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Bridges Over Troubled Water – How Waterway Bridges are Built, How They Fail

Mar 27, 2024 | Blog

At this moment, with the Francis Scott Key Bridge in Boston having collapsed after being rammed by a container ship, bridges are on everybody’s minds. We don’t know any more than you about what happened there (at least at the time of this writing). Regardless, we thought this would be a good time to talk about bridges in general, especially bridges that span waterways like the Scott Key.

These massive structures, especially when they stretch over vast waterways, are marvels of modern engineering that not only facilitate transportation and connect communities, but also showcase human ingenuity and the ability to overcome geographical challenges. Constructing a bridge over water is a complex task that involves careful planning, mathematical precision, and a commitment to safety. Let’s dive into the world of building long bridges over waterways, exploring how they’re constructed, the math behind them, their safety measures, lifespan, and maintenance, while keeping things as light as possible.

The Groundwork: Planning and Design

Before any construction begins, a tremendous amount of planning is required. Engineers and architects must consider the purpose of the bridge, the type of traffic it will support, and the environmental conditions it will face. This initial phase involves detailed surveys and environmental impact assessments to ensure the bridge will be both functional and sustainable.

Environmental Conditions

Water Currents and Hydrodynamic Forces

When constructing a bridge over a waterway, understanding the behavior of water currents is paramount. Strong currents can exert significant hydrodynamic forces on the bridge’s foundations and supports. These forces must be accounted for in the design phase to ensure the stability of the bridge. For instance, in areas with powerful currents, deeper and more robust foundations are necessary to prevent the bridge from being undermined or washed away.

Wind Speeds and Aerodynamics

Wind is a powerful force that can have a significant impact on long-span bridges. High wind speeds can cause a bridge to sway, potentially leading to structural fatigue or failure in extreme cases. Engineers must consider aerodynamics in their designs, shaping the bridge in a way that minimizes wind resistance and ensures stability. The famous Tacoma Narrows Bridge collapse in 1940 is a stark reminder of what can happen when wind effects are not adequately considered.

Temperature Fluctuations and Material Expansion

Temperature changes can cause bridge materials to expand and contract, a factor that’s particularly important in long bridges where even small expansions can accumulate over large distances. This thermal expansion can lead to cracks or structural damage if not properly accommodated. Expansion joints are a common solution, designed to absorb these movements and maintain the integrity of the bridge.

Seismic Activity: Earthquakes and Ground Motion

In regions prone to seismic activity, bridge designs must account for ground motion and potential earthquakes. This involves using flexible materials and joints that can absorb and dissipate seismic energy, preventing catastrophic failure. For example, base isolators can be used to decouple the bridge from ground motion, allowing it to move independently of the earth’s movements.

Flooding and Scour

Flooding can lead to scour, where fast-moving water removes sediment from around bridge foundations, potentially undermining them. Bridge designs in flood-prone areas include considerations for elevated roadways and deep, secure foundations that are resistant to scour. Additionally, adequate drainage systems are essential to manage water flow and prevent water from pooling on the bridge surface.

Mathematical Considerations: The Backbone of Bridge Construction

Math plays a pivotal role in bridge construction, ensuring the structure can withstand the forces of nature and human use. Engineers use complex calculations to determine the load capacity of the bridge, taking into account the weight of the bridge itself, vehicles, and even natural forces like wind and water currents. Geometry is crucial in designing the shape and slope of the bridge, while physics helps in understanding the forces acting on the structure. The kind of math we are talking about is WAY beyond the scope of this article.

One of the key mathematical concepts in bridge construction is the strength of materials, which involves calculating the stress and strain that materials can endure before failing. This is vital in selecting the right materials that can support the weight and tension over long spans without compromising safety.

How Bridges are Built Over Waterways

Constructing a bridge over water presents unique challenges compared to building on land, primarily due to the need to deal with the water itself, the foundation on the waterbed, and the environmental impact. Here’s a somewhat brief overview of how bridges over waterways are constructed, focusing on key steps like building pillars in the water.

Building Foundations in Water

The foundation is the most critical part of bridge construction over water. Two common methods for constructing foundations are cofferdams and caissons:

  • Cofferdams are temporary enclosures built within the water that allow the water to be pumped out to create a dry work environment for the foundation construction. Once the foundation is built, the cofferdam is removed. This method is suitable for shallower waters.
  • Caissons are large, watertight chambers that are floated to the site, then sunk into place. Workers excavate the soil inside the caisson under air pressure until it reaches solid ground. The caisson itself then becomes the foundation or part of it. This method is used for deeper water or more challenging soil conditions.

Constructing the Piers (Pillars)

Once foundations are set, the next step is constructing the piers, which are the vertical structures that support the bridge deck. Materials like reinforced concrete or steel are commonly used. The construction of piers in water often requires specialized floating cranes and barges to transport and place materials.

Installing the Superstructure

The superstructure includes the deck of the bridge, along with any supporting beams or trusses. In some cases, sections of the bridge may be constructed off-site and transported to the location for installation. Methods like cantilever construction (extending sections sequentially from the piers) or using temporary supports can be employed.

How Building a Bridge Over a Waterway Differences from Land Construction

Building over waterways necessitates dealing with several additional complexities:

  • Hydrological impact: Understanding and mitigating the impact of construction on the water flow, local ecosystems, and sedimentation.
  • Access: Construction equipment and materials often need to be transported by barge or floating crane, requiring additional planning and logistics.
  • Foundations: The need for robust foundations that can withstand water currents, pressure, and potentially corrosive environments demands specialized construction techniques like cofferdams or caissons.
  • Environmental considerations: There are often stricter regulations around construction in waterways to protect aquatic ecosystems, requiring additional measures to limit environmental impact.

Building for Safety: Ensuring Longevity and Durability

Safety is the top priority in bridge construction. Engineers incorporate redundancy in their designs, meaning that if one part of the bridge fails, other parts can still support the structure until repairs are made. This is why you often see multiple cables, supports, and foundations in long bridges. Was redundancy not designed into the Scott Key bridge? Or was the catastrophic nature of the incident too great?

The choice of materials is also critical for safety and durability. Steel and concrete are the most commonly used materials because of their strength and longevity. Innovations in materials science, such as the development of high-performance concrete and weathering steel, have further enhanced the safety and lifespan of bridges.

Moreover, designing bridges to be adaptable to their environment plays a significant role in their safety. For instance, in areas prone to earthquakes, bridges are equipped with special joints and supports that can absorb and dissipate seismic energy.

The Lifespan of a Bridge: A Testament to Human Ingenuity

The lifespan of a bridge over water can vary greatly depending on materials, construction quality, and maintenance. Typically, a well-constructed bridge can last over 100 years with proper upkeep. The longevity of a bridge is a testament to the meticulous planning and rigorous standards employed during its construction.

Keeping It All Together: Maintenance and Upkeep

Maintenance is crucial to extending the life of a bridge and ensuring its safety. Regular inspections are performed to assess and repair wear and tear, corrosion, and any structural weaknesses. Advanced monitoring technologies, such as sensors and drones, are increasingly used to provide real-time data on the bridge’s condition, allowing for proactive maintenance.

One of the most common maintenance tasks is the application of protective coatings to prevent rust on metal parts. For concrete structures, sealing cracks and repelling water to prevent freeze-thaw damage are typical practices.

What Can Cause a Bridge to Collapse

Bridge collapses, particularly of the long-span varieties that cross vast waterways, are rare but can be catastrophic when they occur. The reasons behind such failures are often complex and multifaceted, involving a combination of design flaws, material failures, environmental factors, and sometimes human error. Understanding these causes can help in the prevention of future incidents. Here are some of the primary factors that might lead to the collapse of a bridge:

Design Flaws

One of the leading causes of bridge failure is flaws in the design process. This can include underestimating the load the bridge will bear, not accounting adequately for environmental forces such as wind or water currents, or using a design that does not adequately provide for redundancy in the event of a component failure. The collapse of the Tacoma Narrows Bridge in 1940, due to aerodynamic instability, is an often-cited example of a design flaw leading to failure.

Material Failure

Bridges are constructed from materials that are selected for their strength and durability, but these materials can fail if they are of poor quality, used improperly, or subject to conditions beyond what they were designed for. Corrosion is a common issue, particularly in metal components exposed to water and salt (in coastal or winter environments). Concrete can suffer from spalling and cracking, especially if the internal reinforcement begins to rust.

Environmental Factors

Extreme weather events, such as hurricanes, floods, and earthquakes, can overwhelm the design limits of a bridge. For example, floods can cause scour around bridge foundations, removing the support and leading to collapse. Earthquakes can induce forces that are much greater than those considered in the design process, leading to failure, especially in older bridges not designed with modern seismic standards.


Bridges are designed to carry specific loads, and exceeding these can lead to collapse. This might happen if the bridge is used by vehicles that are heavier than those the bridge was designed to support, or if the volume of traffic significantly exceeds the anticipated maximum, putting undue stress on the structural components.

Lack of Maintenance

Regular maintenance is crucial to the longevity and safety of a bridge. Over time, wear and tear, as well as environmental factors, can degrade a bridge’s components. Failure to inspect and maintain the bridge can lead to minor issues developing into critical failures. Rust and corrosion, if not addressed, can weaken structural elements, leading to failure under loads that the bridge would normally be able to support.

Construction Errors

Mistakes made during the construction phase can lead to weaknesses in the structure. This might include using the wrong materials, poor quality control (such as improper curing of concrete), or deviations from the designed specifications. Construction errors might not be apparent immediately and can lead to failure years after the bridge has been completed.

Impact Damage

And now we arrive to what we all witnessed in Baltimore: a calamitous event caused by a gigantic cargo ship crashing directly into the bridge. Bridges can collapse after being hit by vehicles or boats, or due to falling debris from above. While most bridges are designed to withstand typical impacts, extraordinary collisions can cause critical damage, especially if they affect a key structural component. Would it have been possible to build a bridge that could withstand the type of impact as the Francis Scott Key Bridge? We do not have the answer to that in this article.

Age and Obsolescence

As bridges age, materials degrade, and the original design may no longer meet the current standards for load and safety, especially as traffic volumes and vehicle sizes increase. Older bridges might not have been designed with redundancy in mind, meaning that the failure of a single component can lead to the collapse of the entire structure.

Constructing long bridges over waterways is an endeavor that combines scientific principles, engineering excellence, and a deep commitment to safety and sustainability. From the meticulous planning phase to the ongoing maintenance, every step is crucial in creating structures that not only stand the test of time but also serve as symbols of human capability and connectivity.

As technology and materials science continue to evolve, so too will the methods and designs of bridge construction. Yet, the core principles of mathematical precision, safety, and durability will remain the bedrock upon which these future bridges will span the world’s waterways, connecting us in ways we can only imagine.