Working Stress Method of Design

The working stress method is a popular method used in the design of structures in civil engineering. It is based on the assumption that the material used in the structure behaves in a linear elastic manner. This means that the material will deform elastically under load and return to its original shape once the load is removed.

Behaviour of Material

In the working stress method, the behaviour of the material is assumed to be linear elastic. This assumption is made to simplify the design process and ensure the safety of the structure. When a material behaves in a linear elastic manner, its stress-strain relationship remains linear within the elastic region.

Linear Elastic Behaviour

Linear elastic behaviour means that when a material is subjected to a small amount of stress, it deforms proportionally within its elastic limit and returns to its original shape when the stress is removed. This behaviour is described by Hooke's Law, which states that the stress is directly proportional to the strain within the elastic limit.

Assumptions and Limitations

- The linear elastic assumption is valid only within the elastic limit of the material. If the stress exceeds the elastic limit, the material may exhibit non-linear behaviour and undergo permanent deformation.
- The linear elastic assumption also assumes that the material is homogeneous, isotropic, and has a uniform cross-section. These assumptions simplify the calculations and ensure a conservative design.
- The working stress method does not account for the redistribution of stress that occurs after the formation of plastic hinges in a structure. This can lead to a loss of strength and stability in the structure.

Advantages and Disadvantages

The working stress method has several advantages:
- It is a simple and straightforward method that can be easily applied in practice.
- It provides a conservative design by assuming linear elastic behaviour.
- It allows for a direct comparison of the working stress with the allowable stress, ensuring the safety of the structure.

However, the working stress method also has some limitations:
- It does not consider the ultimate strength of the material or the structure, which can lead to overdesign.
- It does not account for the redistribution of stress after the formation of plastic hinges, which can lead to failure in some cases.
- It relies on empirical factors of safety to ensure the safety of the structure, which may be arbitrary and not based on rigorous analysis.

Overall, the working stress method is a widely used design method in civil engineering. It assumes the behaviour of the material as linear elastic, simplifying the design process while ensuring the safety of the structure. However, it is important to consider the limitations and assumptions of this method to avoid potential design flaws and failures.