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|Federal Highway Administration > Publications > Public Roads > Vol. 68 · No. 5 > A Fresh Look at Orthotropic Technology|
Publication Number: FHWA-HRT-05-004
A Fresh Look at Orthotropic Technology
by Alfred R. Mangus
Engineers push for the renaissance of steel deck bridges in the United States.
In the early 1980s, San Francisco's Golden Gate Bridge was in need of a tuneup. Completed in 1937, the landmark bridge spanning the bay between San Francisco and Marin County, CA, began to show signs of deterioration in its concrete deck. Salt fog had reached the rebar, causing corrosion and concrete spalling. Engineers at the Golden Gate Bridge, Highway, and Transportation District made the decision to switch deck systems. In 1985, with assistance from construction engineers at the California Department of Transportation (Caltrans), the Golden Gate Bridge was restored using steel deck panels. The project not only restored the bridge to prime condition but also used fewer materials and reduced the deck weight by 11,160 metric tons (12,300 tons).
The unsung hero in the retrofit is orthotropic technology. Engineers define an orthotropic deck as one that consists of steel plates supported by ribs underneath, overlain by an integrated wearing (driving) surface. An orthotropic deck is a collage of steel plates welded together with a flat, solid steel deck stiffened by a grid of deck ribs welded to framing members like floor beams and girders. By integrating the structural system and the driving surface, orthotropic deck bridges are more lightweight and efficient on long-span structures.
A staple feature in transportation networks in Europe and East Asia, these bridges also are valued for their seismic performance, maneuverability (as in movable bridges), and versatility for construction in cold weather.
First used in Germany in the 1950s, orthotropic technology facilitated the cost-effective replacement of bridges destroyed during World War II. Today, Japan is home to the world's longest suspension, floating, and cable-stayed orthotropic deck bridges. In fact, major orthotropic viaducts in Tokyo are composed of more than 1,100 spans, and there are more than 250 orthotropic deck bridges of various sizes throughout the country.
Despite their popularity overseas, the percentage of orthotropic decks constructed in the United States remains low. But that may be about to change. Orthotropic structures have earned the trust of a few American bridge designers and owners who are pushing the technology forward. In fact, the Federal Highway Administration (FHWA) and many other organizations recently sponsored the world's first conference in nearly 30 years focused exclusively on orthotropic deck bridges.
"Over the last few decades, improved knowledge in regard to the design and performance of orthotropic decks has made them a popular choice for renovating older decks here in the United States," says John Fisher, professor emeritus of civil engineering at Lehigh University. "And if properly designed, orthotropic decks could offer a lifespan of more than 100 years."
Adds Martha Nevai, a structural engineer with the FHWA California Division: "It is important for bridge designers to take a closer look at this technology."
Recovering from a Troubled Past
When first introduced in the United States in the 1950s and 1960s, orthotropic decks represented a new and relatively unfamiliar technology for bridge designers. As a result of inadequate knowledge about the performance characteristics, particularly in regard to fatigue and traffic loading, early designers created bridges that were too light and tended to crack in the welds under repeated use by trucks. An experimental bridge built in the 1960s in Maryland, for example, only lasted a few years. The problem, according to Fisher, was that not enough experimentation had been carried out to define the details.
"Designers had bad experiences with early applications, using deck plates that were too thin," Fisher says. "In the United States, we were using thicknesses of 10 to 12 millimeters [0.39 to 0.47 inch], which is too thin to carry the wheel loads of heavy trucks. Many bridge decks failed, and that turned off owners."
Over the years, however, research in this country and abroad has helped engineers develop a more substantial base of knowledge and data on the performance of orthotropic bridges. According to Benjamin Tang, with FHWA's Office of Bridge Technology, current research on bridge performance indicates that stiffer orthotropic decks with wider ribs, along with prototype testing, could result in good performance and long bridge life.
At Lehigh University, for example, Fisher and his colleagues conducted cyclical traffic loading tests on several orthotropic decks placed during retrofit projects on bridges in New York City, such as the Bronx-Whitestone and Williamsburg Bridges. This research showed that it would be possible to design an orthotropic deck that will provide a service life of more than 100 years.
Fisher notes that full-scale prototype tests are providing a better concept of load distribution and structural detail. "Some early studies were based on small test models," he says. "Engineers relied on direct mathematical proportions of the deck and details, assuming that the distribution of the load would stay the same. Today, the full-scale tests used for bridges like those conducted for the Williamsburg and Bronx-Whitestone Bridges are helping build a comprehensive knowledge base."
Extensive research in Japan and the Netherlands has demonstrated how to locate and repair fatigue cracks, with results indicating that most cracks can be found and repaired relatively easily. Fatigue cracking in weld metal, for example, can be reduced by making rib-to-diaphragm welds more fatigue resistant by using runoff tabs on the diaphragm plates to terminate the welds cleanly. Researchers from Lehigh University confirmed this method in the laboratory tests conducted on the prototype test for the Williamsburg Bridge. They visually detected fatigue cracks at the rib-to-diaphragm welded connections and then repaired them by removing the cracks in the diaphragm plate, modifying the cutout geometry, and enhancing fatigue resistance by local treatment of the weld toes at the region where cracks had developed. During the tests, the researchers used air hammer peening (hammering to bend, shape, or cut) to introduce plasticity and modify the residual stress state. Alternatively, Fisher says, ultrasonic impact treatment could be used to prevent further cracking.
Researchers also improved corrosion protection significantly by switching to closed ribs, which expose 50-percent less surface area than open ribs. Expanding inert foam, made from a chemical compound that expands when injected into a closed area, further enhances the closed-rib design. Bridge engineers use the foam to prevent moisture buildup, condensation, and eventual corrosion on the inside of closed ribs. The deck on the Golden Gate Bridge, for example, features trapezoidal ribs with inert foam placed inside.
Rust-resistant paints and weathering steel, which rusts only to a given point while maintaining its structural integrity, are material options for orthotropic decks. Another technique, hot-dip galvanizing, involves dipping small bridge parts such as railings and lampposts in a zinc solution to make them rust-resistant. "Even in saltwater, galvanized steel piles with an epoxy coating will resist corrosion completely, even after several decades," says Dennis Nottingham, president of PND Incorporated, a civil engineering firm.
Caltrans plans to use a European solution known as "dehumidification" on a project currently under fabrication. With an airtight superstructure, Caltrans will use a dehumidifier to remove moisture caused by condensation within the interior of the superstructure.
Previous experience on several bridges in North America showed that it was difficult for asphalt and concrete wearing surfaces to remain attached to the steel plates on orthotropic decks. Delamination or debonding became troublesome as standard wearing surfaces proved susceptible to the flexibility of 10- to 14-millimeter (0.39- to 0.55-inch) steel deck plates. Modern materials and technologies, however, are providing improved solutions for driving surfaces. Research conducted by State transportation agencies has shown that using thermoset resin-extended asphalt and epoxy concrete can help limit cracking and delamination. And bridges like northern California's San Mateo-Hayward Bridge, built in 1967 using an epoxy asphalt wearing surface, continue to attest that properly researched and developed wearing surfaces can last for decades.
In addition, FHWA continues to support the advancement of high-performance steel technology used in orthotropic decks and other steel bridges. "High-performance steel has improved weldability and much higher toughness to arrest cracking," FHWA's Tang says. "The material possesses enhanced weathering and corrosion resistance characteristics that will make bridges last longer. [FHWA is] working with partners in the steel industry to coordinate and develop robotics and automation for steel fabrication, which will ensure high-quality shop production, rapid deployment, and lower life-cycle cost. We also are supporting research in developing resistance to corrosion in the materials and overlay protective systems that will make orthotropic steel bridges more durable."
Weighing the Benefits And Drawbacks
Today, the United States is home to about 50 orthotropic deck bridges, several of which have earned their place among the Nation's famous spans—including the San Diego-Coronado and San Mateo-Hayward Bridges in California. Projects just completed or under design or construction include the third Tacoma Narrows Bridge, about 80 kilometers (50 miles) south of Seattle, WA, and parts of the skyway on the east spans of the San Francisco-Oakland Bay Bridge.
Orthotropic bridges are appropriate for rail, highway, and pedestrian projects. The opportunity for engineers to mix and match choices for ribs, floor beams, and main girders, while maintaining structural integrity, permits a variety of orthotropic superstructures geared to specific situations.
Their light weight, for example, is ideal for movable bridges that need to be lifted, tilted, or swung to allow ships to pass and for bridges in earthquake-prone areas to reduce seismic forces. "When you design a lightweight deck, you also can use lighter and smaller supporting elements," says Charles Seim, an El Cerrito, CA-based consulting bridge engineer. "This further reduces the weight and cost of the bridge."
For long spans—especially suspension bridges—orthotropic decks provide a lighter, more efficient alternative to concrete decks. In fact, an orthotropic deck can reduce the weight of the entire structure by as much as 25 percent compared to a concrete deck.
"Orthotropic deck bridges provide an excellent solution for large-scale structures where weight is an issue," FHWA's Nevai says, "such as cable-stayed and suspension bridges. A lighter weight superstructure option will reduce seismic loads on the substructure, resulting in smaller or fewer columns, piers, and foundations sizes, which in turn can result in significant cost savings."
The forces acting on the substructure are essentially a function of the mass of the superstructure. "There is a tradeoff between strength and stiffness," Nevai says. "Larger substructures typically are stiffer and attract more seismic load, whereas smaller support elements are more flexible. Typically it is better to have a structural element that is more flexible and detailed to allow movement during an [earthquake or other] event."
By reducing the weight of the superstructure, orthotropic decks have less mass to excite during an earthquake; therefore columns and foundations have lower demands imposed on them from an earthquake's horizontal forces and displacements. Potential cost savings can result from less labor and materials involved in building a smaller substructure.
Says Nottingham, from PND Incorporated: "Orthotropic technology also may be desirable for short-span bridges, especially for situations that require something lightweight, curved, or torsionally resistant."
Orthotropic decks also may offer an alternative to cast-in-place concrete for projects in cold regions. Concrete decks have a minimum curing temperature and require encapsulation and heating for construction projects during the winter. For this reason, orthotropic decks are used in Alaska, Canada, Russia, and Scandinavia.
Integrating the deck, ribs, and floor beams makes for a thin structure suitable for highway interchanges, permitting construction even where overpasses and underpasses allow only minimal structural depth. Replacing at-grade crossings with bridges is simplified by using orthotropic decks, which can reduce backfill height, saving time and cost. "These bridges are light enough that a crane could set a prefabricated short span into place over the tracks quickly," Seim says.
Another advantage, according to Fisher, is that trial installations have demonstrated that even if some of the wearing surface is lost, drivers typically will not detect the deterioration. "The thinner wearing courses used on newer orthotropic decks are not susceptible to pothole effects as any local loss of the surface is not adverse to traffic," he says, "so transportation officials don't need to worry about doing emergency maintenance, especially in the winter when pavement preservation activities are more difficult and patches are less likely to hold." And the hard aggregates specified for the wearing surfaces have proven both skid resistant and durable, Fisher adds.
The biggest barrier to using orthotropic decks today is the initial cost, according to Fisher. "Because the steel plates and wearing surfaces are fabricated in a controlled environment, the costs tend to be higher," he says. "However, if you have to replace other types of bridge decks every 30 to 40 years, the life-cycle cost of using an orthotropic deck that will last more than 100 years becomes much more palatable."
The following three case studies from Alaska, Louisiana, and California illustrate the versatility of orthotropic bridges for solving climatically and functionally diverse transportation problems.
A Crucial Link in a Frozen Land
In the 1970s, engineers were faced with the challenge of linking the oil-producing regions along Alaska's Arctic coast with the ice-free port of Valdez. The most difficult obstruction was the Yukon River, which is 610 meters (2,000 feet) wide at the crossing point and subject to freezing temperatures for almost half of the year. Temperatures routinely drop below the concrete curing point of 4 degrees Celsius (40 degrees Fahrenheit).
In 1972, the Alaska Department of Transportation and Public Facilities began designing the E.L. Patton Yukon River Bridge. Using orthotropic steel technology, the engineers could construct their design even as temperatures plummeted to –15.5 degrees Celsius (–60 degrees Fahrenheit). Opened in 1976, the award-winning bridge is part of the Dalton Highway "haul road," plus it carries the trans-Alaska oil pipeline across the river.
With each road deck measuring 9.1 meters (30 feet) wide, the bridge was built with an eye toward the future, providing space to accommodate a natural gas pipeline. In addition, engineers designed the bridge to withstand the high ice loading of the Yukon River.
Today, Alaska continues to deploy orthotropic decks to meet its transportation needs. In southeast Alaska, for example, the agency uses eight movable orthotropic deck bridges to transfer vehicles from State-run ferries to the shore. Lightweight construction results in smaller motors, counterweights, and support systems, and less energy is required for the smaller motors that raise and lower the spans. "Orthotropic bridges are an excellent solution for . . . the ferry transfer bridges used in many of our facilities," says Elmer Marx, P.E., with the bridge division of the Alaska Department of Transportation and Public Facilities.
A Movable Bridge For a Busy Port
From the frozen tundra in Alaska to tropical Louisiana, orthotropic bridges offer versatility both in terms of climatic durability and application. The Industrial Canal Bridge, also known as the Danziger Bridge, was constructed in 1988 as a replacement for a 50-year-old, double-leaf Strauss bascule bridge in New Orleans. With a 98-meter (320-foot) span and a total deck width of 33 meters (108.75 feet), the bridge carries seven lanes of U.S. 90 across the Gulf Intracoastal Waterway.
In addition to highway traffic, the Danziger Bridge accommodates ships passing through the canal. Vertical clearance for the movable span is 15 meters (50 feet), accommodating about 90 percent of waterway traffic without interrupting traffic flow on the highway. But at least five times a day, an operator must raise the bridge quickly to its maximum clearance of 38 meters (125 feet) above the water to allow larger ships to pass.
After reviewing several alternatives, the Louisiana Department of Transportation and Development chose an orthotropic deck weighing about 2,000 metric tons (2,200 tons) that can be raised 23 meters (75 feet) in less than 2 minutes. In fact, the agency determined that the orthotropic deck system not only would be lighter and smaller, but it also would be comparable in price to a heavier, two-truss design that would require a more extensive lifting system. A solid concrete deck on a bridge of this size would have weighed 3,350 metric tons (3,700 tons).
The larger, movable orthotropic span eliminated the need for expensive protective fenders (wood or rubber reinforcement placed around the pier) and dolphins (small islands built around piers), while providing an unobstructed path for ships and barges using the canal. The longer span enabled the agency to eliminate piers completely from the waterway. After 16 years in service, the Danziger Bridge—along with the nearby Luling-Destrehan Bridge, a cable-stayed orthotropic bridge spanning the Mississippi River--has continued to perform up to expectations. According to Arturo Aguirre, division bridge engineer with FHWA's Louisiana Division, the Danziger Bridge's orthotropic deck was originally chosen over a heavier, truss-based alternative to guarantee a faster lift time. "Since then, the Danziger Bridge, and in fact, both bridges have performed as intended," Aguirre says.
California's Latest Long Span
Home to more than a dozen orthotropic bridges, California leads the Nation in deploying this technology. The new Alfred Zampa Memorial Bridge, constructed between January 2000 and November 2003, became the first major suspension bridge in the United States constructed since 1973. Taking cues from successful European and Asian structures, the Zampa Memorial Bridge features an aerodynamic roadway, using an airplane wing-shaped box girder instead of a stiffening truss to minimize wind resistance.
After the Loma Prieta and Northridge earthquakes in 1989 and 1994, State engineers in California began evaluating the seismic stability of the two existing bridges carrying Interstate 80 over the Carquinez Strait between Crockett and Vallejo, CA. The evaluations showed that the eastern bridge span, built in 1958, would need substantial seismic upgrading. The other bridge, built in 1927, needed to be replaced, in part because it did not comply with current lane widths for interstate highway traffic. The decision was to build a suspension bridge that would allow oceangoing ships more room for maneuvering through the narrow Carquinez Strait.
Caltrans officials determined that using orthotropic technology was the most effective option to address seismic concerns and construct an exceptionally long span within budget. According to Mike Marquez, a project manager for Caltrans, the team considered several factors in the selection of an orthotropic box design. "We wanted a lightweight design to reduce the cost of the foundation and suspension cables, as well as an all-steel ductile superstructure to provide excellent seismic performance," he says. "Finally, we wanted a superstructure that would provide superior resistance to wind and torsional forces, while maintaining visual appeal." The bridge was designed to withstand an earthquake with a magnitude of 8.0 on the Richter scale.
To address the community's desire for an attractive and accessible structure, the bridge also features a 3.7-meter (12-foot)-wide pedestrian and bike path offering views of the Carquinez Strait and San Pablo Bay. The new path closes a major gap in the San Francisco Bay Trail that rings the San Francisco and San Pablo bays.
Caltrans developed a small book and video about the bridge, and copies are available for purchase through the Caltrans History Preservation Committee by contacting Norman Root at 916–227–8208, email@example.com.
Need for Sharing Ideas And Solutions
Bridge engineers are striving to promote cooperation and the sharing of ideas in an effort to push the industry forward. According to Nottingham at PND Incorporated, the lack of new design manuals or publicly available software in North America presents an ongoing challenge, requiring designers to seek guidance from journals, colleagues, and overseas sources.
"Orthotropic bridges traditionally have comprised a niche of long-span, lightweight bridges," Nottingham says. "And [although] engineers are beginning to recognize their versatility, demand is only beginning to push specifications and standardizations forward."
In the United States, designers are creating one-of-a-kind bridges with specific project requirements, including curved overpasses, fast-acting movable bridges, and long-span cable-stayed or suspension bridges. Consequently, U.S. designers have managed to carve a niche for orthotropic deck bridges in the domestic and overseas markets despite the lack of standardization. U.S. engineers are designing orthotropic bridges in other countries, including China.
Orthotropic design in European countries, such as Germany, traditionally has been guided by standard specifications for structural and wearing surface elements. The United States, too, recently stepped up to the plate with the newest version of the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications. According to FHWA's Tang, "The LRFD specifications incorporate improved design methodologies, fatigue resistance details, and material toughness that will ensure adequate performance in orthotropic bridges."
Seim agrees: "Designers will create a successful orthotropic deck bridge if they use these codes," he says. However, more detailed specifications will help U.S. bridge designers and consulting engineering firms join the international movement toward constructing steel long-span bridges with orthotropic decks wherever they are needed. "Orthotropic technology is a powerful device to have in the bridge engineer's toolbox for long spans," he says.
Galvanizing the Industry
Another key factor in pushing ahead the domestic progress of orthotropic deck bridges is communication. "Our knowledge base is limited to a few bridge engineers who have worked on these types of bridges," Seim says. "I am not aware of any universities in the United States that offer a class or seminar focused on orthotropic deck bridge design. Bridge designers and owners should evaluate orthotropic deck bridges for other highway applications in addition to long spans, as a properly designed bridge of this type can offer aesthetics, economy, and a long service life."
To fill this void, Seim and several others recently offered the first college-level course and an advanced seminar through the American Society of Civil Engineers (ASCE). For more information on the course, visit www.orthotropic-bridge.org.
Proponents in the United States took a major step forward in August 2004, hosting the first orthotropic bridge conference in 34 years. Experts from Belgium, Britain, Canada, Denmark, France, Japan, Mexico, the United States, and elsewhere gathered in Sacramento, CA, to share their insights on topics ranging from fatigue resistance and load testing to new design concepts for steel bridge decks.
"We own and maintain the largest collection of orthotropic bridges in North America," says Caltrans Chief Engineer Rick Land. "We continue to fund research on orthotropic technology, and our engineers are staying current with ideas developed elsewhere."
With a followup conference in the works for 2008 and plans to create a permanent specialty subgroup within ASCE to focus on orthotropic bridges, proponents are confident that orthotropic deck bridges will continue to proliferate as a vital component in the toolbox of American transportation engineers.
Alfred R. Mangus is a transportation engineer with the Office of Structures Contract Management at Caltrans. He has 28 years of experience, including 13 years with Caltrans. He received two professional awards from the James F. Lincoln Arc Welding Foundation and has published more than 10 papers and a textbook chapter on orthotropic steel bridges. Mangus and his colleagues at ASCE helped launch www.orthotropic-bridge.org in 2004.
For more information, visit www.orthotropic-bridge.org or contact Al Mangus at 916–227–8926, 916–961–ASCE, or firstname.lastname@example.org.
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