Designing Bridge Protection Systems That Work

By Jason Miles, Contributing Author 

The collapse of Baltimore’s Francis Scott Key Bridge in 2024 brought vessel collision risk back into the public conversation. For bridge engineers, however, it’s something we’ve been thinking about for decades.

Any bridge that crosses a navigable waterway carries some level of collision risk. Most of the time, vessels travel safely through navigation channels and nothing happens. But when a vessel does stray off course and strikes a structure, the results can be serious — sometimes even catastrophic.

To reduce this risk, engineers have long worked to design bridges and protective systems intended to sustain, absorb and deflect the dynamic forces that come from a vessel impact.

In many cases, in particular older bridges, the bridge itself is not intended to take the full force of a ship strike. Instead, design engineers relied on protection systems — such as fenders, dolphins or sacrificial structures — that sit around the bridge piers and act as the first point of contact. They receive the impact and absorb or redirect the energy, keeping the vessel from reaching or causing significant damage to the bridge substructure.

In general, the design of protection systems is not about eliminating risk entirely. It’s about managing it in a feasible and economically viable way that prevents a collision from turning into something much worse.

Why Protection Systems Matter

One thing that has changed over time is the size of the vessels. Shipping has scaled up. Larger ships with heavier loads mean that the kinetic energy in a collision event today can be much higher than what many older bridges were originally designed for. As one comparison, the largest container ships of the 1980’s were typically around 60,000 DWT (dead weight tonnage). Today’s largest container ships have reached 220,000 DWT, an increase of 3.5 times over.

At the same time, a lot of bridge infrastructure is aging. Modern vessel collision design in the U.S. began in earnest after the collapse of the Sunshine Skyway Bridge in Tampa, Fla. in 1980.

The incident caused 35 fatalities and spurred significant research that evolved modern design codes. Many structures in service today were designed before these modern vessel collision criteria were established, and in some cases, before protection systems were commonly included at all.

The combination of larger vessels, more frequent vessel traffic and older infrastructure is driving more owners to take a closer look at how exposed their bridges might be to a vessel strike. A significant part of this analysis is determining magnitudes of impact loads associated with design vessels and comparing them to substructure and protection system capacities.

Even at relatively low speeds, large vessels can carry high kinetic energy, and when that energy is transferred into a fixed structure, the resulting forces can exceed what the bridge piers were originally designed to handle.

Protection Systems

Pier protection systems can be considered the first line of defense between a vessel and the bridge.

If a vessel leaves the navigation channel, these systems are intended to intercept it before it reaches the pier. Depending on the design, they can either absorb the impact energy or attempt to redirect the vessel away from the structure — or some combination of both.

Two of the most common system types are fenders and dolphins.

Fender systems are typically constructed around bridge piers and are made up of driven piles, steel or timber frames, concrete components and other elements designed to distribute impact loads and deform during a collision. When a vessel strikes the system, those components absorb energy and aim to eliminate or reduce forces transmitted to the pier.

Fender systems can also be shaped in such a way to help guide slightly off course vessels back into the navigation channel and have facing elements made of low friction materials to prevent vessel hulls from being snagged.

Dolphins are standalone structures placed near piers or along the channel. They often consist of clusters of driven piles or large-diameter drilled shafts that act as obstructions to vessels that have veered off course. In deeper water or for large design vessels, dolphins can increase in size to 60 or 80 feet in diameter.

These are often constructed of a large circle of sheet piles with soil, loose rock and concrete used to fill the interior. The primary goal for dolphins is to absorb as much energy as possible, slow down the off-course vessel and possibly redirect it away from bridge piers.

Another perhaps less common approach involves artificial islands. These are effective at stopping vessels by causing a grounding event, which dissipates large amounts of energy and is less damaging to the vessels.

However, these islands require large horizontal clearances between the navigation channel and bridge piers and are thus not feasible at many bridge sites.

Designing for Impact

When engineers design these systems, a big part of the work revolves around managing energy. A moving vessel carries kinetic energy. If it collides with something, that energy has to be dissipated in order to bring the vessel to a stop.

Modern protection systems are typically designed to absorb energy through controlled deformation. Piles may bend. Structural members may yield. In some cases, components are intentionally designed to crush in a predictable way.

It’s not about building something rigid enough to stop a vessel instantly, which tends to create larger force spikes. Instead, the system is designed to “give” a little, balancing out the impact and reducing peak loads.

Ideally, the protection systems have enough capacity to initiate hull crushing of the ship or barge, which also dissipates large amounts of energy, not unlike how crumple zones behave in today’s car accidents.

To evaluate these systems, engineers use collision modeling to simulate how a vessel might interact with a protection system. Those models consider things like vessel size, speed, angle of impact and hull geometry. From there, designers can determine the number, size and spacing of piles or shafts needed to achieve the desired performance.

Each Bridge Site is Different

One of the realities of this type of work is that no two bridge sites look exactly the same, and protection systems depend on site conditions.

The types of vessels passing through the waterway may vary widely. Some bridges see only occasional tug traffic, while others cross over major commercial routes carrying large cargo ships.

The shape of the channel also affects the design. Currents, tides, channel width and vessel approach angles all influence the likelihood of a vessel going off course toward a bridge. Bridge foundations may be in shallow or deep water, with deeper water presenting more challenges for protection systems.

Then there are the geotechnical conditions. Free-standing protection systems must be anchored into soils that can resist the forces generated during an impact. In some locations that may require deep driven piles or large drilled shafts.

For those reasons, modern pier protection systems are rarely standard designs. Each system is usually tailored to the conditions of the site and the vessels using the waterway.

Proving Their Value

Collisions between vessels and bridges don’t happen often. But when they do, protection systems can make a big difference.

One example occurred in Casco Bay, Maine, in 2007, when a tanker ship transporting 11.3 million gallons of fuel struck a bridge protection system surrounding a pier. The modern steel and concrete system absorbed the energy from the impact and prevented the vessel from striking the bridge itself. The bridge remained undamaged and there was no environmental petroleum spill.

This outcome was in sharp contrast to an incident in 1996 occurring at an older bridge at the same site. That bridge was protected by a weaker timber system, and an off-course tanker ship went through the protection and collided with the pier.

The result was not only damage to the bridge that had to be repaired, but also a 30-foot gash in the tanker hull that spilled 180,000 gallons of petroleum into the bay. The resulting clean up cost $43 million.

Upgrades Worth Considering

For many bridge owners, the conversation around protection systems starts with existing infrastructure. Since many bridges in service were designed before modern vessel collision design guidelines were in place, that may mean little to no pier protection.

That doesn’t automatically mean a bridge is unsafe. But it does mean conditions should be evaluated, especially if vessel traffic has changed over time. Owners often consider upgrades when:

• Vessel sizes in the waterway increase.
• Vessel traffic increases in frequency.
• Changes occur in the surrounding waterway or when the bridge predates current design practices.

A vessel collision risk assessment is typically the first step. These studies look at the likelihood of a collision and the potential consequences.

In some cases, existing conditions may be acceptable. In others, relatively targeted improvements — like adding dolphins or enhancing fender systems — can significantly reduce risk.

A cost benefit analysis may be performed to assess the effectiveness of structural protection systems compared to operational changes in reducing vessel collision risk.

Common Misconceptions

One misconception about pier protection systems is that they are designed to completely stop large vessels. That’s rarely the goal. Most systems are designed to absorb energy and redirect the vessel rather than bring it to an immediate halt.

Another misconception is that only large bridges need protection systems. In practice, smaller bridges located near active waterways may face similar risks depending on vessel traffic.

Smaller bridges over inland waterways subject to barge traffic may experience smaller strikes more often, and protection systems can be effective in these situations.

Engineers must also ensure that protection systems do not create navigation hazards. Protection systems have to be placed carefully so they protect the bridge while still allowing vessels to pass safely through established channel clearances.

That balance requires coordination between bridge engineers, maritime authorities including the U.S. Coast Guard and navigation experts.

Designing for the Future

Bridge design has shifted from reacting to past events to planning for future ones, and vessel collision engineering is part of that transition.

Today’s designs don’t just consider current conditions. They also account for how vessel traffic and infrastructure demands may change. Vessels are getting larger, traffic in some waterways is increasing and bridges are expected to remain in service for many decades.

Additionally, expectations in bridge performance have changed. Modern bridges are expected to withstand extreme events like vessel collision and return to service quickly.

At the same time, engineers now have better tools for evaluating collision risk and designing protection systems. Improvements in modeling and analysis allow designers to better understand how impacts occur and how protective systems should respond. Modern vessel tracking systems allow detailed traffic data to be collected and analyzed.

Vessel collisions will likely remain relatively rare events. But when they do happen, well-designed protection systems can significantly reduce the damage and help prevent a much larger infrastructure failure.

These failures can lead to costly repairs, significant economic disruptions, environmental disasters and, worst of all, loss of life. Protection systems are a key tool used by bridge designers and owners to help avoid these terrible outcomes.

Jason Miles, P.E., is a senior project Manager at Modjeski and Masters.