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Methods Of Structural Design
Structural design can be accomplished using three primary methods: working stress, limit state, and ultimate load. Initially, the working stress method was utilized for designing both Reinforced Concrete and Steel structures.
However, due to certain drawbacks of the working stress method, the Ultimate Load method was introduced.
In recent times, most countries have adopted the Limit State method of design as it offers the benefits of both working stress and ultimate load methods.
The limit state method is most popular and widely used in the field of structural analysis and design.
What Is Working Stress Method?
The working stress method is the oldest method to design structure. It is rarely used nowadays.
The working stress method depends upon yield strength. In this method, all the structural material behaves as elastic. Stress increases in a linear way as the load is applied to the structure.
In this method, permissible stress is kept below the strength of the material. Sufficient factors of safety are provided for the serviceability of the structure.
The factor of Safety (FOS) = Ratio of material strength to the permissible stress.
FOS = Strength/Permissible stress
Drawbacks Of Working Stress Method
- This method has major defects, as it deals only with the elastic behavior of the member, which means that it cannot reflect the real strength and determine the true factor of safety against failure.
- It assumes that concrete is elastic, which is not true in reality.
- The design method provides a larger percentage of compression steel than the limit state method, leading to uneconomical sections.
- Since the modulus of elasticity of concrete cannot be fixed as one true value due to creep and nonlinear stress-strain relationships, the modular ratio is an imaginary quantity.
- The concept of load factor is not present in this design method, which fails to distinguish the importance of different types of loads, such as dead load, live load, and earthquake load.
- This drawback sometimes leads to over-designed or under-designed structures.Compared to the limit state method of design and ultimate load method of design, the working stress method of design results in a larger section.
- The method does not consider the effect of shrinkage and creep, which are time-dependent.
What Is Ultimate Load Method?
The ultimate load method is just the opposite method of the working stress method. In this method, we provide load until the ultimate strength of the material. The main problem with this method is serviceability. Due to the high load on the structure large deflection occur.
To tackle the problem of large cross-section and uneconomical design in WSM, Ultimate Load Method is used. Cross-section of Beam and column can be reduced significantly by utilizing the ultimate load method.
Drawbacks Of Ultimate Load Method
- The design of structures for ultimate load follows a specific method, but it cannot be guaranteed that a structure designed for ultimate load will perform its function at service load as it can experience large deflections, vibrations, and cracks at working load.
- Instead of relying solely on the linear portion of the stress-strain curve, engineers take into consideration the non-linear portion of the curve. However, analyzing the non-linear stress-strain curve can be a cumbersome process.
- Engineers determine the stress distribution at ultimate load by first calculating the stress distribution at working load and then magnifying it using a load factor. However, this approach is not entirely correct as the inelastic and non-linear behavior of the material can cause significant stress redistribution.
Advantages Of Ultimate Load Method
- The ultimate load method considers the non-linear portion of the curve instead of the idealized straight curve used in the working stress method.
- The ultimate load method provides a fair idea about the margin of safety, which is not present in the working stress method.
- The ultimate load method leads to economical design as sections designed using this method are thinner.
- Different load factors can be assigned to different loads based on their importance, which is not possible in the working stress method.
What Is Limit State Method?
As we know Working Stress Method is concerned with serviceability and the Ultimate load method provides higher strength of material with high loading. But Limit State takes care of both the safety and serviceability of the structure. So we can say the Limit state method is the combination of the working stress method and the ultimate load method.
Limit state methods solve the problem of both serviceabilities like large deflection in the structure and limit state of collapse like a fracture.
In simple words;
The Limit State Method is known for ensuring sufficient safety and serviceability while subjecting the structure to ultimate load.
The concept of limit state involves two primary aspects in structural engineering: the limit state of collapse and the limit state of serviceability.
1. Limit State Of Collapse
Limit state method deals with strength of a structure in terms of its ability to withstand collapse, overturning, sliding, and other forms of failure. Several types of limit states of collapse exist, such as flexure, compression, shear, and torsion.
2. Limit state of serviceability
Limit state of serviceability concerns the extent of deformation that a structure can withstand before becoming unserviceable. Examples of limit states of serviceability include deflection, vibration, cracking, and other factors that may affect the functionality of a structure.
Closing Thought:
Working Stress, Ultimate Load, and Limit State Methods are used for different purposes. The structure is like a water tank where there is no need to limit the width and length of the section, the working stress method can be used. But in common building structures, the limit state method is used.