Biggest Challenges in Bridge Construction

The major challenges for building urban bridges are the availability of skilled labor, access to urban areas and environmental compatibility.

A worker paints the Anzhaite Long-span Suspension Bridge in Jishou, Hunan, China (Source: The Daily Mail)

Building bridges in megacities with the current scarcity of skilled labor will require a massive recourse to prefabrication. In a few circumstances, prefabricated bridge units will be transported on water with tugs and barges, which will allow the use of heavy, large units. In most cases, prefabricated bridge units will be transported on the ground through congested urban roads, which will lead to the use of light, modular units.

A floating crane lifts prefabricated deck sections onto the San Francisco-Oakland Bay Bridge (Source: San Francisco Public Press)

The availability of deck assembly areas and the interference of construction operations with adjacent infrastructure are additional challenges that will govern the bridge design process. As such, incremental launching construction from aerial platforms will see new applications, especially when combined with on-site welding of field splices among modular bridge units. The welding of field splices will also allow for optimized segmentation of bridge units, diminish the cost of field splices, and will relax the fabrication tolerances of the units.

Size Determines Cost, Cost Determines Everything Else

When constructing a bridge for an urban area, the size of a bridge governs the construction process. in turn, the construction cost of a bridge determines the materials and technology. Technology includes labor and investment in special construction equipment. The quantities of structural materials for a bridge depend on the design loads of the bridge, the flexural and shear span of the bridge units, and the mechanical strength of the material.

The Jiaozhou Bay Bridge in China is the longest sea-crossing bridge in the world (Source: The New York Times)

Small and large-scale bridge projects are both necessary in megacities and demand will only increase in light of the newly emerging megacities all over the world. When looking at both the construction of new bridges and the maintenance of existing bridges, the number of small-scale projects will definitely be larger than the number of large-scale projects. The impact these construction projects will have on the mobility of people and goods within a megacity is massive.

Although one may assume large-scale bridge projects with a larger budget will allow for design optimization and the efficient use of high-grade steels, scale economies in competition with other megacities will govern the availability of construction materials and workforce. Eventually, the scarcity of structural materials will lead to the efficient, eco-friendly use of steel and concrete in large and small-scale bridge projects alike.

Prefabrication and Incremental Launching for Bridge Construction

It is true that small-scale bridge projects have smaller budgets for technology, which limit design optimization and construction mechanization and increase the labor demand. Therefore, small-scale bridges will most likely be procured as packages of multiple bridges to acquire scale economies and a more efficient use of materials with the optimized design of modular units.

On the other hand, large-scale bridge projects allow for massive investment in special construction equipment, which will facilitate the prefabrication of modular bridge units in smart, eco-friendly factories. It will also diminish the labor demand of site assembly and the need for complementary infrastructure in an urban environment, as well as enhance the quality and durability of the final product.

Thus, large-scale bridge projects will be designed for modularity and have prefabricated standardized units with asynchronous production lines. Parts of the bridge will likely have different cycle times, just-in-time delivery, and require minimal site operations. Overall, construction technology and risk management of the trans-disciplinary relationships of mechanized construction will dictate the design of large-scale bridge projects in megacities.

Workers assemble a prefabricated bridge in Pennsylvania, U.S. (Source: Roads and Bridges)

Small-scale bridge projects will take advantage of incremental launching technologies. Launched bridges minimize the interference between deck construction and the obstruction to overpass, and this is a major advantage for urban bridges designed to overpass congested infrastructure. Launched bridges do not require extra clearance to support the deck during construction, which simplifies connecting the bridge with existing roads and railways. Launched bridges do not require additional right-of-way as the deck is built behind the abutment and incrementally pushed into position. Additionally, the construction area is far from the infrastructure to overpass, which minimizes the risk for workers and the traveling public.

Incremental launching applied to a bridge deck construction process (Source: CARLOS FERNANDEZ CASADO S.L)

Materials For the Future Generation of Urban Bridges

Steel and concrete are the most common materials for bridges. In the field of steel bridges, high-grade steel will reduce the self-weight of bridge superstructures and the cost of piers and foundations. New composite systems and mechanized plate corrugation will increase the buckling capacity of unstiffened web panels and compression flanges to avoid the use of welded stiffeners.

In the field of prestressed concrete, new steels for rebar will offer higher strength and corrosion resistance to increase the durability and service life of the next generation of urban bridges. Post-tensioning materials are already extremely efficient, and the challenge will revolve around finding new duct systems and passivating materials to able to avoid the quality concerns raised by cement grouts.

Full-span precasting has been employed in thousands of spans of high-speed railway projects and in hundreds of spans of light-rail transit projects. Both steel and prestressed concrete bridges will be present in the mass transit systems of megacities, and both types of bridges are perfectly compatible with steel decks should high-grade steel turn out financially competitive over prestressed concrete in the megacity-oriented life cycle cost analysis.

Modern large-scale bridge projects are designed for 75 or 100-year service life in the USA. The use of renewable protective materials can easily meet this target in steel bridges, but the evolution of design loads and service conditions of urban bridges is hard to predict. Steel bridges offer a major advantage over prestressed-concrete bridges from this point of view, as they are more adaptable and can be modified, strengthened and adapted to new use conditions.

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Thanks to its strong tensile properties, steel is a very practical material, as it can be reused again and again, from one product to the next, while consistently maintaining its inherent qualities. In fact, according to the most recent data compiled by the Steel Recycling Institute (SRI), approximately 80% of steel used today has been previously recycled.

Eco-friendly and Economical Benefits

But durability isn’t the only thing that makes recycling steel so valuable. It’s eco-friendly and cost efficient, too. So much so that it takes 74% less energy to recycle steel than it does to make it from raw materials – enough to power almost a sixth of America’s homes for a year!

It’s also cheaper to reprocess steel than to mine iron ore, or to create new steel, which is an added bonus in today’s budget-conscious society.

How It Works

Typically, when a manufacturing product is no longer considered valuable to its owner, or the metal of a structure meets the end stages of its life, its steel components are picked apart as scraps. The scraps are then melted in high-temperature furnaces, which in turn liquefies the steel and burns off any remaining impurities. Once pure, the liquid metal is molded into new products, such as tools or engines.

Recently, however, some very clever minds have taken the way we use recycled steel to a whole new level.

Subway Cars Turned Underwater Reefs

Along the eastern seaboard, retired New York subway cars have found a new home on the floors of the ocean. And while it may seem that dumping these mammoth vehicles into the sea would be anything but helpful to the ecosystem, the trains that once transported New Yorkers across the Big Apple are transforming into habitats of millions of fish.

The project, which aimed to help the environment, was launched about 10 years ago by New York’s Metropolitan Transportation Authority (MTA).

After being decommissioned, cleaned and stripped of all removable items, some 25,000 cars were transported by barges and dumped off the coast. Although the campaign is no longer in operation, the cars have since been transformed into artificial reefs.

These unlikely habitats continue to provide plenty of space for invertebrates to live, and act as a hideaway for fish seeking protection from predators. The reef also functions as a source of food, offering more viable conditions than the sand bottom for the growth of various nutrients and organisms.

Old Bridge Gets New Life

While the steel which was once used on land is now being repurposed in water on the East Coast, the reverse is happening on the opposite end of the country.

After 77 years of linking San Francisco to Oakland, California’s Bay Bridge remains to be an icon of the region. Its structure, however, was deemed “earthquake unsafe” after a 1989 quake destroyed part of it. In 2013, its replacement opened to traffic and plans to deconstruct the defective bridge were set.

When scraps of the 58,000-ton steel structure were sold and distributed around the country and abroad after its first of three deconstruction phases, members of the community spoke up, demanding that parts be set aside to be reused in the area.

The Oakland Museum, in coordination with the Bay Area Transportation Authority (BATA), began to accept proposals for how the steel should be refurbished. Thus far, proposals have included everything from bus stops to rainwater catchment systems to sculptures that will retain the visual essence of the original bridge.

In a time when recycling is more important than ever, reprocessed steel is being reincarnated into structures of both function and form. Whether it be through urban sculptures or underwater habitats, recycled steel will continue to transform the way we see, use and better the world.

Cable Cars Gripped on Underground Steel Cables
San Francisco is a city with exceptional quantities of steep hills, which are not relatively common in other cities in the U.S. In the early 1900s, when automobiles were not common, pedestrian roads, bicycles, and horse carriages were everything on the street. As the city expanded, increasing number of buildings had to position over the hills while the distance for horse carriages also extended further. In fact, on rainy days, some horses slipped and sometimes died from terrible leg injuries. Such limitations in transportation stagnated quantitative development of the city.

In 1869, Andrew Hallidie witnessed the situation and proposed a cable car that grips on steel cables that are installed on the street. The first line of the cable railway started its construction in 1873 and started operating 3 years later. The operation power at the time was a steam engine, which now have changed to electricity.

There are three tracks on the cable car railway; two tracks on each sides are for cable car wheels and one in the center is for the steel cable. This central track has a space underneath that makes the steel cables flow restlessly in the speed of 15km per hour while making a clamorous sound. Interestingly, the steel cable consists of small steel wires twisted together. To be specific, 19 steel wires of 2.5mm- diameter form one medium size bundle. And six of those medium size bundle make one large steel cable of 3.2cm-diameter. The wires are twisted in a spiral to maximize cohesion among them. Also, to protect the wires and cables from friction with surrounding materials, they are greased amply. These cables are changed every 6 to 8 months.

The cable car system has been running for the past 140 years. Though the power source has been changed from steam engine to electricity, the cable cars – packed with people and luggage – are always traveling throughout the city while providing convenience and extending the city. In fact, the cable cars which allow people to indulge in a vivid and romantic scenery, have been made from a tiny steel wire of 2.5mm-diameter.

Golden Gate Bridge, the Turning Point

In 1929, the world economy faced the Great Depression. Especially, the Golden Gate strait separating the city of San Francisco and Sausalito, restricted nearby cities to interact, leading to limit balanced and holistic development of the cities. Therefore, the general public suggested to create a fundamental frame of city development, resulting in construction of the Gold Gate Bridge – a 2.7km-long suspension bridge between the city of San Francisco and Sausalito.

However, circumstances were not idealistic. The bridge construction had to manage the deep sea level of the San Francisco Bay and the bridge deck had to be installed high enough for large ships to pass through. On top of its notorious reputation for its thick fogs and fierce current, the location is a strategically important location for military purposes, which does not allow any possibility of blockade of straits due to possible collisions in the area.

The suspension bridge system was the final proposal chosen to satisfy the tough conditions. Rather than requiring to place a bridge post in the center, the suspension bridge has bridge post on each end, and the bridge deck hangs perpendicularly on the cable placed on each bridge post. Therefore, its structural functions were maximized by minimizing the thickness and ultimately provided enough height for shipments to travel through.

However, the problem was about building the main cable and the bridge deck with the level of technology and given conditions at the time. In order for the main cable to sustain the weight of the bridge deck, pedestrians and vehicles, a large sized steel of which cross section is 92 cm in diameter was needed. At that time, it was impossible to transport a colossal steel structure of the corresponding size. As a result, it was proposed to use a thin steel wire – slightly thicker than a strand of hair – to make transportable bundles and repeatedly crisscrossed them in between the main posts.

Accordingly, 452 steel wires that are 5.2mm in diameter, were twisted as one bundle, which is 11cm in diameter. These palm-sized bundles are crisscrossed between the two main posts to make even bigger bundles. 61 of these bundles make the upper cable structure which is 92cm in diameter. For the final process, these bundles are knotted regularly and a corrosion-proof steel cover tops the bundles. In total, there were 27,572 wires used. Since a single cable requires 128,748 km length of wires, the total length of the wires used for two cables can presumably rotate the earth 6.5 times. The highest point of the post is about 227m from the water level and is about 67m from seawater.

San Francisco-Oakland Bay Bridge, another Symbol of San Francisco

A year before the completion of the Golden Gate Bridge in 1937, San Francisco-Oakland Bay Bridge was built in 1936. While the Golden Gate Bridge opened a route to Sausalito in the North, the Bay Bridge also led San Francisco to grow as a giant metropolitan area by connecting the city with industrially developed Oakland city in the East.

The total length of the Bay Bridge is 13.5km long and is divided into eastern and western by Yerba Buena Island in the middle. Constructed by C. H. Purcell in 1936, the Bay Bridge, is about five times longer than the Golden Gate Bridge. This bridge has a double-decked structure, of which the lower deck used to be for trains, and now both decks are used as one-way roads for automobiles. The bridge decks are 150m above water level, allowing ships to travel through flexibly. The Bay Bridge was built with Cantilever method, which used a mobile work vehicle rather than a connected pan of the deck. This method is known for its economic efficiency in valleys, rivers, oceans, and areas with high traffic. The Bay Bridge now has more than 250,000 vehicles passing through every day.

San Francisco has everything that a modern city requires. Today, San Francisco has grown into a city that is powerful enough to influence the world economy. It maybe, the hair-thin steel wires that enabled this city to achieve such developments in quality along with its miraculously beautiful scenery.

Lee, Young-joo = A specialist in international cities and architectural design at POSCO A&C Design Center. Participated in new city and architectural design projects in Vietnam, Myanmar, Canada, Australia, and other international cities. She is very interested in the uncovered history and stories behind cities and architecture.