API 650 (API, 1993) provides design criteria for fluid storage tanks with frangible roof joints. The rules are intended for fluid storage tanks used to store flammable liquids which could be subjected to sudden over-pressurization. Over-pressurization can occur due to the ignition of flammable vapors and can exceed the capability of the pressure relief vents specified in storage tank design.

Sudden over-pressurization can lead to the catastrophic loss of tank integrity. One undesirable mode of failure is the loss of the shell-to-bottom joint, which results in loss of containment and spillage of the contents. One such example is shown in Figure 1.1 for a tank not designed to API 650 rules (Gugan, 1978). A similar failure occurred at the Monsanto Company (Morgenegg, 1978), when lightning ignited the vapor above the liquid surface of a 20 foot diameter by 20 foot tall tank, resulting in a tear of the shell-to-bottom joint. To prevent such a failure, the rules in API 650 are intended to ensure the frangible roof-to-shell joint fails before failure occurs in the tank shell or the shell-to-bottom joint. Failure of the frangible roof-to-shell joint provides a large venting area and reduces the pressure in the tank.