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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Here is what bridge expert Marco Rosignoli had to say about emerging megacities, the greatest challenge of providing efficient transportation and what kind of bridges will need to be built to accommodate the changing urban environment.  

The Cities of the Future

Today, megacities are the gateways of globalization. They drive flows of people, goods, knowledge and money around the world, and they also make a large contribution to economic growth at a national level. Tokyo accounts for 28 percent of the Japanese population and generates 40 percent of the country’s GDP. Paris accounts for 16 percent of the French population and generates 30 percent of its GDP. Many metropolitan regions have higher GDP pro-capita than the national average, higher labor productivity, and faster growth rates.

Given their weight in the national economy, the ability of megacities to compete at a global level is paramount. To attract investment, these cities need modern, efficient infrastructures. As the competition shifts from competition among countries to competition among cities, many countries are developing policies to develop their cities into globally competitive megacities.

“Mega” Challenges for “Mega” Cities

Recent studies show that transportation is the biggest infrastructure challenge of megacities and has a big impact on city competitiveness. Transport problems affect megacities at all levels of development and range from obsolete systems and aging infrastructure of mature cities such as London and New York to an insufficient system capacity of transitional cities and even non-existing basic infrastructure of emerging cities such as Karachi.

The mass transport systems of megacities must be capable of transporting millions of people while putting as little strain as possible on the environment. A good quality of life requires a well-functioning infrastructure, and an effective infrastructure contributes to economic prosperity.

Traffic congestion during rush hour at Shanghai, China

Congestion costs are huge for megacities’ economy, employment and the environment. Air pollution and traffic problems are the top two environmental problems of megacities, and road transport alone is responsible for over 40 percent of discharge of suspended particles into the atmosphere. Although water, electricity, health care, safety and security also need investment, recent studies suggest that these sectors are less likely to see a strong link between spending and improved competitiveness of megacities, despite their important impact on the attractiveness of the city for investment. Instead, stakeholders will prioritize spending on improving transportation infrastructure to boost city competitiveness.

Investing in Transportation for Megacities

Stakeholders are split on whether to invest in new transport capacity or reorganize, revitalize and increase the efficiency of existing infrastructure. When new investment is made available, it will likely be used to deliver incremental improvements to existing transportation systems rather than on new infrastructure projects.

The Goethals Bridge in New York under reconstruction (Source: New York Post)

Thus, megacities can expect to see public investment go toward the maintenance of existing bridges, restoring and partially rebuilding instead of constructing from scratch. Maintenance efforts will include the demolition of existing bridges, combined with the conversion of old-generation steels into new families of high-grade steels. This will reduce the environmental impact and achieve a net earning in structural capacity that will pay off part of the energy cost of the reconversion process.

Overall, public investment in traditional roads and roadway bridges will diminish, and will increase for eco-friendly sectors such as mass transit systems, electrified ground transportation for food and public services, light-rail transit bridges serving local districts from mass transit hubs, and high-speed railway networks connecting megacities to airports, ports and other megacities. The environmental impact of private transportation will also lead to new, greener mass transit solutions, which will become a top priority for investment.

Bridges of the Future

Many urban bridges in the U.S. were designed to be as light as possible in light of the scarcity of steel following the 2nd World War. This led to the use of long-span trusses incorporating built-up sections with trusses and lightening holes. Such bridges require constant maintenance and hand painting of large surfaces, which makes their maintenance financially prohibitive. Bridges that are still in acceptable service conditions may suggest replacement just to avoid maintenance costs.

New-generation urban bridges designed for long service life and minimized lifecycle costs will use plate girders and multiple protective layers of replaceable materials. Modern high-grade steels allow for the use of a smaller number of structural members, which diminishes the number of field splices, diaphragms, lateral braces, and steel surfaces to protect.

The Stonecutters Bridge and container port in Hong Kong

Compared with reinforced concrete, steel offers a higher strength-to-weight ratio that increases the design efficiency of modular bridges. These days, steel bridges are becoming increasingly stronger and lighter. As a core material, steel facilitates the delivery of prefabricated units, simplifies the design of bridge piers and foundations, and is easier and faster to recycle or use in smaller quantities.

In the future, these lighter and smaller bridge units will require more field splices, and new types of bolted, welded or hybrid connections will be introduced to increase the structural efficiency of connections, impose less structural constraints on their distribution, and avoid the weakening of the cross-section with drilled holes.

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

Incremental launching construction in London, UK (Source: Knight Architects)

Overall, emerging megacities will translate into an increased demand for bridges, whether it is from building a new bridge or a reconstructing an existing bridge. The challenge will shift from structural considerations to rapid, cost-effective construction processes within a complex urban environment.

Future bridges that will accommodate stricter environmental and sustainability regulations will feature shorter, lighter spans that can be prefabricated in smart factories and transported and erected rapidly in a congested urban environment.

Steel bridges offer many advantages over prestressed-concrete bridges under these new demands. A transition to greener megacities will provide a wealth of business opportunities, eco-friendly technologies and construction materials, and diversification for the coming future.

Continue on to Part Two of Marco Rosignoli’s post on technical and materials solutions for the construction of large-scale bridges in megacities.

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