America's highway system provides continuous, contiguous access to all points within the nation's borders and, through intermodal connections, ties the U.S. to the rest of the world. Highways, and the bridges that connect them, are the backbone of the U.S. economy and essential to national security and disaster response. Many bridges are not only vital links in the transportation system but also are national symbols.
More than 590,000 bridges, tunnels and culverts serve the nation's highways. Today, the Federal Highway Administration (FHWA) lists nearly 160,000 bridges as substandard, and about 3,000 more spans become deficient each year. More importantly, approximately 1 billion crossings of substandard bridges occur every day. Federal and state agencies have made significant progress reducing the number of substandard bridges from more than 250,000 in 1982 to fewer than 160,000 in 2001.
Current projections for traffic growth point to a continuing problem with substandard bridges. Most of the bridges in operation today are more than 40 years old, and most of those under construction today will deteriorate at about the same rate as those that were constructed 20 years ago. Nationally, highway agencies build, replace and rehabilitate about 10,000 bridges per year.
The majority of the new bridges, however, are designed and constructed using today's technologies. In the first article in this three-part series, John Hooks introduced the concept of Bridges for the 21st Century, a comprehensive research and technology (R&T) program proposed by FHWA to identify and deploy cutting-edge solutions to strengthen the bridge infrastructure. Intent on getting ahead of the bridge deterioration curve, FHWA has outlined an R&T strategy to develop the Bridge of the Future, a new generation of cost-effective, high-performance and low-maintenance bridges.
Today's bridges
Data from the National Bridge Inventory helps illustrate the materials and technologies in bridges constructed today. Of the structures built between 1996 and 2000, most are constructed of simple spans (74%), cross water only (80%) and have an average length of approximately 72 ft and an average width of about 46 ft.
The typical bridge is constructed of stringers (the main load-carrying members) that rest on bearings supported by abutments, which are founded on piles. Bearings and a deck joint separate the superstructure from the substructure. Unlike the stringers, which generally are prefabricated, the substructure, deck and parapets are constructed in place.
Construction of the foundations and the substructure accounts for the majority of the total construction time for most bridges. The foundation, or pile cap (which ties all the piles together and supports the weight of the bridge), can be poured only after all the piles are in place. State highway agencies usually install the piles in a group that contains both vertical and battered (angled) alignments to establish a fixed abutment.
Workers spend significant time forming and placing the concrete bridge deck. This operation usually requires assembly of formwork and reinforcement steel mats atop the previously erected stringers. After placing and leveling the concrete with a screed, workers begin the curing procedures. The curing often requires a month or more.
Deterioration of the substructure, stringer ends and bridge deck, often caused by leaking deck joints, governs the service life. The debris and runoff passing through the joints eventually destroy the functionality of the bearings and accelerate corrosion of the concrete abutment or piers. As the superstructure responds to seasonal temperature changes, nonfunctioning expansion bearings also may cause structural damage.
Cracking of the concrete deck can result from poor placement and curing practices, significantly accelerating deterioration. Even decks constructed with the highest quality control may experience cracking due to differential creep and shrinkage, live-load effects and other unexpected actions due to complex geometries.
Water penetrates the deck reinforcement either directly through cracks or eventually simply because of the permeable nature of concrete. Once the reinforcement begins to corrode, deck life decreases significantly. Corrosion-related deterioration is accelerated in those areas of the country where bridges are subject to significant deicing chemicals during winter.
Trends in bridge design
The need for continued renewal of the nation's bridge stock is expected to take place in an environment of unprecedented demands, not only to keep traffic moving but also to increase capacity and reduce delays. The desire to keep initial and long-term costs as low as possible and renewed concerns about homeland security add to the challenge.
Time alone will reveal the exact geometric and material characteristics of the Bridge of the Future. But FHWA has identified definite trends within the National Bridge Inventory database. For example, after analyzing data from the 16-year period between 1985 and 2001, FHWA identified steady and significant increases in main span length (19%), overall length of structure (15%) and deck width (13%). Among the reasons for the longer span lengths is the desire to stretch the limits of existing materials to gain the maximum economy of scale, coupled with the demand to reduce the environmental impact of structures.
As new high-performance concrete increases the strength of spans, engineers can design bridges without intermediate supports. Longer spans require fewer piers and can help reduce the impact on fish, plants and other aquatic organisms.
Higher traffic volumes and increasing safety concerns have contributed to the increase in deck width. "In the future, bridges will carry heavier loads and more traffic," said Myint Lwin, newly appointed director of the Office of Bridge Technology, 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."
Generally, bridges are designed to carry a 72,000-lb design load (HS-20). In 1985, engineers were designing one out of every 50 bridges to accommodate a 90,000-lb (HS-25) load. By 2001, agencies were designing one in five bridges for an HS-25 load. "These trends clearly indicate that we will expect more performance from bridge materials and systems in the future," said Lwin.
The Bridge of the Future
To meet the need for longer-lasting, low-maintenance, high-performance bridges in the decades to come, FHWA has identified specific performance goals to help direct the research initiative for the Bridge of the Future. The goals take into account initial costs, service-life costs and the indirect costs of safety and time.
The proposed performance goals include:
* Achieving a service life that is no longer controlled by corrosion and involves little or no structural maintenance;
* Reducing construction time significantly;
* Designing bridges that can be widened easily or adapted to new traffic demands;
* Reducing life-cycle costs significantly;
* Elevating immunity to attack, flooding, earthquake, fire, wind, fracture, corrosion, overloads and collisions;
* Integrating design and construction of substructure and superstructure; and
* Eliminating vertical and lateral clearance problems.
Wider acceptance and implementation of current best practices can help attain several of these performance goals. One best practice, for example, is building bridges without joints by using integral abutment construction. "One of the biggest problems with corrosion," said Bob Kogler, team leader for design and construction at FHWA's Research and Development Center, "results from water leaking through joints. If we design bridges without joints, we can avoid this common problem altogether." Other practices include using precast or prefabricated components and high-performance materials.
Although greater use of these strategies can help, full attainment of the performance goals for the Bridge of the Future will require research, development and implementation of new technologies. Some areas that FHWA identified as critical include using deep foundations only where other viable options do not exist, extending the service life expectancy of bridge decks and 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 manufacturing methods to improve quality and consistency and ultimately realize the economies of scale that manufacturing can provide.
"We recognize that these goals will seriously stretch our creative and technological capabilities," said King Gee, associate administrator for infrastructure, 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."
A strategic focus
FHWA conducted a detailed analysis of new bridges constructed between 1996 and 2000 to evaluate the market potential for the Bridge of the Future. The goal was to define the most promising systems to pursue and develop in the initial years.
During that five-year period, states and communities built 33,823 new bridges in the U.S. Excluding those that were culverts, two-thirds of the remaining 25,886 bridges fall into six design classifications.
Further analysis of statistics on length and span revealed that the majority have maximum span lengths of 100 ft or less. The geographic distribution of these common bridge types shows definite spatial clustering of the bridges in different regions of the country, such as the Southeast and Upper Midwest.
These findings lead to the conclusion that existing market conditions support a strategic focus on researching a few standard bridge types, simple spans (less than 100 ft), and bridge systems that incorporate standardization and hence can be manufactured in significant numbers.
A program for deployment
An essential element of the FHWA strategic plan is to deploy the results of its research and development programs. The agency plans to broaden and redirect the Innovative Bridge Research and Construction (IBRC) program to become the primary mechanism for transferring new technology to the states. The IBRC has proven successful in incorporating innovative materials and technologies into bridge repair, rehabilitation, replacement and new construction. But the IBRC has been characterized by small, usually incremental, steps, such as replacing steel reinforcement with polymers or standard concrete with higher-performing concrete.
FHWA is proposing a new Innovative Bridge Research and Deployment (IBRD) program with the goal of spurring the development of new and innovative bridge systems. The agency plans to broaden the scope of the program beyond its current focus on new materials to include new structural systems and technologies for strengthening, rehabilitating, repairing, maintaining and preserving bridge infrastructure. The emphasis of the IBRD program will be on deploying and evaluating those technologies that have the potential to become the new standards for the future.
"We plan to emphasize the development and evaluation of technologies that have potential application on thousands of bridges," said Lwin, "not just a few."
In addition to the new program, FHWA plans 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.
"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 said. "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."
The third and final installment in this series describes strategies for ensuring the safety and reliability of new and existing bridges by mitigating the damage caused by extreme events such as earthquakes, flooding, vessel or vehicle impacts and terrorist attacks.
