Ultimate Stress Design Method And Its assumptions with Limitations

Ultimate Stress Design Method :

Ultimate Strength Design method is used extensively and almost exclusively in many countries for structural design practice .The Working Stress Design (WSD) method designs RC sections assuming them to be within their elastic limits, where stresses are proportional to strains. Large margins or factors of safety are assumed on material strengths to ensure such behavior. It is equally, if not more important to predict the ultimate strength of RC sections so that they can be designed to resist the largest loads anticipated during their design lives. The materials are not expected to remain within their elastic limits at such high stresses. More realistic methods of analysis, based on actual inelastic behavior rather than assumed elastic behavior of materials and on results of extremely extensive experimental research, have been performed to predict the ultimate strengths .The Ultimate Strength Design (USD) method, derived from such works .

Assumptions of Ultimate Stress Design Method :

1. Stress in reinforcement varies linearly with strain up to the specified yield strength. The stress remains constant beyond this point as strains continue increasing. This implies that the strain hardening of steel is ignored.
2. Concrete sections are considered to have reached their flexural capacities when they develop 0.003 strain in the extreme compression fiber.
3. Strains in reinforcement and concrete are directly proportional to the distance from neutral axis. This implies that the variation of strains across the section is linear, and unknown values can be computed from the known values of strain through a linear relationship.
4.  Tensile strength of concrete is neglected.
5.  Compressive stress distribution of concrete can be represented by the corresponding stress-strain relationship of concrete.

Limitations of Ultimate Strength Design Method :

Collapse of  FLEXURE,COMPRESSION,SHEAR,TORSION 

Reference : UAP BD ,Design for Flexure By Murat Saatcioglu,Quora

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