Limit State Method and Working Stress Method

Working stress method and limit state method is a technique of designing a structure.

The cardinal difference between the working state method and restrict state technique is that WSM is an elastic design technique whereas LSM is a plastic design technique.

Limit State Method and Working Stress Method:

Working Stress Method:

The working stress design method is a method used for reinforced concrete design, where concrete is treated similarly to elastic steel and concrete wherein the relationship between load and stress is linear.

This method basically assumes that the structural material behaves in a linear elastic manner and adequate protection can be ensured by appropriately restricting the stresses in the material induced by the expected “working load” on the structure.

Advantage of Working stress method of design:

1. This technique is simple in calculation and simple to understand.
2. This methodology provides the worst combination of loadings and higher efficiency throughout excessive conditions.
3. The serviceability requirements are sufficient for the working stress method.

Disadvantages of working stress method:

1. This offers an uneconomical design of the structure.
2. It doesn’t apply the true value of the safety factor.
3. It is a time-consuming method.

Limit State Method:

The limit state method, also known as load and resistance factor design (LRFD), is a technique that considers the ultimate strength of the material which is ignored in the working stress method and also assures that the structure is serviceable for its intended period of design.

Within the limit state method of design, the construction shall be designed to withstand all load libel to behave on it throughout its life with the suitable factor of safety.

It will also satisfy the limits of all serviceability requirements such as deflection and cracking etc.

Advantages of limit state method:

1. It uses a modern way of designing a structure that encompasses a wide range of logical and technical considerations.
2. It includes separate consideration of different kinds of failure, type of material, and type of load.
3. This provides an economical design of the structure.

Disadvantages of limit state method:

1. It uses a possible approach for design so it cannot provide sufficient power and serviceability during the worst combination of loads and peak conditions.

Difference between limit state method and working stress method:

Working Stress Method	Limit State Method
The stress in a component is derived from the working load and compared with the permissible stress.	The stresses are derived from the design load and are compared with the design strength.
This method can also be referred to as the deterministic method as a result of the method assumes that the actual load, permissible stress and safety factors are identified.	This method can also be referred to as non-deterministic because the method is based on a probabilistic approach that relies on real data or experience.
The work stress method is based on elastic theory which assumes that concrete and steel are elastic and the stress-strain curve for both is linear.	The limit state method is based on the actual stress-strain curves of steel and concrete, The stress-strain curve for concrete is non-linear.
Physical capabilities are largely underestimated, Safety factors are used in the work stress method.	The capabilities of the material are not underestimated as much as they are in the working stress method. Partial protection factors are used in the limit state method.
The ultimate load-carrying capacity cannot be precisely predicted.	Ultimate stresses of the material themselves are used as allowable stresses.
Within the work stress method, the material follows Hooke’s rule because the stress is not allowed to exceed the yield limit.	In the limit state method, stress is allowed to exceed the yield limit.
In working stress method, a section which is plane before bending remains plane after bending.	In LSM, a section normal to the axis of the structural element remains on the plane after bending.
In the work stress method, no safety factor is used for the load.	In the limit state method, the design load is obtained by multiplying the load’s partial safety factors to the work load.
The working stress method is less economical as it gives thicker parts.	The limit state method is more economical because it gives thin sections.
Average or data values are used in the work stress method.	The characteristic value limits derived from the probabilistic approach are used in the limit state method.

Also read: Plinth Protection, Modes of shear failure & Moment Resisting Frame

Conclusion:

The working stress method is an old technique of designing within the present era structures, which are normally designed using the limit state method.

Concrete in WSM is considered elastic whereas concrete in LSM is mostly considered plastic.