In the working stress method, the stress-strain curve is assumed to be linear to the yield stress. To overcome the problems and damages due to uncertainty in design, permissible stress is kept as a fraction of the yield stress. As the factor of safety is the ratio of yield stress to working stress, the members of the structure are sized to keep stresses within the permissible value.
The working stress method is the oldest systematic analytical design method. Although IS 800-2007 gives more details and preferences to limit state method, it also permits the use of the working stress method wherever the limit state method is not practically adaptable.
Stress due to DL + LL ≤ Permissible Stress
Stress due to DL + WL ≤ Permissible Stress
Stress due to DL + LL + WL ≤ 1.33 Permissible Stress
In the Limit State Method, the structure should be able to carry and withstand all the loads which will act on it throughout its life. Structure should not suffer a total collapse under any kind of accidental load such as explosions.
The acceptable limit for safety and serviceability requirements before failure occurs is called as Limit state.
|In WSM considers the first yield as design criteria and allowable stress considered as the Upper limit of strength in the cross-section of the member.||LSM considers the functional limit of strength, stability, and serviceability of both single structural members and structure as a whole.|
|Using allowable stress avoids the problem of bulking and yielding.||Post bulking and yielding can happen in LSM|
|WSM does not consider the redistribution of stresses as redistribution occurs after yielding.||In LSM redistribution of stress considered in statically indeterminate structure.|
|Due to greater factor of safety, the allowable stress is the fraction of yield stress ie. allowable stress is significantly less than the yield stress.||Much Smaller material safety factor γm are used for yielding and unlimited stress.|
|It gives uneconomical sections.||It gives economical sections.|
|A greater factor of safety is used.||A smaller factor of safety is used.|
In the working stress method as the overall capacity of steel up-to, the yield stress is not used that is why we require more steel than the limit state method eventually resulting in uneconomical section.
The plane Section before bending will remain plane after bending.
The steel behaves as a linear elastic material.
The working stress design usually results in relatively large sections of structural members, compared to ultimate load. Because of this, structures designed by working stress method gives better serviceability performance under working loads.