Conical Hoppers of Tall Steel Tanks: Case History of Failure and Repair

Abstract

Steel tanks contained wheat in Zaïre, West Africa. They were provided with a conical hopper. After one hopper collapsed abruptly during emptying, inspection disclosed that failure had occurred at the bolted seams. Grain-induced pressures and their distributions were determined. Simple membrane theory and a detailed finite-element analysis calculated hoop and meridional stresses and deformations in the hoppers. They showed that underdesigned, overstressed bolted seams caused hopper failure. The remaining hoppers were strengthened by welding them together. Sixteen mm square steel bars were placed along side the seams to prevent the existing sandwiched neoprene-type gasket from burning and contaminating the fillet welds. Laboratory tests indicated that transfer of forces at the welded seams is through the welded fillets; the bolts are bypassed and become irrelevant. This reduces the maximum stress in the steel to less than half and makes nil the probability for a repeat, abrupt failure of a hopper. All strengthened hoppers are presently used; recent observations show them in an excellent state of repair and performing well.