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|Federal Highway Administration > Publications > Public Roads > Vol. 67 · No. 3 > Getting Ahead of the Curve|
Getting Ahead of the Curve
by Steven B. Chase, Sheila Rimal Duwadi, and John M. Hooks
FHWA proposes a research-packed agenda for preserving existing bridges and building new ones.
Bridges, culverts, and tunnels are the glue that holds the surface transportation network together. Whether designed for motorists, pedestrians, bicyclists, or trains, these structures safely convey travelers above and beneath waterways, through mountains and cities, and across the Nation's varied landscapes.
The Federal Highway Administration (FHWA) is committed to delivering a safe and cost-effective bridge infrastructure to support America's highways. To meet the demand for a 21st century transportation network, FHWA is proposing a robust and aggressive research and technology (R&T) program to preserve the aging and deteriorating U.S. bridge infrastructure and advance new technologies for designing stronger, longer-lasting bridges. Unofficially termed Bridges for the 21st Century, the program will build on and expand the programs pursued under the Transportation Equity Act for the 21st Century (TEA-21) and previous R&T programs.
“FHWA, in partnership with the States, will be developing and deploying innovative technologies,” says Raymond McCormick, senior structural engineer in FHWA's Office of Bridge Technology, “that enable us to get out ahead of the bridge deterioration curve and stay there.”
Three Program Thrusts
To meet current and future needs for bridge research and technology, the Bridges for the 21st Century program will have three major thrusts. The first involves stewardship and management of the current bridge inventory to ensure economical, safe, and continuing service.
The second thrust centers on ensuring the safety and reliability of bridges by improving their ability to resist the impacts of extreme events such as earthquakes, flooding, vessel or vehicle collisions, and intentional attacks.
The third research area involves developing a new generation of cost-effective, high-performance, and low-maintenance bridges. FHWA envisions a new paradigm in design and construction that results in bridges that are built faster and cheaper, require little or no maintenance, and feature a minimum 100-year lifespan.
Focus on Preservation
The National Bridge Inventory shows that more than 480,000 bridges and 110,000 tunnels and culverts serve U.S. highways. The average age of existing bridges is 42 years and rising. In the coming decades, new bridges will replace many of these spans, but in the meantime, bridge owners need cost-efficient ways to maintain and preserve the existing bridges until they are rebuilt.
FHWA lists nearly 160,000 U.S. bridges as substandard. Although many are rehabilitated and taken off the list annually, about 3,000 others are added to the list each year. More importantly, motorists cross over substandard bridges more than one billion times each day. Between 1982 and 2001, Federal and State agencies made significant progress, reducing the number of substandard bridges from more than 250,000 to fewer than 160,000. But much remains to be done.
Says McCormick, “In the past, many bridge owners neglected the structures until they were beyond rehabilitation and in need of replacement. A primary objective of FHWA's stewardship and management focus is to encourage bridge owners to place more emphasis on system preservation based on proven preventive maintenance and rehabilitation techniques.”
Future stewardship and preservation will require new research and technology, as well as innovative tools, strategies, and management practices. Researchers, for example, need to understand the micro- and macro-level mechanisms that cause bridge materials and elements to deteriorate physically, limiting the service life of bridges.
To evaluate and quantify the condition of bridges and their components, the transportation community needs to provide inspectors with improved technologies such as nondestructive testing, remote sensing techniques, and global monitoring. Improvements also are required for the quality, accuracy, and precision of quantitative information on bridge conditions, such as element-level data that support improved decisionmaking processes and tools for bridge management.
The transportation community also must provide breakthroughs in technologies for repairing and rehabilitating bridges quickly to minimize the duration and public impact of work zones. Other research needs include technologies for detecting the condition of bridge decks at highway speeds and methodologies to assess the condition of concrete decks with overlays. Finally, improved modeling for life-cycle cost analyses could lead to cost-effective strategies and techniques for preventive maintenance that extend and optimize service life.
Stewardship and management activities depend on reliable, quantitative data on bridge conditions and the factors that influence performance and deterioration. At present, the data are unavailable, or not available in a format suitable for analysis. A key component of the Bridges for the 21st Century program, therefore, deals with data collection.
FHWA proposed a Long-Term Bridge Performance program that would gather data to support improved methodologies, tools, and programs for bridge preservation and management. The 10- to 20-year program would monitor 1,000 to 2,000 carefully selected bridges representing a cross-section of the existing infrastructure in terms of type, span length, construction materials, traffic volumes, climatic and environmental conditions, and maintenance practices.
The data set for each bridge would start with all available baseline information including design codes, material specifications, as-built plans, and any relevant history of maintenance and major rehabilitation. FHWA would work with highway agencies participating in the program to keep detailed records of maintenance needs, document all system-preservation activities, and track costs.
Each bridge would be instrumented and monitored to collect research-quality data on factors that influence deterioration and affect performance. Wherever feasible and appropriate, FHWA and program participants would deploy advanced bridge-inspection technologies, such as ground-penetrating radar for evaluation of bridge decks. They also would record quantitative data on changes in bridge conditions, including delaminations in concrete decks or corrosion losses in the webs and flanges of steel girders. In addition, they would track operational characteristics such as crashes and congestion on the bridges being monitored.
This quantitative data, collected for thousands of bridges over 10 to 20 years and properly analyzed, would lead to significant advancements in the knowledge of bridge performance. Such data would help bridge owners manage and preserve the condition and capacity of the existing infrastructure at the least-possible cost.
The second focus of the proposed R&T strategy deals with bridge failures due to catastrophic events, both natural and manmade. The goal of this focus area is to deliver the knowledge and technologies that will help ensure that the Nation's bridge infrastructure continues to function safely and reliably, even during extreme or infrequent catastrophic events.
Natural disasters like earthquakes and floods have a high probability of affecting large areas and a large number of highway structures simultaneously, significantly disrupting mobility, emergency response, and local economies. With each major event, engineers learn new lessons about bridge response and performance, and new standards and technologies often follow. Through the Bridges for the 21st Century program, FHWA proposes continued and expanded research in several key areas.
Earthquakes. The seismic research program at FHWA developed and continues to refine guidance for retrofitting existing bridges to help them withstand earthquakes. At-risk structures include all bridges built before 1980 in regions that have moderate to high seismic activity.
To help researchers and engineers understand how structures react during earth movements, FHWA has proposed developing and installing more accurate position monitors that can determine the relative and total movement of critical bridge components. During post-event assessments, inspectors need to have better tools to evaluate the residual strength and structural integrity of damaged sections. New technologies, like the shape-memory alloys that can return to their original shapes after they are deformed, may find application in cable restrainers and seismic isolation bearings.
Floods. Approximately 85 percent of the structures listed in the National Bridge Inventory cross waterways. In the United States, flooding and scour cause more bridge collapses than all other causes combined. FHWA, therefore, has an active program to study the hydraulics and hydrology of bridge structures. The agency has a state-of-the-art hydraulics laboratory where researchers conduct scale-model tests. Through a series of hydraulic engineering circulars published by the FHWA Hydraulics Laboratory and an ongoing training course offered by the National Highway Institute, FHWA helps States and consultants evaluate and mitigate the effects of scour.
Engineers need improved techniques for physically and numerically modeling unique scour problems and accounting for exposure to various flood levels over the life of a bridge. They also need advanced monitoring systems to record the depth of scour during individual events. Improved hydrology and hydraulics research could help improve the design of bridges in tidal waterways, and spatial radar technology used in weather forecasting could improve predictions of flood runoff. In addition, the development of portable instrumentation is critical to assess foundation damage before reopening a bridge after a major flood.
Wind. Since the dramatic collapse of the Tacoma Narrows Bridge near Tacoma, WA, in 1940, the effect of wind on structures has become a significant concern for bridge owners. FHWA is confronting wind-induced hazards by developing comprehensive guidelines for designing and retrofitting bridges, creating specifications for assessing the aerodynamics of new designs, and identifying a rational method for wind-climate analyses. Researchers will conduct extensive experimental and analytical work relative to vortex-induced vibration of bridge decks. In addition, they will study the wind- and rain-induced vibration of bridge cables and explore aerodynamic surface treatments. FHWA also is considering the development of a suite of software tools to analyze the effects of wind and vehicle gusts on support structures.
Transportation agencies and the driving public expect highway and bridge infrastructure to withstand everything from the normal wear and tear from years of service operation to unanticipated events like collisions, fires, and acts of violence.
Overloads. Today, trucking accounts for 80 percent of expenditures on freight transportation in the United States. In May 2002, the Transportation Research Board (TRB) released a study that suggests reforming size and weight regulations to allow larger trucks on roadways. The report, Regulation of Weights, Lengths, and Widths of Commercial Motor Vehicles—Special Report 267, recommends a program of basic research to determine fact-based regulations for truck size and weight.
These factors have a significant impact on maintenance and construction costs for highway pavements and bridges. Research is necessary to assess the ability of existing bridge structures to carry heavier loads. The large percentage of bridges identified as functionally obsolete also presents a challenge if trucks exceed weight or size limits, posing potential structural and safety risks.
Collisions. In May 2002, a barge struck an unprotected pier on a bridge on I-40 in Oklahoma, collapsing the bridge and killing 14 people. For bridges over navigable waterways, the piers are the most vulnerable elements to damage from vessel collisions. Therefore, in heavily trafficked rivers with ships and large barges, bridge owners should protect piers and install monitors to track movements caused by collisions. Design codes should be revised to provide more effective provisions for collisions involving commercial crafts or enemy vessels.
To reduce the impact of vehicle collisions on bridges, FHWA is conducting crashworthiness tests on barriers. More research is necessary to improve analytical capabilities of predicting the performance of barriers during impacts. Researchers at FHWA are employing finite element analysis to predict the crashworthiness of various concrete barrier shapes and developing mathematical formulas that describe the outcomes of crash testing without the need for multiple full-scale crash tests.
Fire. Although fires damage all types of highway infrastructure, fires in bridge tunnels often are more dangerous to motorists and disruptive than those on bridges and overpasses. Following the tragic fires in the Mont Blanc and Tauern tunnels in Europe in 1999, highway agencies began devoting renewed attention to fire safety.
FHWA proposes further study of the properties of materials, especially high-performance materials, at elevated temperatures. Researchers must determine the temperatures generated by the combustion of large volumes of flammable materials, such as gasoline, diesel fuel, liquid propane, and rendered animal fat. Based on heat input and temperature gradients, they can verify the immediate and hour-by-hour effects of exposure on the structural properties of materials. As with the other hazards, guidelines are needed to assess the remaining strength of fire-damaged structures and develop rapid-repair techniques.
National Security. More than 2 years after the tragic events of September 11, 2001, transportation agencies continue to define strategies to protect the Nation's highways and bridges from acts of violence. To develop a resilient physical infrastructure that can withstand these large-scale strikes, researchers need to understand the threats, identify specific vulnerabilities, and develop technologies to help eliminate or protect against these vulnerabilities. Engineers need better methodologies for assessing the safety and residual capacity of structures after an incident, and new techniques for repairing and restoring bridge infrastructure quickly.
In this area, FHWA proposed that the R&T program covers systems analysis and design, material improvement, detection and surveillance, post-event assessment, repair and restoration, and evaluation and training. Also, FHWA is partnering with the defense community to draw on the military's knowledge and experience and will transfer applicable technologies.
Trends in Bridge Design
In addition to managing the existing bridge inventory and identifying strategies to protect structures from natural and manmade hazards, FHWA also plans to develop a new generation of high-performance bridges.
Nationally, highway agencies build, replace, and rehabilitate about 10,000 bridges per year, but the majority of these new bridges are designed and constructed using today's technologies. New materials and methods are necessary to counter or mitigate the impact of deterioration processes in new structures, structural elements, and existing bridge members.
The demand for increased mobility, reduced congestion, enhanced safety, and improved homeland security will present unprecedented challenges for transportation agencies in the coming years. Only time will reveal the exact geometric and material characteristics of tomorrow's bridges, but the National Bridge Inventory offers some definite trends. For example, data from 1985 to 2001 show steady increases in span length (19 percent), overall structure length (15 percent), and deck width (13 percent). Longer span lengths stretch the limits of existing materials to gain the maximum economy of scale and simultaneously reduce the environmental impact of structures.
Higher traffic volumes and increasing safety concerns contribute to the increase in deck widths. “In the future, bridges will carry heavier loads and more traffic,” says Myint Lwin, director of the Office of Bridge Technology at FHWA. “More lanes and wider bridges help accommodate increased traffic, and broad shoulders make it safer for motorists to change a flat tire or for incident response teams to handle emergencies.”
Engineers typically design bridges to carry a 32,660-kilogram (72,000-pound) load (HS-20). In 1985, 1 out of every 50 bridges was designed to accommodate a 40,820-kilogram (90,000-pound) load (HS-25). By 2001, agencies were designing 1 in 5 bridges for an HS-25 load. “These trends clearly indicate that we will expect more performance from bridge materials and systems in the future,” says Lwin.
To evaluate the market potential for the bridges of the future, FHWA recently conducted a detailed analysis of the more than 33,800 new bridges that were constructed in the United States between 1996 and 2000. The goal was to define the most promising systems to pursue and develop. The study showed that two-thirds of the bridges fell into one of six design classifications. (See “Bridge Design Classifications.”) The study also revealed that these common bridge types show definite geographic clustering in different regions of the country and that most of the bridges have maximum span lengths of 30 meters (100 feet) or less.
Based on these findings, FHWA concludes that existing market conditions support a strategic research focus on a few standard bridge types, simple spans (less than 30 meters), and standardized bridge systems that can be manufactured in significant numbers and distributed regionally.
Bridges of the Future
FHWA identified several specific performance goals to guide the agency's proposed research initiative. These goals, which account for initial costs, service-life costs, and indirect costs like time and safety, are as follows:
Wider acceptance and implementation of best practices and innovative techniques, such as using precast or prefabricated components and employing integral abutment construction, can help attain several of these performance goals. Also critical is developing and delivering bridge systems rather than separate foundations, substructures, superstructures, and decks. Moving toward widely adopted standards for design and construction can help State highway agencies take advantage of the economies of scale that manufacturing offers.
“We recognize that these goals will seriously stretch our creative and technological capabilities,” says King W. Gee, associate administrator in the Office of Infrastructure at FHWA, “but we plan to build on a decade of research in high-performance materials and seriously pursue the development of structural systems that will meet these performance objectives.”
Getting the Word Out
Another critical aspect is sharing the results of the R&T program. Since 1998, the Innovative Bridge Research and Construction (IBRC) program successfully promoted the use of innovative materials and technologies in constructing new bridges and repairing, rehabilitating, or replacing existing ones. But implementation under the IBRC program has been characterized by small, usually incremental steps, such as replacing steel reinforcement with polymers or standard concrete with higher-performing concrete. FHWA plans to broaden and redirect the IBRC program to become the primary mechanism for transferring new technologies to the States.
FHWA has proposed replacing the IBRC with a new program called the Innovative Bridge Research and Deployment (IBRD) program, which will have a goal of spurring the development of new and innovative bridge systems. The new program will expand the scope of the IBRC program to include new structural systems and technologies for strengthening, repairing, rehabilitating, and preserving bridge infrastructure.
According to Lwin, the emphasis of the IBRD program will be on deploying and evaluating technologies that are likely to become new standards in the future. “We plan to emphasize the development and evaluation of technologies that have potential application on thousands of bridges,” he says, “not just a few.”
FHWA also plans to launch several demonstration projects to introduce new bridge technologies. The demonstration projects will include both educational and hands-on elements to help move technology from the laboratory into practice.
Committed to Safety
FHWA's proposal to refocus and revitalize the R&T program sets a strategic direction for developing and deploying breakthrough technologies to deliver Bridges for the 21st Century.
“In the pursuit of the bridge of the future, we must not lose sight of the need to manage the existing bridge inventory effectively and efficiently,” Lwin says. “Once FHWA and our partners in the States and private sector develop the new technologies, we will engineer the Nation's bridges to meet the new demands for safety, security, reliability, and durability.”
Steven B. Chase has more than 25 years of experience with FHWA and has worked in the Office of Infrastructure Research and Development since 1992. As technical director for bridges, Chase oversees all research and development activities related to highway structures. He is founder and past chairman of TRB's Subcommittee on Nondestructive Evaluation of Bridges. Chase has a bachelor's degree in civil engineering from the University of Hartford, a master's in civil and environmental engineering from The George Washington University, and a Ph.D. in philosophy and civil and environmental engineering from the University of Rhode Island.
Sheila Rimal Duwadi manages several research programs, including bridge and tunnel security and timber bridges, in the FHWA Office of Infrastructure Research and Development at the Turner-Fairbank Highway Research Center (TFHRC). Duwadi chairs the Technical Committee on Timber Bridges at the American Society of Civil Engineers and is an associate editor for the society's Journal of Bridge Engineering. She is a member of TRB's Committee on General Structures and the American Association of State Highway and Transportation Officials' Subcommittee on Bridges and Structures. Duwadi is a registered professional engineer in Virginia.
John M. Hooks is the team leader for the Inspection and Information Management Team and director of the Bridge Management Information Systems Laboratory at TFHRC. He also manages the IBRC program. Hooks has spent 26 years at FHWA headquarters and TFHRC helping develop and implement innovative bridge technology and management systems. Before coming to the Washington, DC, area, he served as assistant division bridge engineer in the FHWA New York Division Office. Hooks has a bachelor's degree in civil engineering (1966) and a master's degree in structures (1968) from Clarkson University.
For more information on bridge research at FHWA, visit www.fhwa.dot.gov/research/tfhrc/labs/structures/.
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