API 650 Storage Tank – Critical Zone in the Bottom; 

API 653 Storage Tank Inspection, Repair, Alteration, and Reconstruction Code requires the tank bottom to be within 3 inches from the tank shell to be a critical zone because the area must support the weight of the tank shell that is placed near the welding line. Shell-to-Bottom makes Tank-Bottom in this area highly stressed. Critical Zone will have special requirements for inspections, repairs or reconstruction.