Question

The stress condition at the state of impending failure of the section is analyzed and the non-linear stress-strain curves of concrete and steel are made use of, this method is known as:
1. Working stress method
2. Ultimate load method
3. Stress method
4. Limit state method

Answer 1

Correct Answer - Option 2 : Ultimate load method

Explanation: 

Ultimate load method:

• This is also known as the load factor method or ultimate strength method. 
• In this, we make use of the nonlinear region of stress-strain curves of steel and concrete and takes into account the plastic behaviour of these material.
• The safety is ensured by introducing a load factor. “Load factor is the ratio of ultimate strength to the service loads”
• The method is more realistic as compared to the working stress method because the ultimate load method takes into account the non-linear behaviour of concrete.

Limitations:

• The method gives very thin sections which lead to excessive deformations and cracking thus making the structure unserviceable.
• No factor of safety for material stresses.

Working stress method:

• Working stress method is used for the design of Reinforced concrete, Steel and Timber Structures. 
• The main assumption in the WSM is that the behaviour of structural material is restricted within the linear-elastic region and the safety of it is ensured by restricting the stresses coming on the members by working loads.
• It assumes that both steel and concrete are elastic and obey hook's law.

Drawbacks:

• It assumes the concrete is elastic which is not true.
• It uses a factor of safety for stresses only and not for loads.
• It gives uneconomical sections.

Limit State Method:

Limit states are the acceptable limits for the safety and serviceability requirements of the structure before failure occurs. The design of structures by this method will thus ensure that they will not reach limit states and will not become unfit for the use for which they are intended. Limit State Design requires two principal criteria to be satisfied:

• Limit states of  Collapse: It ensures that the structure you are going to build is stable and strong enough against any loads
• Limit state serviceability: It ensures that your designed structure is comfortable and useable enough for human use.