Bridge Safety in the Spotlight After Baltimore Collapse

To view the figures and tables associated with this article, please refer to the flipbook above.

The recent collapse of the Francis Scott Key Bridge underscores the critical need to safeguard bridges from vessel collisions and highlights the impacts a major bridge failure can have.
In addition to the tragic loss of life, a major bridge collapse can have far-reaching adverse effects on the local transportation infrastructure and economy. Bridge failures not only snarl roadway traffic and slow down critical freight, but they can also block shipping channels and major ports for weeks or even months. While vessel collisions are more common than the public may realize, most incidents occur on inland waterways when barges impact bridge piers or their protection systems. Most of these collisions do not significantly damage the bridge or result in closures.

A large-scale collapse such as the Key Bridge is an exceedingly rare event—yet, it has raised awareness of the engineering challenges involved in bridge safety and prompted global concerns about similar vulnerabilities.

The Key Bridge collapse shares several similarities with the major collapse of the Sunshine Skyway Bridge in Tampa Bay, Florida, that occurred in 1980. The Sunshine Skyway Bridge was also a long-span steel truss, and the collapse was caused by an ocean-going vessel striking a primary pier. The incident resulted in 35 fatalities, drawing global attention and becoming a turning point for bridge design in the United States. It also spurred significant research which evolved into the bridge design codes for vessel collision assessment and protection that are still in use today. After that collapse, Modjeski and Masters developed the “Criteria for Design of Bridge Piers Against Ship Collision in Louisiana Waterways,” which was used by several states while the “AASHTO Guide Specifications for Vessel Collision Design” was developed, based in part on Modjeski and Masters’ work. Since then, Modjeski and Masters has continued to perform site-specific vessel surveys, risk assessments, and design of protective measures against vessel collisions.

Bridges that were built before the 1980s—prior to the development of unified vessel collision design guidelines—may be particularly vulnerable to damage or collapse. Many factors can increase vulnerability, including bridge geometry, superstructure type, pier type and locations, frequency and size of marine vessels, and waterway characteristics like current speeds and turns in the channel.

It is important that existing bridges, especially older ones, are assessed for vulnerability by a method that considers all these factors. It is also important that these assessments are updated over time as conditions change, such as increases in marine vessel size or changes in the waterway. If a bridge is found to be vulnerable, appropriate countermeasures specific to the bridge site can be employed to reduce the level of risk.

Container ships in particular have dramatically increased in size over the last 40 years. As open ocean port capacities increased with larger cranes and deeper dredging, shipping companies pushed designs of container carrying vessels larger to increase efficiency. In 2016, the expansion of the Panama Canal was completed, giving rise to new classes of “Panamax” vessels, further increasing the number of large container ships in use.

In vessel collision, ship size is typically expressed in terms of “dead weight tonnage,” or DWT. DWT is the total weight in metric tons that a ship can carry when it is fully loaded, including cargo, fuel, ballast water, and provisions. In the 1980s, large container ships generally ranged up to 50,000 to 60,000 DWT. The largest container ships being produced today exceed 200,000 DWT, representing a 400% increase in mass.

Designing piers and pier protection systems to handle collision loads from large ocean-going vessels can be challenging, and the construction costs associated with these systems can be quite high. Current AASHTO LRFD Bridge Design Specifications express collision load in terms of DWT and vessel impact speed. As the excerpt from the code shows, lateral design loads for collisions from ocean-going vessels can be extremely high.

As we gain a better understanding of the risks our structures could face—and our capabilities of mitigating those risks increase—there is a growing expectation that our infrastructure should be designed and managed to provide resiliency.

Resiliency refers to the ability of a bridge or other section of infrastructure to recover from adverse events in a timely manner. It is important to consider the bridge in the context of the infrastructure system it is a part of. The Key Bridge is not only an important section of the highway network around Baltimore and the east coast, but it also spans a critical waterway and allows access to one of the busiest ports on the east coast.

Engineers have many tools at their disposal for mitigating vessel collisions. For a new bridge, the location and geometry of the crossing can be optimized to reduce the probability of a collision. For example, increasing the span length and locating the crossing away from sharp curves in the channel can reduce the chances that a vessel will strike the piers. Once a pier location is set, design options are available to mitigate the effects of an impact, many of which can be retrofitted to existing bridge sites. The most practical and cost-effective method in the long run is usually to design the bridge pier to resist the loads from an impact directly. Piers that are appropriately sized for vessel collision minimize future maintenance issues, as no external mitigation systems that require inspection and periodic repair or replacement are needed. Additionally, stand-alone pier protection systems can make up 25% or more of the total cost of a new bridge.

For existing structures, engineers frequently employ the use of dolphins, which are large structural elements placed around the pier intended to deflect vessels and absorb the energy of a collision. Fender systems, intended to guide vessels away from piers while also absorbing energy, are also commonly used. Other options include dikes and artificial islands around piers that a vessel would ground on prior to striking a pier. Dikes and artificial islands can be very effective, even for the largest vessels, but they require significant clearances between the pier and navigational channel. This makes them not a feasible option in many cases.

Designing structures for extreme events, such as vessel collisions, hurricanes, earthquakes, and tsunamis, presents a significant challenge for engineers. As time goes on, issues like climate change and increasing vessel size only add to this challenge.

In the past, damage from extreme events was generally accepted, including the loss of services that went along with that damage. Now, expectations have shifted, and we expect our infrastructure to be able to resist extreme events, or be able to return to service quickly after damage occurs. Engineers have the knowledge and tools to meet these challenges, but this requires a commitment to long-term planning and appropriate funding levels.

In the past 40 years, one of the most significant advances in bridge engineering has been the shift from deterministic approaches to probabilistic methods.

A deterministic approach is one-size-fits-all; a design load is determined, and the structure must be designed to withstand that load. A probabilistic approach looks at overall risk and can consider many factors and their chances of occurring. This allows engineers to adjust the target performance level of a structure during design to best meet expectations of the owner. Bridges considered to be critical can be designed to higher performance levels. This approach acknowledges the economic choices owners are often faced with, allowing limited resources to be spent where they are needed most.

As infrastructure ages and environmental conditions evolve, it is imperative that we continue to prioritize rigorous assessments, innovative engineering solutions, and strategic investments to safeguard these critical structures. By doing so, we can enhance resilience, protect lives, and ensure the function of our vital transportation networks for generations to come.