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|Federal Highway Administration > Publications > Public Roads > Vol. 62· No. 2 > The First Channel Bridges|
The First Channel Bridges
by Christopher J. Allen and Frank Naret
In the transportation industry, we are always on the lookout for new construction materials or methodologies that will allow us to improve our transportation systems at a lower cost, reduce construction time, and increase the performance life of our investment. Bridge engineers are no strangers to this quest. From high-performance concretes and steels to today's exploration into the use of composites for bridge construction, researchers, designers, materials suppliers, and contractors continue to seek innovative ways to build more durable bridges more quickly and less expensively.
One of the newest entrants into the bridge-building field is the Channel Bridge. The Channel Bridge is a patented, precast-concrete superstructure system that uses post-tensioned segmental construction. The Channel Bridge is appropriate when a replacement or new alignment structure requires an increase in the vertical under-clearance.
J. Muller International (JMI) developed this system in 1990 for the French Highway Administration, and 11 Channel Bridges have been built in France. Jean Muller of JMI is the system patent holder, and Bridgetek of Saratoga Springs, N.Y., is the U.S. licensee.
The Channel Bridge concept moves away from the typical bridge configuration of a deck supported underneath by a transverse or longitudinal support system. The Channel Bridge features two edge beams that function as the main load-carrying elements with a deck supported between them. This completely eliminates the need for a below-deck support system. The longitudinal edge beams serve a secondary purpose as the traffic barrier.
The Channel Bridge's advantages are an increased vertical under-clearance, decreased construction time, and a lower life-cycle cost.
Replacing an existing structure with the Channel Bridge can result in 0.6-meter (m) to 0.9-m increase in vertical under-clearance without affecting the approach grade. This increase in clearance is directly attributable to the deck's attachment to the bottom of the longitudinal edge beams, instead of placement of the deck on top of an underlying support system. (See figure 1.)
Another advantage of the Channel Bridge is faster construction. Segmental construction uses precast components that can be prefabricated or fabricated off-site concurrently with the substructure construction. JMI states that the time required to replace an existing overpass structure is approximately 100 days.
An anticipated life-cycle cost advantage results from increased concrete durability, the need for fewer bearings, and the elimination of painting. Concrete durability should be enhanced from longitudinal and transverse prestressing, which is expected to maintain compressive stresses in the concrete. Therefore, any cracks that develop will remain tight. Durability is also anticipated to be enhanced through precast operations in a controlled plant environment and through the application of a high-density overlay in the field. Fewer bearings should mean less maintenance and replacement of bearings. And because the Channel Bridge does not require corrosion-protection applications, such as painting, some significant life-cycle savings can be realized.
FHWA's and HITEC's Involvement
The Intermodal Surface Transportation Efficiency Act of 1991 established a new program of funding to be used to "accelerate the testing and evaluation of new technologies, both foreign and domestic, which are designed to increase the durability, efficiency, productivity, environmental impact, and safety of highway, transit, and intermodal transportation systems." To carry out this provision of the legislation, the Federal Highway Administration (FHWA) created the Applied Research and Technology (ART) Program. The ART Program consists of four elements: the Highway Innovative Technology Evaluation Center (HITEC), Applied Research, Designated Technologies, and the Priority Technologies Program. FHWA, through HITEC, provided an initial investment of $180,000 to help compensate for the initial investment in reusable forms and erection equipment for the Channel Bridge project.
HITEC was established in 1994 by the Civil Engineering Research Foundation through a cooperative agreement with FHWA. HITEC's mission is to accelerate the time-consuming process of implementing new technologies in the highway industry. Prior to HITEC's creation, the introduction of innovative technologies usually required new product demonstrations to each state highway agency. Normally, this would involve design and construction time followed by an extensive evaluation period in each state. This pre-HITEC process was inefficient, time-consuming, and costly to both the entrepreneurs demonstrating the new technologies and to the state highway agencies that performed evaluations.
The HITEC process provides consensus-based, nationally accepted performance evaluations by establishing technical evaluation panels with members from state and local highway agencies, academia, the private sector, and regulating agencies. The panelists are usually experts in their fields and represent themselves, not their employer's specific interests. The technical evaluation panel, in cooperation with the entrepreneur, identifies the specific concerns that must be evaluated and resolved prior to the application of the new technology by the highway industry. An evaluation plan is developed and executed to address these specific concerns. Upon completion of the HITEC evaluation, a report is typically generated to facilitate the use of the new technology.
The Channel Bridge evaluation panel consists of five state highway engineers, four consulting engineers, three industry representatives, one academic representative, and three FHWA engineers. The evaluation consists of two phases: a technical design analysis phase (Phase I) and a demonstration phase (Phase II). The technical design analysis phase was completed and was documented in a HITEC report entitled Evaluation Findings: The Segmental Concrete Channel Bridge System, dated March 1996. Phase II is currently underway.
Phase I Evaluation
The HITEC Phase I report evaluated the Channel Bridge's performance history and design concept. The performance history focused on the Channel Bridges in France. The design evaluation focused on specific design issues related to segmental bridge construction in general and to the unique aspects of the Channel Bridge system.
The evaluation of performance history included interviews with French Highway Administration representatives and a visit to a Channel Bridge site. The panel determined that the Channel Bridge technology was acceptable for further review.
The design evaluation included a review of unique features, segmental concrete construction details, fatigue, and redundancy. In the unique features section of the report, the dual role of the edge beam as the main supporting member and as a traffic barrier was evaluated, and the deck overlay protection system was examined. In the segmental concrete construction details section, the evaluation focused on concrete cover, anti-corrosion measures, protection of the prestressing strands, and the comparison of post-tensioning and pretensioning. Fatigue related to deck reinforcement, post-tensioning, and the interface of the edge beam and the deck was also evaluated. Redundancy was evaluated assuming a vehicular impact load to the edge beams and piers. Redundancy was also evaluated for the loss of the edge beam and the loss of two tendons.
The HITEC Phase I report concluded that the Channel Bridge is a unique application of precast segmental design and construction technology. The report also concluded that the information evaluated by the panel forms a sound basis for considering the use of the Channel Bridge by the highway industry. The report, HITEC 96-01, is available through the Publication Department of the American Society of Civil Engineers at (800) 548-2723. With the conclusion of the HITEC Phase I report, the Channel Bridge evaluation transitioned into Phase II.
Phase II Evaluation
The HITEC Phase II report will include the evaluation of two full-scale, Channel Bridge technology demonstrations, and it will include a review of the construction process and a verification of design assumptions.
It is anticipated that the construction portion will document the on-site and off-site preparation requirements, actual fabrication and erection of the segments, and the solutions for problems encountered. On-site preparation includes the design, fabrication, and erection of the temporary erection piers, and the special attachment of the temporary erection beam's seat to the permanent abutments, the erection beams, and winches. The off-site preparation includes the design, fabrication, and erection of the adjustable and reusable forms. The report will document special details, such as using the combination of stainless steel plates, segment hanger beam shoes with teflon sole plates, and a lubricant to ease segment erection. Also, the report will document the learning curve associated with the use of an unfamiliar segmental construction technology.
The design assumptions, such as load capacity, will be verified by instrumentation and a full-scale load test.
New York State Involvement
The New York State Thruway Authority was originally slated to be the first owner of this type of bridge in the United States. However, the project selected by the authority was submitted twice for bids, and both times the bids were rejected because they were substantially higher than the cost of a conventionally designed multigirder bridge.
When it became apparent that the authority's plans would not materialize, the New York State Department of Transportation (NYSDOT) proposed replacing two state bridges with the Channel Bridge system. It was subsequently agreed to replace two bridges within NYSDOT Region Eight, which stretches from the just north of New York City to just south of Albany and encompasses the counties of Columbia, Dutchess, Orange, Putnam, Rockland, Ulster, and Westchester.
In January 1997, the project was awarded to A. Servidone Construction Corp. of Castleton, N.Y. Spancrete Northeast of South Bethlehem, N.Y., was chosen to precast the segments. Two bridges are being replaced with the Channel Bridge system: Carpenter Road over Metropolitan Transportation Authority Metro North Railroad in East Fishkill in Dutchess County and state Route 17M over state Route 17 in Wallkill in Orange County.
In addition to these two structures, a third bridge was included in the project. This bridge, county Route 302 over state Route 17 in Wallkill, is being replaced with a conventional steel girder with a composite concrete deck.
Carpenter Road over Metro North is a 26.3-m, single-span overpass bridge. The bridge is straight, but it is on a 582.2-m-radius horizontal curve, a 194.2-m vertical curve, zero degrees skew, and is superelevated 4 percent. This bridge was to be built in six months.
Route 17M over Route 17 is a 73.8-m, three-span continuous overpass bridge. The span lengths are 32.1 m, 31.9 m, and 8.7 m. The bridge is skewed 39 degrees 42 minutes and 9 seconds in relation to Route 17, and it is superelevated 3 percent. The bridge is straight; however, it is integrated into a 155.4-m-radius horizontal curve and a 182.9-m vertical curve. The bridge was to be built within 14 months, which included a winter shutdown period.
The Route 302 bridge over Route 17 is a two-span, continuous overpass bridge. The span lengths are both 34.1 m. The bridge is skewed 20 degrees 45 minutes in relation to Route 17. The bridge is straight and the section is a normal crown section.
The engineer's estimate and the low bids were compatible. (See table 1.)
The Channel Bridge design included several unique components. Segments were to be cast using a short-line match-cast process with a fairly high-strength concrete of 41.4 megapascals (MPa) and longitudinal and transverse post-tensioning. Standardization of segment configuration is essential to realize cost savings with segmental construction. As an example, the 17M structure was designed with two segment configurations: an abutment segment and a typical interior segment. (See table 2.)
The abutment segments have a greater depth due to the use of a transverse end beam. The transverse end beam provides support and torsional rigidity for each deck end. The abutment segment is shorter in length due to the added weight of the transverse end beam.
The concrete design requirements are a 28-day strength of 41.4 MPa, a seven-day strength of 31.0 MPa, a post-tensioning transfer strength of 29.0 MPa, and a form release strength of 17.2 MPa.
The concrete mix included 3.0 kilonewtons (kN) of cement, 0.5 kN of fly ash, 12.5 kN of aggregate, 4.7 liters (L) of high- and mid-range water reducer, 133 milliliters (mL) of air entrainment, 177 mL of retarder, 20.4 L of calcium nitrate, and 106.1 kilograms of water. This concrete strength is a relatively high design strength and includes calcium nitrate with a retarder as a corrosion inhibitor. The corrosion inhibitor was included because the design does not include epoxy-coated reinforcement.
The deck thickness for each segment varies between 228.6 millimeters (mm) and 330.2 mm. (See figure 2.) Each segment has a transversely oriented rib that is 330.2 mm thick and 1.1 m wide and is located longitudinally in the segment's center. The rib provides additional space for the three transverse, 63.5-mm, corrugated polyethylene post-tensioning ducts. Each transverse post-tensioning duct receives seven 15.2-mm-diameter strands that provide the structural capacity for the deck's approximately 9.8-m span. The deck also has fifteen tendons, each containing seven 15.2-mm-diameter strands for the longitudinal post-tensioning. The ducts are 60.3-mm-diameter galvanized steel.
Each edge beam has five draped, longitudinal, continuous tendon ducts. Four of the tendons contain 19 15.2-mm-diameter strands through 101.6-mm galvanized steel. The fifth tendon duct is for future post-tensioning and will accommodate seven 15.2-mm-diameter strands through the 63.5-mm galvanized steel duct.
There are two temporary post-tensioning bars in the deck and one in each edge beam. The bars are 34.9 mm in diameter, and the ducts are 76.2 mm in diameter. The temporary bars are used to draw the segments together prior to final post-tensioning. All post-tensioning anchorages originally were cast trumpet-type anchorages, except for the temporary post-tensioning anchorages. The temporary post-tensioning anchorages were flat-plate-type anchorages. The deck anchorages were changed from the cast trumpet to a flat-plate-type anchorage.
The design also includes a 38.1-mm micro-silica concrete overlay, pedestrian fencing on top of the edge beams, and an aesthetic treatment to the edge beams' fascias.
The construction process contains many unique features of what would normally be considered ordinary phases of bridge design and construction. The ordinary phases are final design, fabrication and erection plan approval, fabrication, site preparation, and erection of the structure.
The fabrication process included fabricating the forms, segments, segment hanger beams, erection girders, and temporary piers. Form fabrication was performed by Southern Forms Inc. of Chattanooga, Tenn. The forms required assembly in Spancrete's precast facility. The forms themselves are unique in that they are reusable and very adjustable. They can be adjusted for segment depth, length, width, and roadway geometry, such as cross slope, superelevation, and grade.
A segment's fabrication includes setting the form geometry, building the reinforcement cages, placing aesthetic treatments and anchorages, installing the cages into the forms, placing ducts, and placing the concrete into the forms. After curing, a post-casting survey is completed; the forms are stripped; the segment is moved into the match-cast position; and then the next segment is fabricated. After the next segment reaches its release strength of 17.24 MPa, the match cast segment is transversely post-tensioned and grouted. The segment is then prepared for shipping.
The segment hanger beams were fabricated by Fiedeldey Fabricators Inc. of Cincinnati, Ohio, and the erection girders were fabricated by Enerfab Inc. of Cincinnati. A. Servidone fabricated the temporary erection piers. The segment hanger beams were installed as part of the segment's shipping preparation and were adjusted for elevation at the site. Four erection girders were fabricated at 12.2 m, and two were fabricated in 21.3-m, 18.3-m, and 15.2-m lengths. The erection girders were spliced to provide the span required. The different lengths provided span flexibility. The erection girders were spliced and erected at the site after the temporary piers were erected.
Segment erection begins with segments arriving via tractor-trailers. A 444-kN crane is typically located on the approach near an abutment. The crane picks the opposite abutment's segment first, the bearing tops are attached, and the segment hanger beams are set in teflon-coated shoes that are resting on the erection girders. Two winches, located at the opposite end of the erection girders, are then connected to the segment hanger beams, and a lubricant is added to the top of the erection girders. The sliding surface of the erection girders is a stainless steel plate. With the combination of the stainless steel plate, teflon-coated shoes, and lubricant, the segment is pulled across the erection girders. Traffic is stopped during the segment's traversal of the travel lanes below. This process is followed until most of the segments are resting on the erection girders.
In preparation for joining the segments, each exposed side of the transverse joints are taped. The tape eases the cleanup of excess epoxy. Epoxy is then spread across both transverse faces of adjacent segments. The segments are connected by the temporary post-tensioning bars and an initial post-tensioning force of 524.9 kN is applied to each of the bars. Segments are adjusted for alignment and checked by the survey team. The alignment is adjusted by jacking and adjusting the segment hanger beams. A final post-tensioning force of 738.4 kN is applied, and the process of squeezing the segments together is completed. Each duct is then cleared of any excess epoxy by a swab fabricated by A. Servidone. The swab consists of a small-diameter reinforcement bar or a strand with five rubber disks of the proper diameter. The temporary post-tensioning process is followed until all the segments have been joined.
The preparation for post-tensioning starts with threading the strands through the post-tensioning ducts. The strands are then pulled according to the design sequence and elongation. Each 19-strand tendon receives 3958.7 kN of post-tensioning force, and each seven-strand tendon receives 1334.4 kN. Eventually, the structure begins to support itself and lifts to the desired profile and off of the erection girders. The erection girders and the segment hanger beams are then removed. The tendons are grouted; the micro-silica overlay is placed; and the pedestrian fencing is installed. With the completion of the approach roadway work and final clean up, the Channel Bridge is ready for service.
At the time this article was written, the Carpenter Road bridge over Metro North was complete, except for the high-density concrete overlay placement. The Route 17M bridge over Route 17 was under construction. The results of this Channel Bridge demonstration will be published in HITEC's Phase II Evaluation Report. As stated earlier, the report will evaluate and document the final design and construction of this technology, including lessons learned and JMI's and Bridgetek's extensive field involvement. The Channel Bridge is another step forward in the industry's relentless pursuit for improvement in our transportation system. If the advantages of this technology interest you, look for HITEC's Phase II Evaluation Report to be published in the fall of 1998.
1. "Channel Bridge: New Method for Bridge Replacement Using Post-Tensioned Concrete Segments," presentation at the American Segmental Bridge Institute Seminar "Design and Construction of Segmental Concrete Bridges," April 1996, Albany, N.Y., presented by James D. Lockwood, P.E., of J. Muller International, Chicago, Ill.
2. "Channel Bridge Presentation," presentation at the New York State Association of Transportation Engineers Conference, August 1997, Albany, N.Y., presented by Roger W. Laime, P.E., of the New York State Department of Transportation Structures Division, Albany, N.Y.
3. Evaluation Findings: The Segmental Concrete Channel Bridge System, a HITEC Technical Evaluation Report, Report Number HITEC 96-01, March 1996.
Christopher J. Allen is a structural engineer in the FHWA New York Division's Bridge Office. He has been with the New York Division since 1993 and with the FHWA since 1990. He has a bachelor's degree in civil and environmental engineering from Clarkson University.
Frank Naret is an associate civil engineer in the New York State Department of Transportation Structures Design and Construction Division. He has been with NYSDOT since 1977. He holds a bachelor's degree in civil engineering from Pennsylvania State University. He is licensed as a professional engineer in New York.
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