Case Study: Molasses Tank Failure

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ultimate strength, material selection

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On January 15, 1919 a steel tank used for storing molasses burst killing 19 people, injuring 40 people, and destroying nearby houses and businesses. The tank, which was located near Boston Harbor, measured 90 feet in diameter and 50 feet tall. The tank contained 2,300,000 gallons of molasses at the time of the accident. The tank was constructed of steel plates with thicknesses varying from 1/2 of an inch to 5/8 of an inch. The plates were connected with rivets measuring 1 inch in diameter with flat top heads on the inside and conical heads on the outside. Vertical joints failed first by the shearing off of the rivets causing other plates to be torn apart between rivets. Photographs illustrate the collapse of the tank and the magnitude of the clean up effort. Note, the tank failure photo is from a similar failure that occurred in 1979! Molasses weighs 11.75 pounds/gallon or 90 pounds per cubic foot; therefore, the stress on the steel plates averaged 31,200 pounds per square inch, a value which approached the ultimate strength of the steel. Each of the rivets supported a load of 18,000 pounds per square inch even though they were designed to support 10,000 per square inch. Consequently, the rivets were over stressed by 1.8 times. As the chief investigator explained, "My conclusion from this evidence is that this tank was wholly insufficient in point of structural strength to handle its load, insufficient to meet either legal or engineering requirements. I am satisfied that the adequate and predominating cause of this accident was a bursting from internal pressure exceeding its structural strength."

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Author:
Dave Goodman
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