The elasticity theory is basically correct.

However, there is a conceptual error in the basic theory of concrete design. The strength design value of steel bar is far less than its yield strength value (for example, the standard yield strength value of HRB400 steel bar is 400Mpa, but its strength design value is 360Mpa). How does the structure "produce great plastic deformation after reaching yield strength" in normal use? If the reinforcement reaches the yield strength, it is judged as structural failure.

The so-called plasticity means that the compressive stress of concrete in the compression zone of beam increases with the increase of load, which no longer obeys Hooke's law, and its stress change has developed from triangle (linear proportion) to approximate rectangle (curve nonlinear distribution). Like pictures