In flexural analysis of reinforced concrete cross-sections, which assumption is NOT correct?

The assertion regarding the variation of concrete stress with strain is critical in flexural analysis. In reinforced concrete, concrete is considered to have a nonlinear stress-strain relationship, particularly beyond the initial linear elastic region. While it does exhibit a linear relationship in the elastic range up until about 30% of its compressive strength, this relationship cannot be generalized for all strain levels due to concrete's behavior under stress, especially as it approaches its ultimate capacity.

In flexural analysis, the assumption is typically that strain varies linearly across the depth of the section, corresponding to the curvature of the element due to bending. Steel stress, on the other hand, is assumed to behave elastically and linearly proportional to strain until it reaches its yield point, at which point it will follow a different model.

Concrete's inability to sustain tensile stress further solidifies that its tensile stress cannot equate to its yield stress because concrete experiences tensile failure in a brittle manner, usually occurring at much lower stress levels compared to its compressive strength. This intrinsic characteristic underlines why stating concrete tensile stress equals yield stress contradicts fundamental material behavior principles in reinforced concrete design.