Data published by the U.S. Department of Transportation finds there are 597,479 bridges in the U.S. This count includes all bridges longer than 20 feet. Of the total, 12.35%, or 73,798, are currently classified as structurally deficient.

IAC Transportation researched the Federal Highway Administration (FHWA) National Bridge Inventory. The FHWA lists all of the structurally deficient bridges by State and by Type. The FHWA has 23 different types of bridges.

In response to the collapse of the I-35W steel deck truss bridge in Minnesota, U.S. Secretary of Transportation of Mary Peters said “we want states to immediately and thoroughly examine all similar spans out of an abundance of caution”. According to the U.S. DOT, “there are 756 of the relatively unique steel deck truss bridges in the United States”. This number refers to the number of Truss-Deck bridges in the National Bridge Inventory. Among the FHWA’s 23 different types of bridges, there are two types of Truss bridges: 1) Truss-Deck bridges and 2) Truss-Thru bridges. Secretary Peters did not request inspection of the Truss-Thru bridges. However, of the 12,325 Truss-Thru bridges in the U.S., there are 7,217 (58.6%) currently classified as structurally deficient. By contrast, of the 756 Truss-Deck bridges which will receive an inspection, 264 of them are classified as structurally deficient (34.7% of truss-deck bridges are structurally deficient).

If the goal is to ensure the safety of all bridges with a focus on structurally deficient bridges, then an inspection of only "Steel Deck Truss" bridges may not be broad enough.

High-Risk Bridge Types

This analysis sets out to define "high-risk" bridges as those types of bridges having over 30% which are classified as structurally deficient. This designation implies that these types of bridges are prone to exhibiting the tendencies of structurally deficient bridges. There are 5 “high-risk” bridge types:

· Truss-thru bridges: 58.6% are structurally deficient (7,217 structurally deficient bridges out of the total 12,325 truss-thru bridges)
· Movable-swing bridges: 43.9% (100 out of 228)
· Truss-deck bridges: 34.7% (264 out of 760)
· Girder and Floorbeam system bridges: 34.7% (2,668 out of 7,697)
· Suspension bridges: 32.6% (31 out of 95)

Combined, there are 21,205 bridges which are part of these “high-risk bridge types”.

Within these “high-risk bridge types”, we believe it would be prudent to address the structurally deficient bridges within these “high-risk” types. According to this analysis, there are 10,280 bridges in the U.S. which warrant immediate attention.

It should be recalled that in 1967, the Silver Bridge collapse sparked the creation of the national bridge inspection program. The Silver Bridge was a suspension bridge with truss-like characteristics. All truss and suspension bridges should be inspected carefully and frequently. Furthermore, the “high-risk” bridge types specified here should be researched to determine methods that can be used to reinforce existing structures and to develop improved designs for construction of new bridges in the future. Additional research may also be warranted into the redundancy of existing bridge designs.


For more information about the following bridge analysis, contact: Marcus Bowman at mb@iacdctransportation.com or 202-223-7040.