[en] A global method for the analysis of the failure of non elementary structures is examined. The type of failure are: instantaneous or delayed deformation, plastic instability of the material and delayed rupture. The steel structure studied is the lower support of a reactor core composed of two concentric rings, a bottom plate, a top plate and 15 stiffening radial plates. For calculation an annular sector representing 1/30 of this structure is modelized. The behaviour of the structure is examined for the following conditions: an instantaneous temperature rise from 400⁰C to 650⁰C, then the temperature is maintained for 400 hours and decreased from 650⁰ to 400⁰ at a rate of 0.2 degree/hour. The results obtained by elastic analysis, limit analysis and viscoplastic analysis are compared. The reference stress methods is reliable for the determination of structure dimensioning and is less expensive than the viscoplastic analysis. 35 drawings and 19 tables are given