What is working stress method?
Working Stress Method:
The working stress method is a traditional method of structural design that aims to ensure that the stresses induced in the structure under
the worst credible combination of loads do not exceed the allowable working stresses assigned for the materials involved. In this approach, the design is based on the concept of ensuring the structural integrity by providing sufficient strength to resist the anticipated loads without exceeding a predefined limit on stress. The working stress method is commonly referred to as the Allowable Stress Design (ASD) method. Here are the key points regarding the working stress method:
1. Allowable Stresses:
In this method, each structural material has a specified allowable stress, which is a fraction (typically between 33% to 66%) of its ultimate strength. These allowable stresses are established based on empirical testing and safety factors that account for uncertainties and potential variations in material properties and loading conditions.
2. Design Philosophy:
The design philosophy of the working stress method involves verifying that the stresses in the structure under any load combination remain below the corresponding allowable stresses for the materials used. This ensures that the structure has adequate strength to resist the expected service loads and provides a margin of safety against failure.
3. Load Combinations:
The design must consider different load combinations that can be critical to structural safety, such as dead loads (permanent weights), live loads (variable loads like occupancy and equipment), wind loads, seismic forces, temperature variations, and other accidental loads. Each load combination is multiplied by a respective load factor to account for uncertainties and possible extreme conditions.
4. Design Process:
a) Determine the design loads for various load combinations.
b) Calculate the stresses induced in the structure for each load combination.
c) Compare the calculated stresses with the allowable stresses for the respective materials.
d) If the calculated stresses are less than or equal to the allowable stresses, the design is considered satisfactory. If not, the structural design must be revised or strengthened.
5. Advantages:
a) Simplicity: The working stress method is relatively straightforward to understand and apply, making it suitable for simple structures and routine design tasks.
b) Established Practices: It is a well-established method backed by historical empirical knowledge, making it familiar and widely accepted within the engineering community.
6. Disadvantages:
a) Conservatism: The working stress method can be considered conservative as it applies significant safety factors, which may lead to overdesign and inefficient use of materials.
b) Lack of Refinement: It does not consider the actual behaviour of the structure beyond the elastic range, which could be relevant in certain complex structural applications.
c) Complexity for Complex Structures: The working stress method becomes more challenging to apply in complex or indeterminate structures, requiring extensive calculations and manual iterations to satisfy multiple stress constraints.
In summary, the working stress method is a traditional design approach that ensures stresses remain below predefined limits, providing a safety margin against structural failure. While it is simple and widely understood, it can be conservative in certain situations and less efficient compared to more advanced methods like the limit state design method, which is commonly used in modern structural engineering practices.