Difference between limit state method and working stress method


There is a very conceptual difference between limit state method and working stress method. The working stress method of design is based on the behavior of any structure at the working load.

Difference between limit state method and working stress method

Difference between limit state method and working stress method

Working stress methods- Basic assumption

  1. The materials of the structure are homogenous and isotropic.
  2. The behavior of the structure is linear and elastic.
  3. The factor of safety in seel is taken as 1.67 to 1.92.
  4. Load is taken as characteristic load.
  5. The factor of safety for concrete is taken as 3.
  6. The factor of safety for timber is taken as 3 to 5.

Advantage of Working stress method of design

  1. This method is simple in the calculation and easy to understand.
  2. This method gives a better performance during the worst combination of loading and during extreme conditions.
  3. Serviceability requirements are sufficient for the working stress method.

Disadvantage of working stress method

  1. It gives the uneconomical design of structure.
  2. It does not apply the true value of factor of safety.
  3. It is a time-consuming method.

Limit state method

Limit State Design Method is the latest approach in structural design which is based on not exceeding the limiting conditions of probable load and probable strength of materials through a probabilistic approach.

Thus, the philosophy of limit state design method is to see that the structure remains fit for use throughout its designed life by remaining within the acceptable limit of safety and serviceability requirements based on the risk involved.

Different limit states for design Limit states are the states beyond which the structure no longer satisfies the specified performance requirements.

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The various limit states to be considered in design may be grouped into the following two major categories:

a) Limit state of strength

b) Limit state of serviceability

a) Limit state of strength:

The limit state prescribed to avoid the collapse of a structure which may endanger the safety of life and property are grouped under this category. The limit state of strength includes:

  • Loss of equilibrium of whole or part of the structure.
  • Loss of stability of structure as whole as part of it.
  • Failure by excessive deformation.
  • Fracture due to fatigue.
  • Brittle Fracture.
b) Limit state of serviceability:

The limit state of serviceability includes: ) Deformations and deflections adversely affecting the appearance effective use of structure are casing improper functioning of equipmem or services or causing damage to finishing.

  • Vibrations in structures or any part of its components limiting functional effectiveness.
  • Repairable damage or crack due to fatigue:
  • Corrosion
  • Fire.

Advantages of limit state method

  1. It uses modern method of design of structure which involves wide range of logical and technical consideration.
  2. It involves separate consideration of different kinds of failure, types of materails, and types of load.
  3. It provides economical design of structure.

Disadvantages of limit state method

It uses the probable approach of design so it may not provide sufficient strength and serviceability at the worst combination of loads and during extreme condition.

I hope this article on “Different between limit state and working stress method” remains helpful for you.

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