The Silver Bridge collapsed on December 15, 1967, after a fracture in the lower limb of the eye of eye-bar 330 at joint C13N of the north eye-bar suspension chain in the Ohio side span. The National Transportation Safety Board found that the collapse resulted from a cleavage fracture caused by a critical size flaw that developed over the forty-year life of the structure through the joint action of stress corrosion and corrosion fatigue.