WORKING STRESS DESIGN 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 relation ship between loads and stresses is linear . This was the traditional method of design not only for reinforced concrete, but also for structural steel and timber design. 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 loads” on the structure. As the specified permissible stresses are kept well below the material strength, the assumption of linear elastic behavior is considered justifiable. The ratio of the strength of the material to the permissible stress is often referred to as the factor of safety. However, the main assumption linear elastic behavior and the tacit assumption that the stresses under working loads can be kept within the ‘permissible stresses’ are not found to be realistic. Many factors are responsible for this such as a long term effort of creep and shrinkage, the effects of stress concentrations, and other secondary effects. All such effects resulting significant local increases in a redistribution of the calculated stresses. The design usually results in relatively large sections of structural members, thereby resulting in better serviceability performance under the usual working loads.
Assumptions of Working Stress Design Method
- Plane Section before bending will remain plane after bending
- Bond between steel and concrete is perfect with in elastic limit of steel
- The steel and concrete behaves as linear elastic material
- All tensile stresses are taken by reinforcement and none by concrete
- The stresses in steel and concrete are related by a factor known as “modular ratio
- The Stress-strain relationship of steel and concrete is a Straight line under working load
Limitations of working stress method :
- The assumptions of linear elastic behaviour and control of stresses within specially defined permissible stresses are unrealistic due to several reasons viz., creep, shrinkage and other long term effects, stress concentration and other secondary effects
- Different types of load acting simultaneously have different degrees of uncertainties. This cannot be taken into account in the working stress method
- The actual factor of safety is not known in this method of design. The partial safety factors in the limit state method is more realistic than the concept of permissible stresses in the working stress method to have factor of safety in the design.