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.
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
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
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
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;
construction time significantly;
bridges that can be widened easily or adapted to new traffic demands;
life-cycle costs significantly;
immunity to attack, flooding, earthquake, fire, wind, fracture, corrosion,
overloads and collisions;
design and construction of substructure and superstructure; and
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
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
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
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
"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.
FHWA outlines a research strategy for building high-performance bridges