Comparison Between Working Stress Method & Limit State Method? ~CivilWork.

Summary

The comparison between the working stress method and the limit state method highlights two distinct approaches in structural engineering design. The working stress method relies on permissible stress limits of materials, offering a conservative approach with fixed safety factors. In contrast, the limit state method is a modern, probabilistic approach, considering ultimate and serviceability limit states. It optimizes design by utilizing materials to their full capacity, offering greater flexibility and safety through adjustable load and resistance factors. The limit state method is widely adopted globally and offers a more efficient, cost-effective design compared to the older working stress method.

Takeaways

• 😀 The Working Stress Method (WSM) is a traditional design approach based on permissible stress limits for materials like concrete and steel.
• 😀 The Limit State Method (LSM) is a modern design approach that focuses on the concept of limit states, ensuring safety and serviceability of structures.
• 😀 WSM assumes that materials behave elastically up to certain stress limits, while LSM considers the ultimate material capacities.
• 😀 In WSM, safety is ensured through predetermined stress limits and the inclusion of safety factors, but it may lead to underutilization of materials.
• 😀 LSM uses a probabilistic approach, minimizing the probability of failure by applying appropriate load and resistance factors.
• 😀 WSM provides limited flexibility in design optimization as the stress limits are fixed, whereas LSM offers more flexibility with adjustable factors based on specific requirements.
• 😀 WSM is typically based on older design codes, which are still used in some regions and for certain types of structures.
• 😀 LSM is based on modern design codes, which are more widely adopted and considered more advanced in terms of safety and design requirements.
• 😀 The Working Stress Method may result in less efficient designs, as materials may not be used to their full capacity.
• 😀 The Limit State Method allows for optimized, more economical designs by utilizing materials up to their ultimate capacities.
• 😀 Safety in WSM is achieved through the use of fixed safety factors, while LSM provides a more comprehensive safety approach by considering different limit states (ultimate and serviceability).

Q & A

What is the main difference between the working stress method and the limit state method?

The main difference is that the working stress method is based on permissible stress limits for materials, while the limit state method uses limit states, which define conditions under which a structure may fail to perform its intended function.

How does the working stress method ensure structural safety?

In the working stress method, safety is achieved by ensuring that the stresses in structural elements do not exceed the permissible stress limits, with additional safety factors often incorporated.

What are the types of limit states considered in the limit state method?

The limit state method considers two types of limit states: ultimate limit state (which involves failure or collapse) and serviceability limit state (which deals with performance issues like excessive deflections).

How does the limit state method improve upon the working stress method?

The limit state method allows for the use of materials up to their ultimate capacities, optimizing the design and making it more cost-effective. It also takes a probabilistic approach, considering various limit states and load factors.

Why might the working stress method result in conservative designs?

The working stress method is conservative because it uses fixed stress limits and assumes materials behave elastically up to those limits, which often results in underutilization of material strengths.

What is the advantage of using the limit state method over the working stress method in terms of flexibility?

The limit state method offers greater flexibility by adjusting load and resistance factors based on specific conditions, allowing for more efficient and optimized designs.

Which method is based on older design codes?

The working stress method is based on older design codes that are still used in some regions or for specific types of structures.

What is the main approach used in the limit state method?

The limit state method uses a rational and probabilistic approach, considering structural reliability and minimizing the probability of failure by evaluating different limit states and associated load and resistance factors.

What does the working stress method assume about the behavior of materials?

The working stress method assumes that materials behave elastically up to certain stress limits, and designs are based on those limits.

How do the safety factors differ between the two methods?

In the working stress method, safety is mainly ensured by applying predefined stress limits and safety factors, while in the limit state method, safety is incorporated by considering various limit states and using load and resistance factors.