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Repair Windows errors before they cause bigger problemsFix Now →Scan for outdated or missing drivers - takes under a minuteDriver Scan →Bridges collapse when a critical member, connection, pier or foundation can no longer carry the forces acting on it. Corrosion, vulnerable details, construction problems, flood-driven scour and other extreme loads can start that failure. Whether a local defect becomes a disaster also depends on what inspections detect, whether owners act on findings, and whether the structure has another way to carry the load.
How does a bridge go from damage to collapse?
A bridge carries traffic and other loads through a chain of connected components to its foundations and the ground. Engineers call this a load path. A collapse can begin when one part of that path loses enough capacity that it cannot safely transfer the forces it receives.
The initiating problem might be a cracked or corroded member, a vulnerable connection, a foundation losing support, or damage from extreme conditions. What happens next depends on the structure: neighboring components may take on more load, or the initial failure may trigger a sequence of failures. A local defect is not automatically a collapse, but a defect in a critical, poorly redundant part of the load path can have consequences far beyond that part.
Why redundancy matters
Redundancy is a structure’s ability to redistribute forces or preserve a load path after a member is damaged. It can limit the consequences of a local failure, but it does not make inspection or maintenance unnecessary. A bridge with little redundancy may be especially vulnerable if a single critical member fails.
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What are the main ways bridges lose capacity?
Corrosion and neglected deterioration
Corrosion does more than make steel look worn. It can reduce the amount of material carrying load, weaken a connection, or generate forces that move connected components out of position. If deterioration affects a critical member and goes unnoticed or unrepaired, the bridge may lose a vital part of its load path.
Scour and foundation loss
Scour is the removal of sediment around a bridge foundation, often by moving water. It can erode support beneath a pier or abutment and destabilize the structure, even when the problem is difficult to see from above the waterline. Flood conditions can make a vulnerable foundation more dangerous, but underwater inspection and attention to scour protection matter outside a single flood event, too.
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Design, detailing and connection vulnerabilities
A bridge’s large members can appear substantial while a connection or local detail remains vulnerable. Geometry and the way forces concentrate in a detail can affect its resistance to cracking or fracture. The specific design matters: a finding about one bridge detail should not be treated as proof that every bridge with a broadly similar connection is unsafe.
Extreme loading and damage
Bridges must carry expected loads, but unusual conditions can create demands a structure cannot withstand, especially if it is already weakened or its capacity has been misjudged. The cases discussed below show distinct mechanisms—corrosion, foundation erosion and vulnerable members or details—rather than establishing a current ranking of causes.
What do bridge failures teach about prevention?
| Case | Failure mechanism and affected part | What the investigation shows |
|---|---|---|
| Mianus River, 1983 | Corrosion-related forces displaced hangers in a pin-and-hanger suspension assembly. | The NTSB found that deficiencies in Connecticut’s bridge safety inspection and maintenance program meant the displacement was not detected. A connection can deteriorate in a way that changes how it carries load, and a missed defect can remain consequential until failure. |
| Silver Bridge, 1967 | A small crack developed in a suspension-linkage eyebar. | ASCE describes the bridge as having no structural redundancy, allowing the local failure to propagate into a total collapse. The disaster prompted the first national bridge safety inspection program in the United States. |
| Schoharie Creek, 1987 | Erosion beneath spread footings undermined foundation support. | The NTSB attributed the collapse to inadequate riprap maintenance that led to severe erosion. It also cited ambiguous construction plans, inadequate inspection and oversight, and a lack of structural redundancy as contributing factors. |
| I-35W bridge investigation | Design errors and the condition of gusset plates were at issue. | The NTSB’s review of major infrastructure failures concluded that visual inspection alone was inadequate to detect the design errors and assess the gusset plates; non-destructive evaluation should have been used for those critical elements. Inspection needs to match the failure mode and component. |
| Fern Hollow, Pittsburgh | A transverse tie plate on one bridge leg failed after corrosion and section loss. It was a fracture-critical, nonredundant steel tension member. | The NTSB identified repeated maintenance and repair recommendations that were not acted on, along with poor inspection quality, incomplete identification of fracture-critical members, inaccurate load-rating calculations and insufficient state oversight. The case shows how deterioration, missed or mischaracterized risk, and failure to make repairs can combine. |
| Hoan Bridge | A particular lateral-bracing connection detail reduced fracture resistance. | FHWA concluded that the specific detail made the bridge vulnerable to premature failure. That finding concerns the Hoan Bridge’s particular details; it should not be generalized to every bridge using a broad connection class. |
Historical figures also need a clear time and geographic frame. An FHWA National Highway Institute underwater-inspection reference manual reports a compilation by the Structures Division of the New York State Department of Transportation and Texas A&M University: at least 1,502 documented bridge failures during 1996–2005, with 58% attributed to hydraulic conditions. This is a historical count for that period, not a current annual rate or a worldwide estimate.
How do inspections help prevent collapse?
Inspection helps owners find deterioration, understand a bridge’s condition and make informed maintenance and asset-management decisions. In the United States, the National Bridge Inspection Standards (NBIS) provide a coordinated framework for safety inspection of highway bridges on public roads. FHWA says the standards were most recently updated in 2022. They are a framework, not a guarantee that every defect will be found or that every bridge has the same risks.
Match the method to the component
A routine visual inspection may not reveal a defect hidden underwater, within a connection, or in a critical member that needs closer examination. FHWA’s underwater-inspection guidance notes that scour damage can be difficult to see from the surface. The NTSB’s I-35W findings illustrate a different limitation: visual inspection alone was inadequate for detecting design errors and assessing gusset plates, for which non-destructive evaluation should have been used.
Give critical members the required attention
Under FHWA’s NBIS FAQ, fracture-critical members are tension members whose failure could cause part or all of a bridge to collapse. The FAQ says such members or components must receive a hands-on inspection every 24 months or less under the applicable criteria. That interval applies to these designated members under those criteria—not automatically to every bridge component.
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What engineering and construction practices reduce risk?
Design for loads, details and consequences
- Account for expected loads, material behavior, fatigue and fracture risks, and how forces pass through members and connections.
- Pay particular attention to connection geometry and local details; a bridge’s overall size does not establish the capacity of every connection.
- Where practical, provide alternate load paths so one member failure is less likely to cause disproportionate collapse.
Build and maintain what the design depends on
Construction plans, details and inspection practices must be clear enough to support sound work and later evaluation. The Schoharie Creek investigation cited ambiguous plans as a contributing factor. For existing bridges, maintenance must address deterioration and protective measures, including riprap where it is used to protect foundations from erosion.
Evaluate scour and flood vulnerability
Owners need to assess scour potential, inspect underwater components, and plan for bridges vulnerable to scour or flooding. A surface view alone may not show whether sediment has been removed from around a foundation. If a bridge’s support may be compromised, the condition needs evaluation and an appropriate response rather than an assumption that visible damage would be obvious.
Turn findings into repairs and decisions
Inspection results reduce risk only when owners understand their significance and follow through. That means accurately evaluating capacity, setting any needed restrictions, documenting deficiencies, prioritizing repairs and communicating risk to the responsible decision-makers. Fern Hollow demonstrates how inaccurate load ratings and unaddressed recommendations can compound deterioration.
Support oversight and accountability
Effective programs need qualified inspection work, clear identification of critical features, appropriate review and resources to make repairs. Investigations of Mianus River, Schoharie Creek and Fern Hollow show that technical standards alone cannot prevent failures if inspection weaknesses, maintenance needs or oversight problems are left unresolved.
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The cases and standards here chiefly concern U.S. highway bridges and public-road inspection practice. They show why prevention needs several layers: sound design and detailing, construction that preserves the intended load path, suitable inspection, accurate evaluation, timely repair and accountable oversight. They do not establish a current worldwide bridge-collapse rate or a globally comparable share of collapses caused by each mechanism.
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