In the field of structural engineering, two commonly used methods for designing structures are the working stress method and the limit state method. Both methods have their own set of advantages and disadvantages, and it is important to understand the difference between them before deciding which method to use for a particular project.
Working Stress Method
The working stress method is an older method of designing structures that is still used in some parts of the world. In this method, the design is based on the assumption that the material used in the structure behaves elastically within a certain range of stress. This means that the structure will not fail as long as the stress on the material does not exceed a certain limit. The allowable stress is calculated based on various factors such as the type and quality of material used, the safety factor, and the load that the structure will be subjected to.
One of the advantages of the working stress method is that it is a simpler method of design that is easier to understand and implement. However, one of the main disadvantages of this method is that it does not take into account the possibility of plastic deformation or failure of the material due to stress concentration or fatigue, which can lead to unexpected failure of the structure.
Limit State Method
The limit state method is a newer method of designing structures that is widely used in most parts of the world. In this method, the design is based on the concept of limit states, which are the conditions under which the structure will no longer be able to perform its intended function. The design is based on the ultimate limit state, which is the condition when the structure will collapse, and the serviceability limit state, which is the condition when the structure will no longer be able to perform its intended function due to excessive deformation or vibration.
One of the advantages of the limit state method is that it takes into account the possibility of plastic deformation or failure of the material due to stress concentration or fatigue, which makes it a more reliable method of design. However, one of the main disadvantages of this method is that it is a more complex method of design that requires a higher level of expertise and knowledge to implement.
Differences Between Working Stress Method and Limit State Method
There are several differences between the working stress method and the limit state method:
- Assumption: The working stress method assumes that the material behaves elastically within a certain range of stress, while the limit state method takes into account the possibility of plastic deformation or failure of the material due to stress concentration or fatigue.
- Design Criteria: The working stress method is based on the allowable stress, while the limit state method is based on the limit states.
- Safety Factor: The working stress method uses a safety factor to ensure that the structure will not fail, while the limit state method uses a partial safety factor to account for the uncertainties in the design process.
- Load Factors: The working stress method uses load factors to account for the uncertainties in the load that the structure will be subjected to, while the limit state method uses a combination of load factors and partial safety factors.
- Reliability: The limit state method is generally considered to be a more reliable method of design than the working stress method.
Conclusion
Both the working stress method and the limit state method have their own set of advantages and disadvantages, and it is important to choose the method that is best suited for a particular project. While the working stress method is simpler and easier to understand, it does not take into account the possibility of plastic deformation or failure of the material due to stress concentration or fatigue. The limit state method, on the other hand, is more complex but more reliable, and takes into account the various limit states under which the structure will no longer be able to perform its intended function.
