Substructure Condition Rating

What It Means

The substructure includes all structural elements below the bearings: abutments (end supports), piers (intermediate supports), pile caps, footings, and piles that transfer loads from the superstructure to the underlying soil or rock. Substructure Condition Rating is recorded as NBI Item 60 on the 0-9 scale, where 9 indicates Excellent condition, 5-6 indicates Fair, 4 is the threshold for structurally deficient, and 0 indicates Failed. Inspectors evaluate for differential settlement, tilting or rotation, cracking (particularly shear cracks and flexural cracks), spalling, scour undermining at piers in waterways, corrosion of reinforcing steel, deterioration of bearings, abutment backwall damage from thermal movement, and erosion or undermining of footings. Substructure problems are often the most expensive to repair because many components are below waterline, underground, or require cofferdam construction and dewatering to access, substructure repair projects frequently cost 3 to 10 times more per square foot than equivalent deck work. Scour, the erosion of sediment around piers and abutments by flowing water, is the leading cause of bridge failures in the United States and a primary substructure concern, responsible for roughly 60% of all U.S. bridge collapses with known causes. The Schoharie Creek Bridge collapse on the New York Thruway in 1987, which killed 10 people, was caused by scour undermining a pier footing during a spring flood. FHWA requires all bridges over water (roughly 480,000 of the 617,000+ NBI bridges) to have a scour evaluation recorded in NBI Item 113, and bridges identified as scour-critical must have a Plan of Action on file with the state DOT. In the BridgeWatch Condition Score, Item 60 carries 34% weight, slightly higher than deck and superstructure, because substructure failure modes are often catastrophic and difficult to detect visually before significant movement occurs.

Frequently Asked Questions