Short Answer
Limit State Method is preferred over Working Stress Method because it provides a more realistic and safe design. It considers both failure conditions and normal working conditions, while Working Stress Method only focuses on elastic behavior.
It is also more economical because it uses materials efficiently and applies proper safety factors to both loads and material strength. This makes structures safer, stronger, and cost-effective.
Detailed Explanation:
Limit State Method preferred over Working Stress Method
Limit State Method is widely used in modern civil engineering because it gives better results compared to the Working Stress Method. It provides a balanced approach by considering both safety and serviceability of structures. Engineers prefer this method as it is more scientific, reliable, and economical.
Better safety approach
The Limit State Method ensures higher safety by considering the actual failure conditions of a structure. It checks the ultimate limit state, which means it ensures that the structure will not collapse under maximum loads. In contrast, the Working Stress Method only keeps stresses within permissible limits and does not directly check the failure stage.
In Limit State Method, partial safety factors are used separately for loads and materials. This makes the design more accurate and reliable. It accounts for uncertainties in loads and variations in material properties more effectively.
Consideration of serviceability
Another major advantage of Limit State Method is that it checks serviceability conditions. This means it ensures that the structure performs well during normal use without excessive deflection, cracks, or vibrations.
Working Stress Method mainly focuses on elastic behavior and does not clearly address serviceability issues. Because of this, structures designed using WSM may sometimes face problems like cracks or discomfort during use.
Economical design
Limit State Method is more economical compared to Working Stress Method. It allows better use of material strength by considering the actual capacity of materials. As a result, less material is required, which reduces the cost of construction.
Working Stress Method uses higher factors of safety, which leads to overdesign and higher material usage. This makes it less economical in modern construction practices.
Realistic behavior of materials
Limit State Method considers the actual behavior of materials up to failure. It does not limit the design to only elastic conditions. This gives a more realistic picture of how structures behave under different loads.
On the other hand, Working Stress Method assumes that materials always behave elastically, which is not true in real conditions. This makes WSM less accurate.
Use in modern design codes
Most modern design codes are based on Limit State Method. Engineers follow these codes for designing structures like buildings, bridges, and other infrastructures. This shows that Limit State Method is widely accepted and trusted in engineering practice.
Working Stress Method is now rarely used, except in special cases where elastic behavior is important, such as water-retaining structures.
Balanced design approach
Limit State Method provides a balanced approach by considering both safety and usability. It ensures that the structure is not only strong enough to avoid collapse but also comfortable and durable for everyday use.
This balance is not fully achieved in Working Stress Method, which focuses mainly on safety within elastic limits.
Conclusion
Limit State Method is preferred over Working Stress Method because it provides better safety, economy, and realistic design. It considers both ultimate and serviceability conditions, making it more reliable and suitable for modern engineering practices.