An extended essay on the advantages of concrete and segmental construction would probably not present the key points as clearly as a simple listing will. The advantages listed in the box at left became evident as segmental construction was used in Europe starting with post-World War II reconstruction and introduced in the U.S. in the 1970s, and as it spread throughout the world as one of the most competitive methods for bridge construction. It is important to note that the versatility of segmental construction applies not only to highway structures, but also to rail structures and even movable bridges, such as the West Seattle Swing Bridge (described below).
Span Ranges for Various Methods of Erection
Starting with smaller range spans for girder bridges — between 24 m and 46 m (80 feet and 150 feet) — span-by-span construction is the most cost-competitive and the quickest method of erection. Record speeds of a 46-m (150-foot) -span being completed in one day have been met quite a few times. This was the case for bridges in the Florida Keys, where contractors managed to erect one span a day by erecting the precast, prefabricated segments with either an under-slung or overhead gantry, and elsewhere in Florida and in Texas.
The range of spans for girder bridges can be extended to up to 122 m (400 feet) for the precast segmental applications and up to 228 m (750 feet) or even 259 m (850 feet) for cast-in-place segmental construction. An example is the Houston Ship Channel Bridge in Port Arthur, Texas, which has a main span of 228 m (750 feet). Of course, cable-stay bridges, which are able to span beyond the limits of girder bridges, have been built all over the world with spans of between 152 m (500 feet) on the short end and up to 914 m (3,000 feet) on the long end.
Case Studies of Advantages of Segmental Construction
Now, let us look at some examples of U.S. bridges to illustrate how the many advantages of concrete and segmental construction became apparent. PB participated in the design and/or construction in different roles with the exception of the Natchez Trace Bridge. PB's role on the Baldwin Bridge was in both the design and construction. PB was part of the general engineering consultant team on the H-3, Green Line and West Seattle Swing Bridges, and part of the construction engineering and inspection team for the Sunshine Skyway Bridge.Keep in mind that these advantages are also illustrated by some of our other recent projects, including the I-15/U.S 95 (Spaghetti Bowl) Interchange and Vietnam Veterans Memorial Bridge in Virginia; however, these two projects are described in detail in following articles, so are not included below.
Baldwin Bridge, Connecticut. The Baldwin Bridge was bid in competition with a steel alternative, which was also designed by PB using plate girder construction. The segmental option was selected, and Connecticut got its first segmental bridge. The bridge was built with no interruption to the navigational traffic of the Connecticut River and no interruption to any roadway traffic on the two approaches. It presented enhanced environmental advantages, such as no need for mitigation measures in the river beyond the cofferdam construction for the pier construction, and resulted in an aesthetic and pleasing structure (Figure 1).
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| Figure 1: Baldwin Bridge in Connecticut |
H-3, Hawaii. Other good examples of segmental construction are the two bridges on the sides of the tunnel structure that, together with the tunnel, make up the overall H-3 project. The North Halawa Valley structure was built using cast-in-place balanced cantilever construction with travelers supported by overhead gantries (Figure 2). On the other side, the Windward Viaduct structure was built using precast segmental construction. This project has received numerous awards because it mitigated the disturbance of this pristine environmental area in the beautiful island of Oahu. PB was program manager.
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| Figure 2: North Halawa Valley structure in Hawaii |
Natchez Trace Bridge, Tennessee. Although not a PB project, an interesting structure that illustrates some of the other advantages of precast segmental construction is the Natchez Trace Arches. This segmental bridge was built over a national park area where, again, main project goals were to address environmental issues and minimize disturbance to this site and the natural habitat. This bridge also illustrates the aesthetic value of segmental construction (Figure 3) and its practicality. The same type of equipment, construction means and methods were used to build both the arches, and the girder structure supported by the arches.
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| Figure 3: Natchez Trace Arches in Tennessee |
Green Line Bridges, Los Angeles. The Los Angeles Metro’s Green Line Bridges were built over busy intersections in highly traffic-congested areas in the vicinity of the Los Angeles Airport. One of these bridges, the Rosecrans Bridge, followed a diagonal alignment to span Rosecrans and Aviation Avenues. Another, the Kramer Bridge, avoided traffic interruption and reduced right-of-way intrusions in this highly industrial area (Figure 4).
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| Figure 4: Kramer Bridge in California |
West Seattle Swing Bridge, Washington. The West Seattle Swing Bridge in Washington (Figure 5) proves the versatility of segmental construction in an application not normally even considered for concrete bridges — movable bridges. Segmental construction allowed for a controlled and even distribution of dead load in the bridge’s final configuration and during construction. It is the only method that gives the inherent balance of dead load during construction, which allows erection on the permanent mechanism required for rotating a swing bridge. Under any other construction scheme, the rotating mechanism has to be locked and construction has to proceed on a temporary support condition.
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| Figure 5: West Seattle Swing Bridge, Washington |
Sunshine Skyway Bridge, Florida. One of the most recognizable segmental cable-stay bridges in the U.S. is the Sunshine Skyway Bridge with its signature 365-m (1200-foot) single plane of cables. This bridge is one of the best illustrations of the beauty and effectiveness of segmental construction (Figure 6).
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| Figure 6: Sunshine Skyway Bridge in Florida |
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