Three factors influence reinforced concrete beams after firing are analyzed

and primary and secondary relations between these factors are determined using the mathematical model of in grey degree of incidence in grey theory

which provides a theoretical basis for evaluation of the safety of the reinforced concrete after firing. Damage degree of reinforced concrete structures after firing is difficult to be determined accurately because the influence of fire on the reinforced concrete structures can not be precisely evaluated. This paper attempts to find out the most important factors influencing the development of the system and grasps the primary character through the comparison of many factors and the analyses of their relative orders. In this paper

the relevant factors of the damage degree of reinforced concrete structures after firing have been put in order through calculation of related degree between "mother" factors （surplus bearing capacity） and "children" factors （ultimate temperture

primary loads

and firing time）. The primary load has a primary influence on the residual bearing capacity of the reinforced concrete members

the ultimate temperture is the secondary and the firing time is the last. The analysis in this paper argue that the influence of the primary loads is the most important factor due to Load Induced Thermal Strain（LITS）. Concrete structures will be cracked by LITS. Meanwhile

damage of concrete members could not be restored when ultimate temperate exceeds 600°C and has a significant influence on residual bearing capacity. It is shown that the theory of grey degree of incidence is worth to be applied in finding out the primary and secondary relations among many factors due to its simple calculation.