What It Means
Every bridge in the National Bridge Inventory has a unique Structure Number (NBI Item 8) assigned by the state transportation agency that owns or has inspection responsibility for the bridge. The format varies by state: some use numeric-only identifiers, others combine state, county, and sequence codes, and still others use alphanumeric schemes tied to route mile markers or county road systems. For example, Minnesota uses 5-digit sequential numbers, Pennsylvania uses a 14-character format encoding district and county, and New York uses a 7-character BIN (Bridge Identification Number). Despite the variation, each Structure Number is unique within its state and permanent for the life of the bridge, it does not change when the bridge is rehabilitated, when ownership transfers between agencies, or when the road it carries is renumbered. This permanence is essential for longitudinal tracking: inspection history, load ratings, maintenance records, and condition trends are all keyed to the Structure Number. BridgeWatch uses the Structure Number as the canonical identifier for each bridge detail page, ensuring that a user bookmarking a specific bridge will continue to reach the correct record even if the route designation changes. In addition to the state-assigned Structure Number, the NBI includes Item 1 (State Code, based on FHWA region codes) and Item 5A (Record Type, typically route carried on or under). Together these items uniquely identify every bridge record nationally. FHWA publishes a merged structure identifier by concatenating state code and Structure Number for cross-state analysis, which BridgeWatch uses in its URL slugs.
Structure Number is one of the bridge-engineering or FHWA-policy concepts that recurs across BridgeSafety. Below is how the concept connects to the National Bridge Inventory data behind every page on the site.
Within the BridgeSafety Condition Score, each primary component (deck, superstructure, substructure) contributes about a third of the rating, with an age penalty applied to bridges past their typical design life. The methodology page describes the scoring in full detail.
Frequently Asked Questions
What does "Structure Number" mean?
A unique identifier assigned to each bridge in the National Bridge Inventory by the owning state or agency (NBI Item 8).
Why does Structure Number matter for bridge safety?
Every bridge in the National Bridge Inventory has a unique Structure Number (NBI Item 8) assigned by the state transportation agency that owns or has inspection responsibility for the bridge. The format varies by state: some use numeric-only identifiers, others combine state, county, and sequence co...