America’s gigantic $28 trillion economy is built atop an infrastructure network featuring some of the most impressive engineering feats in history. Everything from its 600,000 bridges and 49,000 miles of Interstate highway to the city-defining Golden Gate and Brooklyn bridges support the hundreds of millions of cars and trucks transporting millions of passengers and goods. However, a significant portion of this infrastructure is starting to age. Many of these bridges were built in the sixties and are now surpassing eighty years of age, the typical lifespan of steel and concrete constructions. Meanwhile, the US population has nearly doubled since they were built, undoubtedly adding more stress and load to some already frail bones. 

The coming few decades will test the resilience of America’s infrastructure and its ability to rebuild. In 2021, the American Society of Civil Engineers estimated that there is an “infrastructure investment gap” of $2.6 trillion, which if unaddressed could end up costing the United States $10 trillion in foregone GDP by 2039. Bridges are a key facet of that infrastructure. While up to a third of American bridges may be ailing, it also presents the country with the opportunity to rebuild these structures in a way far stronger, more resilient and beautiful than before.

The Aging of American Bridges 

Most bridges are like organisms. Over decades of use, they inevitably begin displaying significant wear and tear. Just as the costs of healthcare begin piling up in old age, sometimes the largest infrastructure expenses balloon at the end of the asset’s life cycle, when maintenance becomes constant until rebuilding or replacement becomes essential. So it shouldn’t come as a surprise that most new bridges built in America today are in fact replacements for old ones.

In 2023, the American Road & Transportation Builders Association released a report saying that more than 76,000 bridges in the United States need replacing, while a third — over 200,000 — require major rehabilitation or replacement.

Although that’s quite a lot, the United States has been doing a fairly good job of playing whack-a-mole over the past few years to repair or restore some of the oldest structurally deficient bridges. Still, the number of bridges coming to term will only continue to grow, especially as the Eisenhower-era bridges start exceeding the 80-year point – often considered the terminal age for steel and concrete construction.

Source: American Road and Builders Transportation Association

Hoping to add fuel to the restoration effort, President Joe Biden signed the Bipartisan Infrastructure Bill into law in November 2021, a $1 trillion bill devoted to modernizing and building net-new infrastructure across the country. $110 billion has been specifically devoted to repairing and replacing outdated bridges and roads. How effective this bill will prove at refurbishing and updating American bridges and roads is yet to be determined. However, as an interesting point of comparison, the initial budget to build the entire Interstate highway system was roughly $215 billion in 2023 dollars.

Another increasingly salient criticism of U.S. bridge construction and maintenance concerns the amount of time it takes to complete major construction projects. A 2015 book entitled Bridges: Their Planning and Engineering offers a damning description of the situation:

“From the time that a bridge is proposed through final construction, the state or locality has to go through a labyrinthine process. When the bridge just uses an existing right-of-way and has no effects outside that narrow band, the process can take as little as three years. With lawsuits, budget shortfalls, and environmental controversies, the process can take two decades, if the bridge is ever built at all.” 

A contemporary example of these frustrations followed the aftermath of the collapse of Baltimore’s Francis Scott Key Bridge on March 26, 2024. A container ship named Dali rammed into one of the bridge’s pillars, causing the bridge to buckle and collapse, killing six construction workers. The bridge not only acted as a massive thoroughfare connecting Baltimore across the Patapsco River, but its collapse effectively cut off goods access to the Port of Baltimore. 

When the Secretary of Transportation Pete Buttigieg gave a speech the following day promising federal funds to fix the bridge, he noted that though building the bridge took five years “does not mean it will take five years to replace.” When the assessments were completed on how restoring the bridge would take, it turned out that it would, in fact, take nearly five years — and that the restored bridge should be expected at some point in 2028.

Clearly, the urgency of equipping the nation to better restore, design and build its bridges is escalating. As it does, perhaps the country should look to both its ambitious past, and abroad to seed the inspiration necessary for a revival in its ability to build efficiently and durably.

The ABCs of Structural Theory

The oldest bridge in the world is the Arkadiko Bridge, in Greece. It was built an estimated 3,300 years ago as part of a road connecting the neighboring city-states of Argos and Mycenae with the port city of Palais Epidavros. The war against Troy was waged by the mythological king Agamemnon, who ruled from his seat in Mycenae, ostensibly around the time that the Arkadiko Bridge was built!

Though we may have more sophisticated building technologies, new materials, and new methods that go along with them today, a great deal of the intuition involved in modern bridge building was already around in these ancient times and even far prior. Architecture was really an art before it was a science, and the intuitive and experiential knowledge required to do it was passed on from master to apprentice for centuries before any of the rules were formalized. 

Today, this knowledge is codified into structural theory, but the core idea that there are only a handful of structural elements at play in the world has been an observation we’ve carried with us for millennia. Man-Chung Tang, a notable structural engineer and bridge builder, calls these elements the ABCs of the structural world, the axial, bending, and curved structures capable of deflecting loads and forces through them. You can try yourself to look around your environment and decompose the structures you find into axial, bending, or curved elements! 

Source: Science Direct

John Stanton, in a wonderful lecture called “Spanning the Gap,” very elegantly describes a lot of the intuition behind how this structural theory is applied to bridge building, specifically.

The goal of every bridge, at the end of the day, is to not collapse under the force of itself or the load it bears, while extending over an empty span. One of the most obvious ways to cover a span is with a beam. “Every bridge is just a beam,” says Stanton.

Given the forces imposed on the beam, there is a quadratic relationship between the span the beam needs to cover and the thickness of the beam. This means the longer you want your bridge to span, the thicker the beam you’ll need to resist the compressive forces pushing down on it. For large spans, the required beam thicknesses become unrealistically huge. In beam bridges, we see the bending element at work.

Stanton emphasizes how effective design can decrease the density of material you need through clever application of structural elements. Trussed bridges, for instance, that transfer power through axial forces, can be far more sparing with material and, as a result, have been in use since the Bronze Age.

Another ingenious and ancient design is the arch, which derives its eon-spanning dependability from the element of curvature, which evenly deflects compressive forces along its sides. Flip an arched design upside down, and you get a suspension bridge. Rather than a compressive force pushing down, the load on the bridge is now balanced by tensile forces pushing outward.

Though all these designs have existed in some form for millennia, not all were possible to implement at scale. The limiting factor for designing and constructing large infrastructural works from bridges to aqueducts has always been the material available. So, the history of our ability to build infrastructural projects is really the history of materials.

A Brief History of Bridges

For the majority of human history, stone was the strongest material available. It was also the most durable, able to persist where timber would rot or dry and fracture. And so, it was the material of choice for the great builders of ancient Egypt, Greece, and Rome.

However, stone was not a good material to build simple beam bridges out of. Stone cannot withstand great tensile stresses, and will simply crack if too great a load is imposed on a span too long. To create longer bridges, the Romans turned to arch bridges. This design decision, based on the structural characteristics of stone, is remarkably what gave the Roman civilization its iconic look, as exemplified by everything from the arches of its aqueducts to the arched dome of the Pantheon. 

The limited ability of stone to withstand tensile stresses still meant that there was a limit on how long the span of the arch could get, so to cover long distances, multiple arches had to be pushed together, like in the Pont du Gard in southern France.

Source: Encyclopedia Britannica

It was only in the past 170 years that new architectures for bridges became possible with the introduction of a new material: steel.

Steel can resist a great deal of tensile stress, which meant that girders could be fashioned out of it to span large distances. However, steel is also ductile, meaning it can be extruded to form wires. High-strength steel cables allowed for the introduction of the first long-spanning suspension bridges and cable-stayed bridges. 

America began building prominent steel bridges just ten years after the invention of the Bessemer process in England, which allowed for the mass production of steel. The first major steel bridge built in America was the Eads Bridge which spans the Mississippi River, connecting Missouri to Illinois. It was the first of what would become a number of iconic American constructions, defining the identities of major cities across the US.

The Brooklyn Bridge is perhaps the most notable of these. When completed in 1883, it was the longest suspension bridge ever built and was referred to as the “eighth wonder of the world.” However, it’s really the story of its construction that most strikingly imprints itself on the imagination.

Its architect was John Roebling, an immigrant who came to the United States from Prussia, filled with utopian ideals and dreams of building a technological paradise on the Western shore of the Atlantic. After starting an agricultural commune in Pennsylvania, Roebling began drawing up designs for aqueducts and bridges across the Eastern United States. His commissioned works include the Allegheny Aqueduct Bridge in Pittsburgh, the Niagara Falls Suspension Bridge, the John A. Roebling Suspension Bridge in Kentucky, and more. 

By 1959, Roebling developed a vision for a bridge spanning the entire East River, connecting the island of Manhattan to Brooklyn. It would take ten years to convince Congress to do it, but in 1869, three weeks after the approval was signed, Roebling died. A foot injury for which he actively avoided treatment led to a tetanus infection and his demise.

In his absence, John Roebling’s son, Washington, took over. Little did he know, he was about to preside over one of the most grueling and deadly modern construction projects in America. The central span of the bridge was to be supported by two twin towers, both of which had to be anchored at the bottom of the river upon caissons. Caissons are retaining structures dropped to the bottom of the river, with compressed air pumped in to keep the water out. Workers would be lowered into the caissons, so they could excavate the mud from the bottom of the river, ultimately lowering the caisson down to the bedrock.

The caisson process was grueling. Men were paid $2 a day, around $50 today, to enter a hot, highly pressurized, dark cage from which they had to dig up mud for 8 hours a day. If they rose to the surface too quickly, they could contract decompression sickness, from which many died. Washington Roebling himself contracted decompression sickness one too many times and found himself bedridden before the construction of the towers could even begin in earnest. His wife Emily had to supervise the operation on his behalf. 

Every part of the bridge construction, from establishing the foundation below the river to building the towers to stretching the seven million pounds of steel cable across the spans, produced casualties. And after fourteen years, it was finally over. Washington, bedridden, never got to step foot on it, but his bedroom had a window from which he could gaze at it with binoculars. For a while, it was the tallest thing on the Manhattan skyline, as it far preceded the era of steel skyscrapers, but when construction began to climb ever higher, the Brooklyn Bridge, presciently, fit right in. 

The next material innovation in bridge-building was the introduction of reinforced concrete, which combined the tensile strength of steel and the compressive strength of concrete into one material. The nature of this material meant that extremely slender and elegant bridges could be built, the most famous of which might be Robert Maillart’s reinforced concrete bridges, found across Switzerland.  

Source: Salginatobel Bridge, Omrania

While incredible constructions were going up across the United States, most of the bridges were being built to decongest traffic in cities. It was the cities or states themselves that commissioned and financed such projects when there was a clear internal benefit, however, for larger projects that could connect states to each other, there was no clear funding body. 

This produced difficulties and security risks. In 1919, a military convoy sought to demonstrate what it would take to move critical people and goods across the United States along the sole coast-to-coast highway at the time, the Lincoln Highway. Due to poor road conditions, the 81 cars traveled at an average speed of 5 miles per hour, finally arriving in California, after setting off from Washington, D.C., in 62 days. One of the Lieutenant Colonels on this expedition was a young Dwight D. Eisenhower, and when he became president in 1953, made it clear that he never forgot the sorry lesson of the transcontinental motor convoy from his youth.

In 1956, his administration passed the National Interstate and Defense Highways Act, a gargantuan bill that would become one of the largest infrastructure projects in the history of the world. Nearly 42 billion cubic yards of earth were moved over thirty-six years to pave 46,000 miles of interstate roads and build 54,663 bridges.

A key element of the act was the standardization of all road and bridge design parts of the Interstate Highway system. Everything from the width of roads, the number of lanes, and the clearances required set constraints on how America’s bridges should look had to be standardized. This was the basis of the girder and composite bridges that now feature in most of America’s highways. With a guidebook set, and the government’s guarantee to foot 90% of the bill, the 1960s onwards marked a true boom in bridge building. By the time the Interstate system was complete in 1992, a trip across the country would only take 42 hours.  

Source: Bridges: Their Engineering and Planning

Steel and concrete were, of course, the materials of choice when building out this vast expanse of American infrastructure. The bridges could be longer, stronger, and more cheaply built than having to build them out of, say, stone. However, there were still trade-offs involved. 

Namely, steel and concrete are not as durable as stone. Both are susceptible to weathering and deterioration from water damage, heat, and other environmental elements and have an average service life of just over 80 years, at the later end of which, they require considerable maintenance and eventually, replacement. The bridges completed during the sixties are now coming upon that eighty-year mark, which means that the United States is moving into a predicament where it will need to evaluate at scale what to do with quite a number of bridges reaching their maturity dates.

The Future of Bridges 

Innovation in construction materials has not yet stopped, nor are we at the limits of what we can do with them. The strongest steel cables available today are shockingly tough. They can withstand 270,000 pounds of force for every square inch, or roughly 20,000 kilograms per square centimeter. This has allowed suspension and cable-stayed bridges in particular to feature longer and longer spans. 

Source: Science Direct

The 1915 Çanakkale Bridge is the longest suspension bridge in the world today, with its longest central span reaching just over 2 kilometers in length. Completed in March 2022, it is the first fixed crossing across the Dardanelles, symbolically connecting the continents of Europe with Asia.

Source: Daily Sabah

As time goes on, however, it seems that the spans we build only continue to shoot up. Based on the strongest steel currently available, which can reach 20,000 kilograms per square centimeter, Man Chung-Tang has estimated that the maximum span length of a suspension bridge might be something like 10 kilometers. We’re only one-fifth of the way there now.

In our era of high-strength steel and concrete, it’s no longer the United States building record-breaking infrastructure. China, whose decade-long infrastructural boom has stunned the world, now has that distinction. China is now home to the highest bridge in the world, the Duge Bridge, the longest bridge in the world, a 102-mile viaduct for high-speed rail along the Yangtze River, and the longest sea bridge, the 34-mile long, Hong Kong-Zhuhai-Macao Bridge, integral to connecting the Greater Bay Area, which consists of Macao, Hong Kong, and Shenzhen and is home to 68 million people. 

The speed at which China is capable of erecting these massive structures is also record-breaking. In May 2023, builders of the Shenzhen-Zhongshan Bridge set a world record for paving more than 22,000 square meters of asphalt in a single day. 

However, give these bridges another eighty years, and China will have to make the same kind of evaluations the United States is deliberating over now. The coming years, as the United States considers whether to rebuild its bridge infrastructure, present an exciting opportunity. It’s an opportunity to re-evaluate how we construct infrastructure and a chance to incorporate some of the most advanced technologies today into the bridges that will be servicing tomorrow. The Bipartisan Infrastructure Law has already earmarked $110 billion in federal funds to rebuild the most economically important bridges, alongside a few thousand smaller bridges. 

Our goal should be to let this investment open the door to considering the use of new materials, from new high-performance concretes to carbon fiber-reinforced polymers, materials that could create stronger, longer-lasting bridges. We should also make room for better sensors and simulators capable of better predicting and monitoring the structural conditions of bridges so that maintenance needs can be identified sooner and fixed faster. 

Bridges are some of the most impressive and awe-inspiring engineered structures in the world. As we look to renew our infrastructure according to existing standards and modest economics, we should aspire, also, to seek beauty within the constraints.

Just as steel and concrete were materials that went on to inspire generations, the new structures we build should inspire pride in future generations, reminding them that engineering is not merely a calculating science, but an art.