Short Answer

Working Stress Method (WSM) is a traditional method used in structural design where materials are designed to work within safe stress limits. In this method, the actual stresses developed in the structure are kept below the permissible stress of the material.

It is based on elastic theory and uses a factor of safety to ensure safety. The structure is designed so that it behaves safely under normal working loads without reaching failure conditions.

Detailed Explanation:

Working Stress Method

Working Stress Method is one of the oldest methods used in civil engineering for designing structures. In this method, structures are designed in such a way that the stresses developed due to applied loads remain within the safe limits of the material. These safe limits are called permissible stresses. The main idea of this method is to ensure that the structure remains safe and does not fail under normal working conditions.

Basic concept of method

The Working Stress Method is based on the principle that the stress in a material should not exceed its allowable or permissible value. This permissible stress is obtained by dividing the ultimate strength of the material by a factor of safety. This factor of safety provides a margin to account for uncertainties in load, material properties, and construction quality.

In simple terms, the structure is designed to carry loads in such a way that it always remains in the elastic range. This means that when the load is removed, the structure returns to its original shape without any permanent deformation.

Assumption of elastic behavior

A key assumption in the Working Stress Method is that materials behave elastically under working loads. This means stress is directly proportional to strain, following Hooke’s law. Because of this assumption, the method is simple and easy to understand. However, it does not consider the behavior of materials beyond the elastic limit.

Factor of safety

The factor of safety plays an important role in this method. It ensures that even if there are variations in material strength or unexpected loads, the structure will still remain safe. Higher factors of safety are generally used in this method, which makes the design more conservative.

Advantages of working stress method

The Working Stress Method has some advantages. It is simple to use and easy to understand. It provides a good level of safety due to the use of high safety factors. It is also useful in situations where elastic behavior is important, such as in water-retaining structures like tanks and dams.

Limitations of working stress method

Despite its advantages, the method has several limitations. It does not consider the actual failure condition of materials. It assumes linear behavior even when materials may behave differently at higher loads. It also leads to uneconomical designs because it uses higher safety factors, resulting in more material usage.

Comparison with modern methods

Modern methods like Limit State Design are more advanced compared to the Working Stress Method. While WSM focuses only on elastic behavior, modern methods consider both safety and serviceability. Because of this, WSM is now less commonly used in most structural designs.

Applications of working stress method

Even today, the Working Stress Method is used in certain types of structures where crack control and elastic behavior are important. Examples include water tanks, retaining structures, and some prestressed concrete structures.

Conclusion

Working Stress Method is a traditional design approach that keeps stresses within safe limits using elastic theory and safety factors. Although it is simple and safe, it is less economical and less accurate compared to modern methods. However, it is still useful in specific cases where elastic behavior is important.