Question

Identify the correct statement about working stress method and limit state method of design?

• A. Working stress method reflects the true safety margin of a structure at failure.
• B. Limit State Method is generally more conservative, resulting in heavier, less economical sections.
• C. Design process of Limit State Method is based on plastic, or ultimate behaviour.
• D. Working stress method accounts for the simultaneous variation of loads and material strengths using multiple partial safety factors.

Hint:

- Working Stress Method (WSM): Deterministic, assumes strictly elastic behavior, often results in heavier structures.
- Limit State Method (LSM): Probabilistic, utilizes plastic/ultimate behavior, produces lighter, more optimized sections.

Answer 1

The Correct Option is C

Concept: Structural engineering has evolved through two primary design philosophies for reinforced concrete and steel structures: the traditional Working Stress Method (WSM) and the modern Limit State Method (LSM). WSM is a deterministic approach based on linear elastic theory, while LSM is a probabilistic approach that evaluates structural behavior up to ultimate failure conditions.

Step 1: Analyzing the mechanics of the Limit State Method.
The Limit State Method represents a more realistic design approach by considering how materials perform near failure:
• Unlike older methods that assume stress remains strictly linear, LSM accounts for the non-linear, plastic, and ultimate behavior of steel and concrete as they approach failure.
• It designs the structure to withstand ultimate factored loads, ensuring safety up to the point of collapse while also verifying serviceability criteria (like deflection and cracking) under normal everyday conditions. This makes the statement in Option C correct.

Step 2: Deconstructing errors in the other choices.

• Option A: WSM assumes materials remain purely elastic and applies a single large factor of safety to their yield strength. Because it ignores the plastic reserve capacity of materials, it does not accurately reflect the actual safety margin at collapse.
• Option B: LSM optimizes material sizes by considering their full structural capacity, which generally leads to lighter and more economical sections than those designed using WSM.
• Option D: It is the *Limit State Method* that uses multiple partial safety factors to account for variations in loads and material strengths. WSM relies on a single safety factor.