Question

The stress level in concrete in limit state method when compared to working stress method is

• A. Same
• B. Lower
• C. Higher
• D. no comparison

Hint:

Remember the core philosophies:
- WSM = Elastic design. Stresses under service loads are kept low.
- LSM = Ultimate strength design. Stresses at ultimate load are high, utilizing the material's full capacity.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question compares the stress levels that concrete is designed to handle in the Limit State Method (LSM) versus the Working Stress Method (WSM).

Step 2: Detailed Explanation:

• Working Stress Method (WSM): This is an elastic design method. The stresses in the materials under actual service loads are kept well below their ultimate strengths by using a large factor of safety (approx. 3 for concrete). The material is assumed to behave linearly elastically, and stresses are kept in the low, elastic range.

• Limit State Method (LSM): This is an ultimate strength design method. It considers the behavior of the structure at the point of collapse (the "limit state"). The design is based on the ultimate, non-linear strength of the materials. A partial safety factor (1.5 for concrete) is applied to the characteristic strength to get the design strength. The design stress distribution is non-linear (rectangular-parabolic) and utilizes a much larger portion of the concrete's actual strength capacity. For example, the maximum design stress is $0.446 f_{ck}$.


Comparing the two, the permissible stress in WSM (e.g., $0.33 f_{ck}$) is much lower than the design stress used in LSM (up to $0.446 f_{ck}$). LSM allows the concrete to be stressed to a much higher level, closer to its actual failure strength, resulting in more economical designs.

Step 3: Final Answer:
The stress level in concrete in the limit state method is higher when compared to the working stress method.