The Pennsylvania Turnpike has chosen an innovative bridge
type — known as the precast concrete segmental bridge — for a new, six-lane
signature bridge across the Susquehanna River just south of Harrisburg between
Interchanges 18 (Harrisburg West) and Interchange 19 (Harrisburg East). The
structure would become the first major vehicular bridge in Pennsylvania to
employ segmental design and construction.
In segmental construction, precast concrete "box" sections are manufactured at a concrete plant located at or near the construction site and linked together to form a structure. Precast bridge pier sections are stacked one atop another, and bridge deck sections are connected in a row and hoisted or lowered onto the piers. In concept and in execution, the technique rather resembles the assembly of a child’s Lego structure.
But the precast concrete structure is anything but juvenile in its performance benefits.
"Besides the distinctive, streamlined profile of the segmental structure, these bridges — in certain situations — can be less expensive to build, cheaper to maintain and completed in much less time than competing bridge-construction techniques," said Bradley J. Heigel, P.E., the Turnpike Project Manager overseeing the Susquehanna River Bridge project. "The Susquehanna River Bridge, because of geometric factors such as its length, width and height, is one instance for which the segmental approach is especially well-suited."
Bridge Engineer Manager Gary L. Graham, P.E., added that the repetitive span length — or the equal distance between bridge piers — was another factor that drove the selection of the segmental process. "The consistent, multiple spans of the Susquehanna River Bridge really caters to this type of design and construction," he said.
The new Susquehanna River Bridge will be built parallel to and just 50 feet north of the existing four-lane, 50-year-old bridge. Depending on the design firm selected, it may be constructed as a single, wide bridge or as two completely independent structures — one for eastbound traffic and one for westbound traffic.
Cost to replace the original 4,526-foot Susquehanna River Bridge — the longest wholly owned bridge on the Turnpike — is estimated at $40 million. The replacement project also involves $55 million of construction work to the roadway approaching the bridge, bringing the total project cost to $95 million.
"We at the Turnpike are quite proud to pioneer the precast segmental bridge type in the Commonwealth of Pennsylvania," said Executive Director John Durbin. "The new bridge will enhance safety and convenience for our customers while at the same time serve our increasing traffic volume needs well into the new millennium."
The Turnpike Commission expects that the Susquehanna River Bridge will become a "signature" structure.
"Since it was built nearly 50 years ago, the Pennsylvania Turnpike has traditionally been known as the ‘tunnel highway,’" said Graham. "Now, with the impressive bridges being built on the Mon/Fayette Expressway and with the segmental bridge across the Susquehanna River, the Turnpike may eventually also be known for its bridges."
Turnpike engineers are currently reviewing letters of interest from potential bridge design consultants, and they plan to make a final selection sometime this summer. Design of the structure is expected to last approximately two years.
"We’ve received seven responses to our advertisement for consultants that was issued earlier this spring," said Heigel. "Among those seven are the prominent names in the worldwide segmental-bridge market."
The construction contract is scheduled to begin in early 2003, with construction of the new structure to commence shortly thereafter. Heigel said that he anticipates construction will last approximately three years.
The old bridge will remain open as the new one is being built, thus minimizing any impact to the flow of Turnpike traffic over the Susquehanna River. Once traffic is rerouted over the new structure, the old bridge will be disassembled and the piers removed below the water.
During the past year and a half, an engineering consulting company has been studying the feasibility of either redecking the existing 50-year-old bridge or building an entirely new structure.
A detailed study had determined that re-decking the existing bridge would be too lengthy (up to six years) and too costly. Because all work would have to be completed at night, one lane at a time, the contractor would have a limited window to shut down the lane to be redecked, complete the work scheduled, and then reestablish the two-lane pattern before the morning deadline. If the contractor was unable to reopen the lane due to weather or other factors, traffic delays would certainly have ensued. In addition, shutting down a lane on the bridge during high traffic volume periods posed significant risks in case of a traffic accident or vehicular breakdown.
The decision was made to build a new bridge in January and approved by the Turnpike Commission in February 2000.
Another issue driving the construction of an entirely new bridge related to capacity of the existing structure.
On average, almost 28,000 vehicles per day cross the Susquehanna River Bridge (eastbound and westbound), 18 percent of which is truck traffic. According to Traffic Engineering Manager Tim Scanlon, that average daily traffic volume is expected to double in the next 20 years. "By 2020, we could see as many as 50,000 to 60,000 vehicles per day using the Susquehanna River Bridge," he said. "We need to add one lane in each direction to efficiently manage such rapidly increasing volumes."
INDUSTRY TOUTS SEGMENTAL BRIDGE ADVANCES
The industrial approach used in segmental construction ensures a high level of repetitive operation that, according to segmental bridge industry reports, leads to exceptional productivity. As a result, segmental structures, in the right application, can be less expensive to erect than competing bridge-construction methods. In the case of the Susquehanna River Bridge, the Turnpike could realize estimated potential cost savings of approximately $3 million as compared to other construction techniques.
Another advantage of the segmental method is that of construction duration. In certain cases, industry reports say, construction time has been halved in comparison to conventional bridge-building methods. The Susquehanna River Bridge is expected to take approximately three years to construct as a segmental project, as opposed to more traditional methods that would likely require additional time for construction.
Advocates also tout the construction quality advantage of the segmental structures, due largely to the fact that the modules are manufactured in a controlled environment, or factory, under stringent industrial rules with quality materials, equipment and labor. Such controls, the industry experts claim, lead to superior performance, low maintenance and improved durability.
"Because of the bridges’ increased durability, they typically require less maintenance," said the Turnpike’s Bridge Engineer Manager Gary L. Graham, P.E. "Maintenance engineers for other segmental bridge owners have told us they love these structures; they have been the strongest advocates of the segmental approach."
Another interesting feature of the Susquehanna River Bridge will be the ability to easily and safely inspect the underside of the bridge deck by walking through the enclosed box configuration. The concrete box shape — the design feature that enhances a segmental structure’s strength — is like an enclosed causeway that runs beneath the deck. An added benefit is the ability to easily run public utilities — such as utility lines and fiber-optic cable — through the enclosed box structure.
Precast segmental bridges were invented in France in the early 1960s and used widely throughout Europe during the 60s and 70s. The bridge type was introduced in the United States in the late 70s, when the precast segmental technology was used for the Keys' bridges in southern Florida. During the 80s, the precast segmental construction was applied domestically in many states.
Because of the newness of this type of construction method in Pennsylvania, the Turnpike’s engineering department is planning a series of meetings during design with construction contractors and precast concrete manufacturers who may be interested in bidding on the Susquehanna River Bridge project. During the meetings, these contractors will learn about the details of this particular project.
"The meetings will serve as a mechanism for these firms to understand what we’re trying to accomplish," Graham said. "We also hope that these meetings will help spur interest among perspective bidders within the state and region.
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