Risk Mitigation for Bridge Strikes by Large Vessels

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The USA NTSB report, “ Safeguarding Bridges from Vessel Strikes: Need for Vulnerability Assessment and Risk Reduction Strategies,” highlights the critical need for proactive measures to protect vital infrastructure. In alignment with these findings, CROSS and CHIRP’s "Safety Alert- Bridge Strikes by Large Vessels," November 2024, identifies similar risks and the urgent necessity for comprehensive risk reduction strategies. This work reinforces the importance of vulnerability assessments and proactive mitigation efforts to enhance bridge safety worldwide.

Bridges are highly engineered structures, and as a result, major failures are rare. However, when they do occur, bridge failures not only present a risk to life but also cause significant disruption in road, rail traffic, and shipping. This outage of critical infrastructure has a tremendous economic and societal impact.

The history of bridge collapses due to ship or barge collisions is concerning due to the significant risks involved. Between 1960 and 2015, 35 major bridge collapses occurred worldwide from such incidents, resulting in 342 fatalities.

Bridge vulnerability to ships depends on vessel size and control. It is impractical to design every bridge to withstand a strike for every vessel traveling at its top speed; so, a risk-based approach is required, as set down in current international design standards.

Typically, a bridge’s open span is defined by the minimum height of the deck above the high-water level and the clear distance between piers. These factors determine whether the bridge is inherently protected by its geometry or if other considerations must be taken into account. Figure 1 illustrates the various potential impact points for a bridge, both at the superstructure and substructure levels.

A bridge’s deck height is determined by natural topological constraints and by the required clearance for traffic beneath it. In the case of maritime traffic, consideration needs to be given not just to the nature of the existing traffic but also for potential future growth.

Changes in maritime traffic will continue to be driven by the waterway upstream of the bridge. The extent of containerization of freight traffic in response to globalization and the relative ease of moving vast quantities of goods over long distances has generated huge changes. The expansion of the Panama Canal ten years ago and the introduction of Panamax-size container ships to U.S. ports have prompted concern among port authorities with some bridge modifications undertaken to accommodate these larger vessels. This raises risks as vessels of increasing size pass under existing bridges.

Below is a brief overview of increases in size:

Large span bridge decks over water are primarily designed to resist vertical loading and dynamic effects, such as traffic and wind forces. Designing the superstructure to withstand lateral impact forces from marine traffic traveling under such bridges is impractical. Instead, design philosophy is to ensure that the bridge deck is higher than the tallest vessels allowed in the navigational channel.

Over time, the navigational requirements will change. Therefore, a central part of bridge management is periodically reviewing and assessing changing risks to the structure. These risks will involve ship impact, but other risks should also be considered, such as changes to water levels, road and rail traffic, and the capacity of safety features. Global warming will play an increasing role as sea levels rise, and hence, safe clearance heights diminish.

Bridge substructures may appear to be substantial, but they still possess significant vulnerabilities. Substructures are not tolerant of lateral movement, and arresting a ship’s kinetic energy is likely to involve movement and distortion of both the ship and the substructure. Features installed during construction will be proportional to the vessels in use around the bridge at that time. The protection systems need to be regularly reviewed to ensure they remain appropriate for the size of vessels, changes to the navigational demands of the location, and the navigational controls in place.

In response to past bridge collapses, significant efforts were made to provide codified guidance to bridge engineers. In 1991 in the United States, the American Association of State Highway and Transportation Officials (AASHTO) produced the “Guide Specification and Commentary for Vessel Collision Design of Highway Bridges.” This has been updated several times, and the current 2009 version is compatible with the AASHTO 2014 LRFD general code.

In Europe, the forces for ship impact are contained within the Eurocodes. More significant guidance is provided in Ship Collision with Bridges from the International Association for Bridge and Structural Engineering (IABSE). The code drafting committees for the AASHTO and IABSE documents overlap, which helps to ensure consistency of the design rules.

Future merchant shipping designs will be optimized to ensure the Energy Efficiency Design Index (EEDI) for new vessels aligns with the Carbon Intensity Indicator (CII), which focuses on carbon emissions compliance. Similarly, existing ships will need to meet the Energy Efficiency Existing Ship Index (EEXI) to reduce their environmental impact. Meeting these emissions standards may come at the cost of engine power, as vessels might need to lower power output to comply with the requirements for engines and auxiliary systems. This reduction in engine capacity can introduce significant risks, particularly in challenging situations such as adverse weather conditions or strong currents, where greater power may be necessary for safe navigation.

The challenge lies in balancing the need for compliance with environmental regulations and ensuring that ships retain sufficient engine power to operate under difficult maritime conditions safely.

Part of the challenge is that the greatest risks will be for older structures that were designed with shorter spans and lower clearances to accommodate much smaller vessels. Additional risks may be associated with the inherently reduced robustness within such structures. Also, modern practice has a conflicting requirement of tying structures together continuously as opposed to having single, simply supported structures which would result in fewer spans failing.

Difficulty arises in how to upgrade or protect elements that are ill suited to modern shipping requirements. The provision of new passive devices to protect the substructure of a bridge from impact should not require major works to the bridge itself but may have an impact on the navigational clearances.

It is recommended that both maritime and highway or rail authorities controlling bridges over waterways conduct periodic reviews to assess changes in risk over time due to variations in vessel types and cargos using the waterway. This should be included in management standards and guidance for marine, road, and rail bridge maintenance, operation manuals, and incident management procedures.

Navigation errors, loss of power, or loss of directional control can result in catastrophic consequences for both the vessel and the bridge. Reviewing the navigational environment around critical bridges is essential to ensure that risk exposure is adequately controlled.
The highest risk for bridge contact is when a total loss of propulsion or electrical power occurs. Therefore, maintaining the machinery responsible for propulsion, energy, and navigation is critical. Power loss often happens after maintenance in port, underscoring the need to mitigate this risk.

The MARPOL (International Convention for the Prevention of Pollution from Ships) regulations on sulphur emissions require ships to carry multiple fuels and switch between them depending on location. The Alert strongly recommends that port authorities prohibit fuel changes near all navigational hazards.

Climate change has increased the frequency and severity of extreme weather events, such as torrential rains and tropical revolving storms. These conditions lead to stronger river currents that may destabilize vessels near bridge abutments. Additional control measures, like tugs, may be necessary in such situations.

Autonomous ships are becoming more common, and their use must be factored into planning new bridges. These vessels should have built-in redundancy and be equipped with advanced sensors, real-time data, and AI to detect mechanical failures or navigation issues quickly.

Even with sound navigation, engineering, and robust procedures, the safe passage of vessels beneath bridges ultimately depends on people. Human factors play a crucial role in both prevention and response.

Capability and Competence: There is significant variation in crew competence across the industry, especially in high-risk, high-consequence environments such as busy ports or narrow bridge passages. This applies equally to deck and engineering crews.

Engineers, in particular, play a vital role in maintaining propulsion, electrical, and auxiliary systems. A power loss, often following maintenance, has frequently contributed to past bridge strikes so ensuring that crews are both capable and familiar with vessel-specific systems is crucial to preventing such failures.

Bridge and Engine Room Teamwork: Safe navigation depends on coordination between the bridge and the engine room. Poor communication, especially under time pressure or when language barriers are present, can cause delayed or inappropriate responses during an emergency. Effective Bridge Resource Management (BRM) and Engine Room Resource Management (ERM) practices, along with clear communication protocols, are essential for managing complex scenarios, such as navigating near critical bridges.

Fatigue and Decision-Making: Fatigue, workload, and commercial pressures can impair judgement. Decisions to proceed with unresolved technical issues or in marginal weather conditions can increase the risk of an incident.

Bridge owners and port authorities should have contingency plans for handling the aftermath of a major bridge strike by whatever type of vessel. If the waterway is blocked, traffic and commerce will be disrupted, affecting businesses and local communities and the major suspension of road or rail transport. All the providers who might be affected should be aware of any risks posed by a possible bridge strike or other emergency and have contingency plans in place.

The rarity of major incidents involving vessels striking bridges should not lead to complacency. The potential for such catastrophic loss of life and disruption to society underscores the importance of risk-based design processes and the need for periodic review of risk as maritime traffic evolves.

It has been reported that the final cost of the Francis Scott Key Bridge collision will make it the most expensive marine-related loss for the global insurance market on record. Given the stakes, port authorities and designers must collaborate closely to mitigate risks effectively.

There is a need for robust maintenance protocols, well trained and experienced crews, and consideration for environmental factors in passage planning. All of these can reduce the risk of errant vessels.

Even with the best preventive measures, extreme weather, mechanical failures, and human error mean that the potential for collision remains. With advancing engineering standards and technological safeguards, human behavior and decisions remain central to preventing catastrophic bridge strikes. Recognizing the limitations and strengths of human operators is not just supplementary to risk mitigation—it’s essential. ■

About the Authors

Alastair Soane, Principal Consultant and founder, CROSS, has contributed to construction safety, risk management, and structural assessment for buildings and bridges. He has held leadership positions in both industry and academia, shaping the standards and best practices that underpin structural engineering.

David MacKenzie is the Chair of the foundation that owns the COWI Group. In this role he is responsible for the oversight of the research and development programme run by the foundation which sponsors work in the built environment. He is also a Senior Technical Director of COWI.

Dave Watkins, Deputy Director, CHIRP Maritime, spent much of his career at sea, including many years in command of ships of many types and sizes, including VLCCs, Cape Size, Container ships, and General cargo feeder ships, before becoming Fleet Quality Assurance Manager and Designated Person Ashore.