A structurally deficient bridge is classified as such if it receives a low rating of 4 or less by the NBI.  Many cases of structurally deficient bridges are caused by flooding overtop, as well as corrosion that can cause significant section loss of steel supports, movement of structure, or advanced cracking and deterioration in concrete bridge decks.  A structurally deficient bridge may be closed or restricted use to light vehicles because of structural deficiency.