous
- Bthe control on the quality concrete is not as good as that of steelCorrect
- Cthe concrete is weak in tension
- Dthe voids in concrete are 0.5% while those in steel are 0.15%
Solution
Concept
Partial Safety Factors for Materials
Formula
No formula
Step-by-step solution
1. Recall that partial safety factors account for uncertainty and variation in material strength. 2. Compare the manufacturing conditions: Steel is produced in factories under strict quality control, whereas concrete is mixed on-site or at batching plants with greater variability. 3. Match with the correct option: Option B.
Explanation
Concrete is manufactured on-site or at batching plants where quality control varies due to raw materials, mixing, and curing conditions, whereas steel is produced in factories under strict quality control. Because there is a higher chance of variation in concrete strength compared to steel, a higher partial safety factor (1.5) is used for concrete than for steel (1.15).
Shortcut
Remember: Steel = Factory = High Control (1.15), Concrete = Site = Low Control / High Variation (1.5).
Common mistake
Many students mistakenly choose Option A (concrete is heterogeneous while steel is homogeneous) or Option C (concrete is weak in tension) because these statements are factually true about the materials, but they confuse a material property/behavior with the reason for partial safety factors, which specifically account for quality control during manufacturing and construction.
Correct answer: B) the control on the quality concrete is not as good as that of steel