America's structurally deficient bridges

The Term That Sounds Scarier Than It Is

"Structurally deficient" is one of the most misunderstood terms in infrastructure policy. When media reports that a bridge your family crosses daily is structurally deficient, panic can set in. But the reality is more nuanced — and actually more alarming in a different way.

A structurally deficient bridge is not necessarily a bridge that is about to fall. Many SD bridges remain fully open to all traffic, including heavy trucks. The SD label means the bridge has significant deterioration in one or more components that makes it a priority for repair or replacement — not that it will collapse next week.

That said, the SD designation is not meaningless. Bridges with SD status do carry real risks: they are more susceptible to sudden deterioration from weather, flooding, or overloading. They require more frequent inspections. And they are significantly more expensive to repair the longer action is delayed.

What Makes a Bridge Structurally Deficient?

FHWA classifies a bridge as structurally deficient when:

• Any primary element (deck, superstructure, or substructure) is rated 4 or below on the 0–9 condition scale
• The bridge's waterway adequacy or channel condition is rated 2 or below
• The operational status has been restricted due to structural concerns

The most common trigger is a superstructure or substructure rating of 4 ("poor — advanced section loss, deterioration, spalling or scour"). Scour — the erosion of soil around bridge foundations by water flow — is particularly dangerous because it happens underwater, out of view, and can cause catastrophic failure with little visible warning.

The Real Risk: Underinvestment Over Decades

The backlog of bridge repairs in the US is a direct result of decades of deferred maintenance. Bridges have a design life of typically 50–75 years, but many in service today were built in the 1950s and 1960s — the era of the Interstate Highway System buildout. They are now 60–70 years old, well past their design life.

ASCE's 2021 Infrastructure Report Card gave US bridges a C+ grade — the highest of any infrastructure category, but still reflecting significant deferred maintenance. ASCE estimated that one in three bridges needs repair or replacement.

The Infrastructure Investment and Jobs Act (2021)

The Bipartisan Infrastructure Law signed in 2021 allocated $40 billion specifically for bridge repair and replacement over five years — the largest dedicated bridge investment in the nation's history. Under the Bridge Formula Program, every state receives a formula-based allocation to address their worst bridges first.

Progress has been made: the number of structurally deficient bridges has declined from about 56,000 in 2016 to around 35,044 today. But even at current investment rates, eliminating the SD backlog entirely would take decades.

Famous Bridge Failures and SD Bridges

Not all structural bridge failures involve bridges that were already SD-rated. Some high-profile failures occurred without warning:

• I-35W Mississippi River Bridge (2007): Collapsed in Minneapolis, killing 13. The bridge had been rated "structurally deficient" since 1990 but remained open. The failure resulted from a design flaw combined with additional weight from construction equipment.
• Silver Bridge (1967): Collapsed in Point Pleasant, West Virginia, killing 46. This tragedy directly led to the creation of the National Bridge Inspection Program and the NBI database.
• FIU Pedestrian Bridge (2018): Collapsed during construction in Miami, killing 6. This was a newly built bridge, not an old deteriorating one.

The lesson from history is not that all SD bridges will fail — it's that aging infrastructure requires sustained attention, and when it doesn't get it, the consequences can be catastrophic.

States With the Most Structurally Deficient Bridges

The geography of SD bridges follows patterns of industrial history, climate, and infrastructure age:

• Iowa: Leads in raw SD count due to its enormous rural bridge network — tens of thousands of county road bridges that carry modest traffic but require sustained local government funding.
• Pennsylvania: Home to many mid-20th-century urban bridges that have outlasted their design lives.
• Oklahoma and Louisiana: Climate, flooding, and heavy truck traffic from agriculture and energy sectors accelerate deterioration.
• West Virginia and Mississippi: Budget constraints limit the pace of repair despite high SD rates.

Check our rankings page for the current state-by-state breakdown of SD rates.

What Can You Do?

As a driver, the most practical steps are:

• Know your bridges: Use PlainBridge to look up bridges on your regular commute. A Poor or SD-rated bridge on a heavily traveled route deserves attention from local officials.
• Report visible damage: Cracking concrete, exposed steel rebar, sagging, or unusual sounds while crossing are all worth reporting. See our guide on how to report a damaged bridge.
• Engage locally: Bridge repair funding often comes down to county and municipal budget priorities. Local advocacy matters for rural bridges in particular.
• Support infrastructure investment: Federal bridge programs require both federal funding and state matching funds. Support for infrastructure investment at all levels of government translates directly to safer bridges.

The Bottom Line

America's structurally deficient bridges are not about to collapse en masse. But they represent a real and growing infrastructure liability — one that gets more expensive to fix the longer action is deferred. The 35,044 SD bridges in the US today represent tens of thousands of communities, commuters, and supply chains relying on infrastructure that needs attention.

The good news is that the system works: federal inspection requirements catch deterioration, restrictions and closures are imposed when needed, and investment — while never enough — is increasing. The bad news is the pace of repair still lags behind the pace of deterioration in many states.