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|Federal Highway Administration > Publications > Public Roads > Vol. 70 · No. 3 > Get In, Stay In, Get Out, Stay Out|
Publication Number: FHWA-HRT-2006-001
Get In, Stay In, Get Out, Stay Out
by Vasant Mistry and Al Mangus
An ABC solution — prefabrication — offers a faster, cheaper way to build new bridges for addressing the deterioration of some U.S. spans.
Nearly half—more than 250,000—of U.S. bridges according to the National Bridge Inventory are in the 25- to 50-year age range. This is a major concern for many State departments of transportation (DOTs) and the Federal Highway Administration (FHWA) because many bridges have a life expectancy of 50 years—making them near the ends of their anticipated life cycles. The current goal with new bridge construction is a 100-year lifespan.
In addition to aging, about 26 percent of the bridges in the United States are deficient. Highway capacity also has increased little during the past two decades. But traffic demand has grown tremendously, causing increased congestion, with bridge construction projects compounding the problem. Traffic control represents anywhere from 20 to 40 percent of construction costs, and user delays are priced at thousands of dollars per day in heavy traffic areas.
Accelerated bridge construction (ABC) is a swift and economical solution for rehabilitating or rebuilding bridges to address aging, substandard load capacity, safety, and congestion. The following success stories show that many State DOTs, related agencies, and contractors have used ABC, especially prefabrication, effectively.
A Look at ABC
ABC can be defined as replacement or new bridge construction that uses design and construction methods to minimize impacts to the traveling public, river traffic, railroads, and the environment — all while maintaining high levels of quality and safety. Examples include "midnight" lane or road closures, contracting incentives/disincentives, prefabricated elements, and high-performance materials to reduce the typical duration of onsite construction, improving safety and minimizing traffic disruption while producing a higher quality bridge. ABC can be used anywhere along the entire process, starting with planning, where early right-of-way acquisition, expedited environmental permitting, and innovating contracting can speed the early stages. During actual construction, prefabricated elements and cutting-edge equipment and innovations, such as self-propelled modular transporters (SPMTs) and concurrent onsite engineering operations, can hasten the later stages.
One particular ABC method involves maximizing prefabrication — building bridge elements and systems that range from small, discreet parts to entire spans offsite and then moving them into place swiftly — which can greatly cut the time invested in construction, minimizing traffic congestion and safety hazards. Prefabricated deck panels for bridges, for example, can speed construction considerably, with minimal disruption to the traveling public.
"Prefabrication also allows a controlled construction environment that permits safer access by the worker, improves quality, and extends the life and performance of materials," says Byron Lord, program coordinator for Highways for LIFE at FHWA. "Frequently it significantly reduces the man hours of labor required to produce the bridge."
"The need for prefabrication continues to increase due to the growing reconstruction necessary as we celebrate the 50th anniversary of our aging interstate highway system this year," says Mary Lou Ralls, principal at Ralls Newman, LCC, an Austin, TX, consulting firm, and former State bridge engineer and director of the bridge division at the Texas Department of Transportation (TxDOT). "Prefabrication is particularly attractive in addressing this need," she adds, "because of the combined benefits of accelerated construction, so critical in our congested environments, and the improved quality possible with fabrication in a more controlled environment. This improved quality will help these bridges achieve the 75 to 100 years of service that are now needed."
By maximizing prefabrication and minimizing onsite construction time, State DOTs and other bridge owners can reduce costs in time and materials, decrease congestion, and increase safety.
Maximizing Prefab Technology
Nearly all parts of bridges can be prefabricated, theoretically, but what is built ahead of time depends on the particular project. All or part of the construction can consist of prefabricated or precast elements. According to Lord, prefabrication is applicable to most bridge projects, but managers must weigh the cost of materials and the cost of congestion and inconvenience to stakeholders. "As in every engineering decision, it is important to take into consideration the relevant factors that determine the best application for the conditions and needs," he says. (For more information, see the FHWA report Framework for Prefabricated Bridge Elements and Systems (PBES) Decision-Making, available online at www.fhwa.dot.gov/BRIDGE/prefab/if06030.pdf.)
Using precast elements for building larger bridges can be cost effective in materials and alleviating congestion, because precasting concrete "thrives on replication," says Hratch Pakhchanian, a structural design engineer with FHWA's Eastern Federal Lands Highway Division.
Bridge elements might include deck panels, sometimes using fiber-reinforced polymer (FRP) technology. The panels can be moved and placed one right after the other during construction, and later, during repair. With the more traditional approach, an old deck would be removed and a new framework of reinforcement put in place, with the concrete being cast onsite. The waiting time for curing is normally 4 weeks. However, with the use of precast elements that same deck may take only a few days or hours for placement, and be ready for vehicle use immediately.
Likewise, bridge elements such as columns, parapets, box beams, and bent caps can be made from prefabricated materials. Under precasting, the concrete is often prestressed during the offsite curing and bridge parts are threaded together with high-performance steel, which provide added durability.
Entire superstructures including the deck and its railing, striping and signage, the beams or girders supporting the deck, and, in the case of steel bridges, truss spans can be built offsite — sometimes hundreds of miles away — and be moved to a different site for assembly.
FHWA recently held a series of regional workshops on ABC. Applied technology transfer workshops also were held, with agreements between FHWA and the bridge owners to facilitate using ABC techniques for specific bridges. Some examples of ABC successes can demonstrate the advantages of prefabricated elements and systems.
Virginia: Dead Run and Turkey Run Bridges
In northern Virginia, FHWA's Federal Lands Bridge Office refurbished the Dead Run and Turkey Run Bridges on the George Washington Memorial Parkway in 1998 using prefabricated deck panels. Because of the parkway's heavy commuter use — average daily traffic of 43,000 vehicles in 1996 — the bridges needed to be kept open to traffic on weekdays during replacement of the decks.
The three-span Dead Run Bridge consists of two structures, each carrying two lanes of traffic. The Turkey Run Bridge is also two structures of two lanes each, but it has four spans. Both bridges have a 20.3-centimeter (8-inch) concrete deck supported on steel beams with noncomposite action (deck and girders are not structurally linked, and they function independently of one another). This aspect of the original design, along with the use of precast concrete deck panels, facilitated quick deck replacement and allowed the structures to be kept open during weekday traffic. (See also the September/October 2002 issue of Public Roads.)
The construction rate was replacement of one span for one bridge per weekend. The construction sequence involved closing each bridge on a Friday evening, saw cutting the existing deck into transverse sections that included curb and rail, removing those sections of the deck, setting new precast panels, grouting the area beneath the panel and above the steel beam, and reopening the bridge to traffic by Monday morning.
West Virginia: Howell's Mill Bridge
Another deck replacement project involved replacement of an entire superstructure using FRP. In a scenario similar to the Virginia project, when the West Virginia Department of Transportation (WVDOT) overhauled the Howell's Mill Bridge in Cabell County in 2003, significant daily traffic needed to be accommodated.
The West Virginia project also had constructability (difficulty posed to builders by a project's surroundings or circumstances) issues. For example, work on a neighborhood bridge could be complicated by heavy traffic on an interstate that runs underneath the highway. Work on other bridges might encounter different constructability issues such as difficult elevations, long stretches over water, or crowding by adjacent buildings.
For the Howell's Mill bridge project, the requirement to accommodate traffic became a constructability issue, and that issue was largely overcome through use of a prefabricated, full-depth, FRP deck to speed construction.
The 75-meter (245-foot)-long, 9.9-meter (32.5-foot)-wide bridge required a replacement deck of 728 square meters (7,833 square feet). The deck arrived onsite in 2.4- by 9.9-meter (8- by 32.5-foot) panels with a factory-applied skid-resistant surface. All panels were attached in just 3 working days. At about 2.27 metric tons (2.5 tons) each, the panels weighed 20-percent less than their concrete counterparts and required no forms to set up or strip off. And the panels are immune to chloride ion-induced corrosion, making them ideal for environments where deicing chemicals are commonly used.
As an added benefit, panel construction in a controlled environment meant that quality control and sampling of materials was accomplished at the factory, saving time and money.
Texas: Several Successes
In downtown Houston, TxDOT needed to replace two 113-span sections of the IH 45 Bridge in 1997 and used precast bent caps on the existing columns to speed construction and avoid congestion. Designers estimated that a conventional bridge system would require more than a year and a half of construction, with user delay costs of $100,000 daily. Instead, by using precast bent caps, the 226 spans were replaced in 190 days.
The prefabricated elements enabled much of the work to be repetitive, which had its own benefits, according to Kenneth Ozuna, transportation engineer supervisor for TxDOT. "This job was a success due to the repetition of the work and the contract language establishing a schedule coupled with incentives and disincentives," he says. "The work involved repetitive work on more than 60 of the 113 spans — resulting in high efficiency of work crews. An added benefit was convenient access — an entire lane beneath and parallel to the bridge provided unrestricted access for cranes, deliveries, and staging of operations."
Using precast columns on cast-in-place footings or drilled shafts also can greatly reduce bridge construction times. Columns can be segmental and hollow or concrete-filled. In Texas, the Dallas/Fort Worth Inter-national Airport decided to upgrade its Skylink people mover system to accommodate new terminals and more passengers. Casting conventional concrete columns with forms and guy wires for the reinforcing would have been difficult and expensive because of interference with the airport apron. Aircraft terminals and gates would have had to be closed.
Instead, airport officials opted for a precast segmental system of columns. The shorter work time enabled column construction to occur at night, with minimal disruption to taxiing airplanes and baggage movers.
Another Texas project makes use of both prefabricated substructures and superstructures. In the early 1990s, Texas State Highway 249 was upgraded from a four-lane, at-grade road to a limited-access freeway. Two overpasses were built at Louetta Road to carry three lanes in each direction, plus shoulders and ramp transitions. The superstructure consists of trapezoidal, 137-centimeter (54-inch) U-beams as well as precast deck panels supported on the U-beams' top flanges with a cast-in-place composite concrete topping. At the substructure level, at the interior bent, a single segmental pier supports each beam. All beams and piers were precast, designed, and built using high-performance, high-strength concrete.
Developments continue in the use of bent caps and other prefabricated substructures, Ralls notes. For instance, there is a current National Cooperative Highway Research Program project to develop connection details for precast bent cap systems in seismic regions. And FHWA is sponsoring a Multidisciplinary Center for Earthquake Engineering Research project to develop details for segmental columns in seismic regions, she says.
Louisiana: Lake Pontchartrain Bridge
Bridge designers and builders are finding ways to prefabricate entire segments of a superstructure. Units may include steel or concrete girders prefabricated with a composite deck, pieced together near the worksite, and then lifted into place. This scale offers tremendous potential advantages for traffic flow and constructability.
In Louisiana in 2002, the Department of Transportation and Development (DOTD) sought to replace an I-10 bridge span over Lake Pontchartrain. The original span, built using prefabrication, was 19.8 meters (65 feet) long and 14 meters (46 feet) wide and weighed 317 metric tons (350 tons). The new span, with a 19-centimeter (7.5-inch) concrete slab cast on precast prestressed concrete girders, was built on a barge on the north shore of the lake and then floated to the bridge site.
The contract allowed the construction company 24 hours of roadway closure for span removal and replacement under an incentive/disincentive clause. The firm placed the bridge span in much less time than that, enabling it to earn the maximum incentive. For DOTD this approach meant minimal disruption to the main artery into New Orleans and the gulf coast and minimized the use of a 161-kilometer (100-mile) detour route.
Wisconsin: Mississippi River Bridge
In Wisconsin, a new bridge across the Mississippi River has a totally prefabricated superstructure. In 2002, the Wisconsin Department of Transportation (WisDOT) decided to build the 784-meter (2,573-foot)-long, 15-meter (50-foot)-wide bridge, changing U.S. 14/61/WIS 16 from a two-lane to a four-lane route to provide safer, more efficient access to downtown La Crosse and into Minnesota. WisDOT opted to use a central prefabricated tied arch section and float it into place before connecting it to the permanent bridge piers.
The bridge elements were built 145 kilometers (90 miles) from the site in pieces manageable for shipping and erection. They were then assembled on barges near the bridge site. The 145-meter (475-foot)-long, 26.5-meter (87-foot)-high center-span steel arch superstructure was finally floated into place in December 2003.
The prefabrication enabled WisDOT to keep the main channel of the Mississippi open to all river traffic during construction per United States Coast Guard requirements. Contract specifications did not allow temporary structures in the river during navigation season. Erecting the arch on barges allowed the work to proceed without interfering with river navigation. It also enabled the contractor to work on both the river piers and the arch simultaneously, speeding the construction schedule.
Connecticut: Church Street Span
Because prefabrication moves so much of the preparation work for bridge construction offsite, the amount of time that personnel are required to work onsite, frequently in or near traffic, at elevation, over water, or near power lines is greatly diminished, thus contributing to improvements in work zone safety.
In Connecticut in 2003, part of the Church Street South Extension project required replacing a 98-meter (320-foot) steel truss span across a rail yard with active power lines, creating a constructability issue. To address the issue, the largest mobile crane in the world lifted a 771-metric-ton (850-ton), prefabricated replacement span into place in a single night, reducing the time that personnel otherwise would have had to work around power lines and moving trains.
North Carolina: Linn Cove Viaduct
Using prefabricated substructure elements reduces the heavy equipment required and the time that the equipment is onsite. The result is less potential damage to sensitive environments compared with conventional construction.
Adjacent to North Carolina's Grandfather Mountain, the Linn Cove area is one of the State's most sensitive biological areas. In an early prefabrication success, engineers in 1983 completed a bridge at Linn Cove as part of the Blue Ridge Parkway, using prefabricated elements and an innovative construction method that had little impact on the area.
To avoid placement of heavy equipment in Linn Cove, the bridge was built in one direction from the south abutment to the north almost entirely from the top down. The only exceptions to this approach were construction of the initial span on temporary piers and construction of a temporary timber bridge that enabled the micropile foundation drilling machine to prepare several of the foundation sites ahead of the erection of the superstructure. "Use of prefabricated elements allowed the construction to be done with little falsework (temporary elements), which minimized disturbance to the ground underneath the bridge," says Gary Jakovich, a senior bridge engineer with FHWA.
The design included 153 superstructure segments, each weighing 45.4 metric tons (50 tons), along with 40 substructure segments weighing up to 40.8 metric tons (45 tons) each. Today the 379-meter (1,243-foot) long Linn Cove Viaduct winds around Grandfather Mountain in an S-shape at an elevation of 1,250 meters (4,100 feet).
At 2.5 kilometers (1.5 miles) long, with some of its slip-formed reinforced concrete columns taller than the Eiffel Tower and an orthotropic steel deck superstructure 270 meters (885 feet) above the river it crosses, the Millau Viaduct in southern France is the tallest vehicular bridge in the world. It was completed in December 2004 for half its original price estimate. State-of-the-art launching techniques, meaning the prefabricated pieces were assembled and launched out from the bridge's abutments, eliminated most of the risk of working on a high bridge.
The four-lane bridge carries the new A75 toll road over the Tarn River valley, which splits the Massif Central and links the towns of Clermont-Ferrand and Béziers. A driver paying the A75 tolls saves 3 hours when driving from Paris to Montpellier or to the freeway systems in France and Spain.
Seven site-cast, slip-form-reinforced concrete piers support the viaduct. They range in height from 78 meters (256 feet) to 245 meters (804 feet), depending on their placement along the uneven valley floor. A nearly 100-meter (328-foot) pylon caps each pier, making the tallest column 343 meters (1,125 feet), which is just 100 meters (328 feet) shorter than New York City's Empire State Building.
A total of seven cable-stay towers support eight orthotropic steel superstructure spans resting on the seven slip-formed concrete columns. The Millau Viaduct is the world's largest orthotropic steel superstructure, consisting of a 204-meter (669-foot) span at either end of the bridge and six 342-meter (1,122-foot) spans in between. One hundred and fifty-four stays attached to caret- or wedge-shaped steel pylons attached to the tops of the concrete piers support the orthotropic steel spans.
For all its impressive dimensions, the Millau Viaduct appears to onlookers as delicate and transparent. The viaduct uses the least material possible, and the most practicable for each component, also making it less costly. To accommodate expansion and contraction of the deck, each concrete column splits into two thinner columns below the roadway, forming a V-shaped frame.
"The bridge not only has a dramatic silhouette, but crucially it makes the minimum intervention in the landscape," indicates its architectural design firm, Foster and Partners.
The bridge curves slightly on a 20-kilometer (12.4-mile) radius, and the roadway has a slope of 3 degrees. Both features provide improved visibility for drivers and make the bridge less monotonous to drive, increasing both alertness and safety.
Prefabrication and a new type of high-grade steel were instrumental in reducing the total weight of the orthotropic steel superstructure. Thousands of pieces were crafted in Eiffage's (company descended from the Eiffel Tower's creator) steel-fabrication facilities in the towns of Lauterbourg and Fos sur Mer and trucked to both ends of the bridge site. These shop-fabricated deck units of 60 metric tons (66.1 tons), each 4 meters (13.1 feet) by 17 meters (55.8 feet), were field-welded in assembly-line processes at both sides of the valley and launched toward each other, meeting over the Tarn River.
Because of the great distances between piers, temporary steel falsework towers were built between them to take the weight of the deck sections as they were pushed out from the bridge ends. Using a system of American-made, high-capacity hydraulic jacks, drawing on high-pressure cylinders and pumps, and guided by global positioning system (GPS) technology, the deck sections were slid out at a rate of 60 centimeters (2 feet) every 4 minutes. Although American technology launched this amazing structure, it is rarely used in the United States. (For an example in Iowa, however, see "Iowa Issues Video on Innovative Bridge Construction Technology" in the March/April 2005 issue of Public Roads.)
When the sections were all in place, the whole deck weighed 36,000 metric tons (39,683 tons), far less than a conventional concrete deck. Another figure: U.S. $523 million (300 million euros), the total cost of the bridge — half the original estimate.
Get In, Stay In, Get Out, Stay Out
A mantra frequently used by the highway community — "Get In, Stay In, Get Out, Stay Out" — has become popular because of the need to reduce traffic congestion caused by work zones. The slogan also is relevant to the reconstruction of bridges because of the pivotal roles that they serve in most transportation systems and the resulting need to avoid long construction times.
The various projects featured in this article prove the effectiveness of ABC for rehabilitating and building bridges to minimize impacts on stakeholders. Many of the projects contain more than one ABC element, such as contracting mechanisms, use of midnight road or bridge closures, and innovative construction methods and tools.
ABC has helped reduce costs, production times, and other frustrations to stakeholders associated with bridge construction.
Vasant Mistry is the senior bridge engineer at FHWA. He serves as the national technical expert for steel bridges and is responsible for promoting high-performance steel, ABC technology, cost-effective steel bridge design, and the use of innovative bridge technologies and materials. He has been with FHWA for 26 years.
Alfred R. Mangus is a transportation engineer with the Office of Structures Contract Management at the California Department of Transportation (Caltrans). He has 29 years of experience, including 14 years with Caltrans. He has received two professional awards from the James F. Lincoln Arc Welding Foundation.
For more information, contact Al Mangus at 916-227-8926, 916-961-ASCE, or firstname.lastname@example.org. Prefabricated bridge elements and systems are one of FHWA's priority, market-ready technologies and innovations. More information is available at the R&T Web site www.fhwa.dot.gov/crt/.
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