10 Difference Between Working Stress Method (WSM) And Limit State Method (LSM)

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What Is Working Stress Method?

Working Stress Design Method is a method used for the reinforced concrete design where concrete is assumed as elastic, steel and concrete act together elastically where the relationship between the loads and stresses is linear. 

The method basically assumes that the structural material behaves as a linear elastic manner and that adequate safety can be ensured by suitably restricting the stresses in the material induced by the expected ‘’ working load’’ on the structure.

What You Need To Know About Working Stress Method

  1. The stresses in an element is obtained from the working loads and compared with permissible stresses.
  2. This method can also be referred to as deterministic method because the method assumes that the actual loads, permissible stresses and factors of safety are known.
  3. Working stress method is based on the elastic theory which assumes that concrete and steel are elastic and stress strain curve is linear for both.
  4. Material capabilities are under estimated to large extent. Factors of safety are used in working stress method.
  5. Ultimate load carrying capacity cannot be predicted accurately.
  6. In working stress method, the material follows Hooke’s law as stress is not allowed to cross the yield limit.
  7. In working stress method, a section which is plane before bending remains plane after bending.
  8. In working stress method, no factor of safety is used for loads.
  9. Working stress method is less economical as it gives thicker sections.
  10. Average or statistic values are used in working stress method.

What Is Limit State Method?

Limit state method, also referred to as Load and Resistance factor design (LRFD) is a method which considers the ultimate strength of the material (which is ignored in working stress method) and also assures that the structure is serviceable for its intended period of design.

In limit state method of design, structure shall be design to withstand safely all load libel to act on it throughout its life with appropriate factor of safety. It shall also satisfy all serviceability requirements such as limitation of deflection and crackling etc.

What You Need To Know About Limit State Method

  1. The stresses are obtained from design loads and compared with design strength.
  2. This method can also be referred to as non-deterministic because the method is based upon the probabilistic approach which depends upon the actual data or experience.
  3. Limit state method is based on the actual stress-strain curves of steel and concrete. For concrete the stress-strain curve is non-linear.
  4. The material capabilities are not under estimated as much as they are in working stress method. Partial safety factors are used in limit state method.
  5. The ultimate stresses of materials itself are used as allowable stresses.
  6. In Limit State Method, stress is allowed to cross the yield limit.
  7. In Limit State Method, a section which is plane normal to the axis of structural component remains plane after bending.
  8. In limit state method, design loads are obtained by multiplying partial safety factors of load to the working loads.
  9. Limit state method is more economical as it gives thinner sections.
  10. Characteristic values derived from probabilistic approach are used in limit state method.

Also Read: Difference Between Plinth Beam And Tie Beam

Difference Between Working Stress Method And Limit State Method In Tabular Form

BASIS OF COMPARISON WORKING STRESS METHOD LIMIT STATE METHOD
Description This method can also be referred to as deterministic method because the method assumes that the actual loads, permissible stresses and factors of safety are known.   This method can also be referred to as non-deterministic because the method is based upon the probabilistic approach which depends upon the actual data or experience.  
Stresses The stresses in an element is obtained from the working loads and compared with permissible stresses.   The stresses are obtained from design loads and compared with design strength.  
Basis It is based on the elastic theory which assumes that concrete and steel are elastic and stress strain curve is linear for both.   It is based on the actual stress-strain curves of steel and concrete. For concrete the stress-strain curve is non-linear.  
Material Capability Material capabilities are under estimated to large extent. Factors of safety are used in working stress method.   The material capabilities are not under estimated as much as they are in working stress method. Partial safety factors are used in limit state method.  
Ultimate Load Ultimate load carrying capacity cannot be predicted accurately.   The ultimate stresses of materials itself are used as allowable stresses.  
Principle In this method, the material follows Hooke’s law as stress is not allowed to cross the yield limit.   In state method, stress is allowed to cross the yield limit.  
Plane Section In this method, a section which is plane before bending remains plane after bending.   In this method, a section which is plane normal to the axis of structural component remains plane after bending.  
Factor Of Safety In this method, no factor of safety is used for loads.   In this method, design loads are obtained by multiplying partial safety factors of load to the working loads.  
Economical Sense The method is less economical as it gives thicker sections.   The method is more economical as it gives thinner sections.  
Characteristic Values Average or statistic values are used in working stress method.   Characteristic values derived from probabilistic approach are used in limit state method.