Beam stiffness degrades with its age in service and its service performanc e is weakened. According to the fundamental characteristics of the reinforced co ncre te

the influence of stiffness degradation caused by different kinds of damage i s obtained. Among them

the crack is the most direct and obvious factor. Further more

according to the analysis of bending rigidity formula presented in current stan dard

an influence parameter of crack development on the stiffness degradation

i.e.

nonuniformity coefficient of tensile steel strain φ

is extracted. Av erage crack distance and crack depth are taken as crack statistic parameters. Based o n analysis and modeling with ANSYS

the modified bending rigidity formula relate d to crack is obtained.