1. Discuss the salient feature of working stress methods and ultimate load method?
Here are some ideas to investigate and comment.
Working stress is generally more conservative.
It does not take into account of the post-yield behaviour of the reinforcing steel.
Ultimate stress caters to both strenth and serviceability requirements separately.
If properly designed, will ensure gradual failure instead of sudden failure by under-reinforcing.
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The working stress method and the ultimate load method are two different approaches used in structural engineering to design and analyze structures. Let's discuss their salient features:
1. Working Stress Method:
- This method considers the working loads acting on the structure and designs it with a factor of safety to ensure that the stress levels in the structure remain within acceptable limits during its entire service life.
- The allowable stress values for different materials are determined based on extensive experimental data, codes, and standards.
- The design is based on the assumption that the structure behaves elastically, meaning it returns to its original shape after the load is removed.
- The factors of safety are applied to the working loads, material properties, and dimensions of the structure to account for uncertainties like material variability, construction quality, and serviceability requirements.
- The design is conservative, ensuring that the structure won't experience any deformation or failure under working loads.
2. Ultimate Load Method:
- This method focuses on the maximum load that the structure can withstand before failure.
- It considers the ultimate strength of the materials used in the structure rather than the allowable stress.
- The design is based on the assumption that the structure will experience some amount of plastic deformation before ultimate failure. Plastic deformation is the permanent deformation that occurs when the stress exceeds the elastic limit of the material.
- The ultimate loads are multiplied by an appropriate safety factor to ensure that the structure can handle unforeseen variations in loading or material strength.
- The design is more economical compared to the working stress method, as it allows the structure to experience higher stresses and deformations before failure, resulting in potentially reduced material usage.
Both methods have their own advantages and disadvantages, and their applicability depends on various factors such as the nature of the structure, materials used, and specific design requirements. The selection of the appropriate method is usually influenced by local codes, project specifications, and engineering judgment.