Figure 1: Nineteenth century cantilever bridge construction (the Firth of Forth Bridge in Scotland)

Early Developments

Nineteenth-century iron and steel truss bridge builders developed a method for constructing large bridges without the use of falsework. They placed individual members in a sequence outward from the supporting piers toward the midspan in as close to a balanced condition as possible to prevent “tipping” of the entire, partially completed structure. This method required only minimal falsework for support during construction because most of the bridge’s weight was supported by the previously constructed structure. Resistance to tipping was provided by limiting the unbalanced weight and either fixing the superstructure to the foundation or using temporary supporting piers. This construction scheme was referred to as “canted lever” or cantilever erection (Figure 1).

In the 20th century, cantilever methods for building prestressed concrete bridges were developed in response to advancements in construction technology. The first generation of prestressed concrete cantilever bridges was constructed in the 1950s as part of the overall reconstruction and expansion of European post-World War II infrastructure, particularly in war-damaged Germany and France. Reconstruction had created a demand for large highway and railway bridges that could be constructed with a minimum amount of factory- manufactured items, such as large fabricated steel members, because many manufacturing facilities had been damaged during the war and were in the process of reconstruction themselves at that time.

Prestressed concrete was a logical material for development under these conditions. It allowed for construction of concrete spans with reduced cross sections and less weight than reinforced concrete construction, and it had undergone an early period of development in Europe by Eugene Freyssinet and other pioneers in the early century. Post-tensioning of high-strength steel wires, bars and strands had become an established construction method by this time.

Starting with construction of Ulrich Finsterwalter’s Lahn River crossing in Germany in 1951, more than 300 prestressed concrete cantilever bridges were constructed prior to the mid 1960s. These early bridges pioneered development of much of the erection equipment and procedures that remain in use today for cast-in-situ segmental bridge construction. The bridges were constructed using a series of full-depth “slices” or segments that were cast-in-situ. Falsework was not required because the relatively short traveling forms needed for casting the concrete segments were supported by the previously erected portions of the bridge superstructure. Longitudinal post-tensioning tendons were placed in the upper portion of the segments and then stressed to provide the compressive force needed to resist tensile stresses from the structures’ dead loads and loadings applied after construction.

The vast majority of the early concrete cantilever bridges were box girders due to the efficiency of the structural form and the stability inherent in the box cross section configuration. Without the requirement for falsework, these early bridges could be constructed at great heights above the ground. The development of high-strength prestressing steels allowed construction of long spans. By the mid 1960s, the first generation of prestressed concrete cantilever box girder bridges had extended concrete girder bridge construction to spans of nearly 150 m (500 feet).

1960s and 1970s: Refinement and Extension of Technology

A majority of the pioneering work in technology, equipment and structural forms that are in use on contemporary segmental bridge projects was developed during the 1960s and 1970s. Earlier bridges had been constructed with midspan deck hinges that allowed the structure to deform under sustained dead loads without inducing large internal forces in the girders, but resulted in rideability problems after the bridge profile geometry changed over time. This problem was addressed by using closure joints between cantilevers to make the multi-span structure continuous. The first rigid frame segmental bridge was the Bouguen Bridge constructed in France in 1963.

Computer Programs for Design. The use of continuity required an improved understanding of the plastic behavior of concrete under sustained loading and of the mathematical models suitable for design. The development of improved material-behavior-prediction models coincided with early applications of generalized structural analysis computer programs for the design of bridges. Specialized computer programs for designing segmental concrete bridges were in the early stages of development during this time.

Following the early developments in Germany and France, cast-in-situ cantilever bridge construction began to be disseminated to many other parts of the world. The first cast-in-situ concrete cantilever bridge constructed in the U.S., the Pine Valley Bridge near San Diego, California, was opened to traffic in 1974.

Figure 2: Precast span-by-span construction with overhead truss (the Spaghetti Bowl in Las Vegas, Nevada).

Figure 3: Cast-in-place span-by-span construction with self-launching forms (Rosario Victoria Bridge in Argentina).

Figure 4: Incrementally launched deck for Neckarburg Arch Bridge in Germany, 1977.

Precasting. A revolution in concrete bridge construction came about during this era in response to the demands of the post-war economy for a large number of high-quality concrete structures that could be assembled in a short time frame-the application of precasting technology to segmental bridge construction. Precasting bridge segments in a plant environment:

• Offered the advantages of improved curing and better quality control
• Reduced the influence of weather on production rates
• Offered opportunities for schedule compression because portions of the superstructure could be fabricated in the plant and placed in storage while the substructure was being constructed.

Many large construction companies developed increasingly larger capacity equipment for precasting, transporting and erecting concrete segments. Specialized epoxies for sealing the precast segment joints were developed. In addition, match cast technology provided a method to precisely match adjacent segment face geometry, allowing the necessary precision to achieve the required structural geometry in the field. The first segmental concrete bridge with precast segments was the Choisy-le-Roi, constructed in Paris in 1964.

Alternative Construction Methods. Alternatives to cantilever erection were developed to extend the scope of segmental bridge construction to many new applications, including:^ftnt1

• Span-by-span. An entire span of segments is supported by a temporary truss or girder until it is post-tensioned and self-supporting. The truss or girder is launched to the next adjacent pier, and the process is repeated (Figures 2 and 3).
• Progressive Placement. The erection of multiple span bridges proceeds in one heading, often with temporary supports to limit stresses in the structure during erection. This method is particularly well suited to sites with severe access limitations or where environmental issues limited contractor access to the work site. Its first cast-in-situ application was in Finland in 1967.
• Incremental Launching. A segment is attached to the previously completed bridge superstructure, and then the entire completed bridge is launched outward before the subsequent segment is assembled. This method was first applied on the Rio Caroni Bridge in Venezuela in 1963 (Figure 4).

Concrete Cable Stayed Girder Bridges. Another feature of this era was the development of concrete cable stayed girder bridges which, being erected in cantilever from the piers, represented a logical extension of concrete cantilever bridge erection methods except that most of the prestressing tendons were external to the bridge deck cross section. Cable stayed concrete bridges extended the span length of concrete bridges to beyond 360 m (1,200 feet) eventually and made concrete bridges cost competitive with steel trusses and arch bridges. Concrete’s high compressive strength made it ideal to resist large axial forces that stays introduce into the deck.

The development of reliable computer software for predicting structural behavior during construction and under service was a necessary precondition for design and construction of this highly indeterminate bridge type. The first concrete segmental cable stayed highway bridge was Venezuela’s Lake Maracaibo Bridge, constructed in 1962. This bridge was followed by cable stayed bridges in Europe and Africa in the late 1960s and early 1970s. The most notable examples of the first generation of concrete segmental cable stayed bridges in the U.S. are the Pasco Kenewick Bridge in Washington and East Huntington Bridge in West Virginia.

Other Bridge Types. A final feature of this period was the application of segmental concrete construction methods to bridge types other than box girders. Precast segmental arches were constructed on the 385-m (1,280-foot) -span Kirk Bridge in Yugoslavia (1980), while cast-in-situ segmental concrete arches were constructed in cantilever for the 195-m (650-foot) -span Van Staden Bridge in South Africa (1970) and other structures in Europe and Japan.

1980s and 1990s: Dissemination of Technology

These decades saw the expansion of segmental construction technology to an increasing number of bridge projects around the world, along with the transformation of segmental construction from a research and development environment to a widely accepted mass production environment.

The large-scale introduction of segmental concrete construction to the U.S. coincided with the completion of the interstate highway construction program in the 1980s and continued throughout the 1990s, with the increasing volume of construction in the Sun Belt states and infrastructure reconstruction in the industrial states. The growth of segmental construction projects was aided also by the declining market share of steel bridges in favor of prestressed concrete structures.

The rapid growth of the Asian economy in the 1990s created a demand for transit systems and elevated expressways that could be constructed with minimum impacts. Development of specialized erection equipment and large-scale prefabrication yards allowed construction of increasingly larger segmental concrete bridge projects. As an example, the Bang Na Expressway constructed in Bangkok in the late 1990s is a 1200-span, continuous, 50-km (30-mile) -long viaduct with a cost of more than $1 billion.

Mega-bridge projects, such as the Storaebelt and Oresund Bridges in Denmark and the Confederation Bridge in Canada, expanded the range of segmental construction to “mega” applications with the weights of concrete segments exceeding 6803 metric tons (7,500 short tons). This heavy lift technology is largely the continuation of previous projects constructed in Europe for use in the North Sea. Continuing developments in heavy lifting technology offer the prospect of increasing span lengths and mean greater use of prefabrication yards to speed erection time.

Current construction of the San Francisco Bay Bridge replacement includes the precasting, transport and erection of segments greater than 453 metric tons (500 short tons). This represents a quantum leap relative to previous precast cantilever bridge projects in the U.S.

The computer revolution of the 1990s gave us a large number of commercially available design tools for engineering these sophisticated structures. The increasing use of the Internet allows projects to be designed in multiple locations, provides closer links to the construction sites and provides tools to assist in the management of larger scale projects.

Figure 5: Vietnam Veterans Memorial Bridge - cantilever construction of a segmental box girder using traveling forms

Precast cantilever construction with overhead truss.

Current Status and Trends

Current developments in construction materials technology offer the potential for future innovations in segmental concrete bridge construction. Lightweight concrete has been used for concrete box girder spans of up to 300 m (1,000 feet) — the current practical limit — and offers great potential for concrete bridge construction in areas of high seismicity. Client demands for increased durability of structures to extend service life and reduce maintenance costs have led to current developments in cable stay, prestressing system and grouting technology. The application of polymers to concrete bridges offers the potential for dramatic reductions in required concrete cross sections and lighter bridge structures. Condition investigation and rehabilitation of existing structures offers new markets for materials and technology developments.

Past experience has shown that segmental concrete bridges are particularly suited to the following types of projects:

• Environmentally sensitive areas where disruption to the ground surface must be minimized (e.g., wetlands, slopessubject to erosion or areas with hazardous materials)
• Reconstruction of large-scale urban expressways where disruption to existing operations must be minimized or eliminated
• Repetitive bridge projects that lend themselves to standardization of structural components and construction activities, such as long transit viaducts or elevated expressways
• Long-span bridges over navigable waterways, long crossings over water or crossings over waterways subject to scouring or unique hydraulic features
• Bridges over areas with poor construction accessibility, such as steep terrain or canyons
• Large-scale projects where a bridge’s construction schedule needs to be compressed to fit overall project schedule limitations.

We can expect that many future projects will fit into these types of categories.

It should also be noted that the construction technology associated with segmental concrete bridges originated from the design-build contract environment, making it extremely well suited to that project delivery method. There have been a steadily increasing number of design-build segmental bridge projects, including PB’s design of the Vietnam Veterans Memorial Bridge over the James River in Virginia (Figure 5).^ftnt2