Analysis of equivalent substitution of rectangular stress block for nonlinear stress diagram

In addition to parabola-rectangular, bilinear and rectangular concrete stress-strain diagrams for the analysis of reinforced concrete flexural members according to STR 2.05.05:2005 [1] and EC2 [2] parabola stress-strain diagram with descending branch can be used as well. It is well-known that parabola stress-strain diagram with descending branch provides the most precise description of stress-strain behaviour of the concrete in comparison with the others. Direct application of such nonlinear diagrams for engineering analysis of cross-sections of flexural and eccentrically compressed reinforced concrete members is inconvenient. Therefore in analysis of rectangular cross-section the rectangular stress block (RSB) is substituted for nonlinear stress diagram (NSD). The said substitution should be equivalent which means that the carrying capacity of reinforced concrete member determined using NSD should be equal to that determined using RSB. This equivalence is provided by coefficients which depend not only on deformative properties of the concrete, i.e. stress-strain character of the diagram, but on the method of diagrams replacement also. The coefficients, obtained according to various methods are different for identical replaced diagrams [3-6]. However the said substitution should be equivalent. In literature we found only few methods dealing with substitution of RSB for NSD [7-9] in analysis of cross-sections of flexural members. Therefore in present publication the principles of substitution of rectangular stress block for nonlinear stress diagram has been analysed.