A SEISMIC POST ELASTIC BEHAVIOR OF SPHERICAL TANKS

French regulation for seismic verification of structures is separated in two parts : * the "normal risk" for a structure which affects only it's surrounding when it collapses, * the "special risk" for a structure which could be dangerous for persons at long distances The first category includes all buildings. The calculation methods are in general static and the post elastic behavior is taken into account by a reduction factor. This factor allows to divide seismic stresses by a factor, function of the type of structure. For reinforced concrete and steel structures, values of this factor can be found in literature and codes (PS92 in France). The second category concerns the nuclear field and SEVESO's industrial sites (petrochemical industry for example). For the "special risk", the calculation method is a modal spectral analysis, which is an elastic calculation method too. The ability of a structure to have a ductile post elastic behavior is also taking into account by "reduction factors". The spectrum to be considered is regulatory where the seismic level is low (95% of French territory) and must be calculated elsewhere (taking into account site effects). If we consider industrial equipments, only a few values of this reduction factor exist, and they are not specified in codes. The engineer has to define and justify the values he chooses. To find a solution to this problem, the French Ministry of Environment decided in 1997 to make a comparison of French and US approach for this problem. This document gives values for some equipments but concludes anyway that some more calculations are necessary to be sure of these reduction factors. In 1999, The Ministry of Environment decided to undertake specific calculations for some equipments. This paper presents the first calculations performed for this project.