FHWA outlines a plan for minimizing the effects of an unpredictable natural or man-made disaster on U.S. bridges
To meet the demand for a 21st century transportation
network, FHWA is proposing a comprehensive program of bridge research and
The Bridges for the 21st Century program is part of the
surface transportation legislation that will authorize highway and bridge
programs for fiscal years 2004 through 2009.
In the first article of this three-part series on the
proposed R&T program, John Hooks introduced the concept of Bridges for the
21st Century, focusing on "Stewardship and Management" of the
existing bridge inventory.
The second article, by Dr. Steven Chase, outlined an R&T
strategy to develop the "Bridge of the Future," a new generation of
cost-effective, high-performance and low-maintenance bridges.
This third and final article presents a strategy for dealing
with bridge failures due to catastrophic events, both natural and man-made.
Addressing these rare and unusual events is the focus of FHWA's initiative to
ensure the "Safety, Reliability, and Security" of U.S. bridges. The
goal is to deliver the knowledge and technologies that will help ensure that
the nation's highway bridge infrastructure continues to function safely and
reliably, even during extreme or infrequent catastrophic events.
Bridges vs. nature
Natural disasters like earthquakes and floods have a high
probability of affecting large land areas and a high number of highway structures
simultaneously, significantly disrupting regional mobility, emergency response
and regional economies. Each major earthquake and flood teaches engineers new
lessons about bridge response and performance, and new standards and
technologies often result.
The seismic research program at FHWA developed and continues
to refine guidance for retrofitting bridges to make them less likely to fail
during earthquakes. FHWA also continues to explore new design concepts that
enhance seismic performance, however much work still remains. At-risk
structures include all bridges built before 1980 in the moderate- to
high-seismic regions, especially those that are susceptible to vertical
accelerations, span active faults or exist in areas where seismic activity has
Researchers need to develop and install 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 that need further exploration include
shape-memory alloys (used in cable restrainers and isolation bearings) that go
back to their original shape and location after a seismic event.
There is a need to better understand the effects of seismic
activity on highly skewed bridges, poor restrainer details, inadequately
reinforced footings, battered and short-length piles on weak soil foundations,
old flared columns, large piles fixed to pile caps and bridges built using poor
Additional research needs to be conducted to understand
soil-structure interactions. In particular, researchers need to understand risk
levels for the liquefaction of various soil types and the effects of
liquefaction on pile foundations, such as reduced lateral resistance, lateral
spreading, reduced vertical resistance and post-earthquake settlement. FHWA
proposes to continue its multiyear seismic research program to address these
and other relevant needs and to develop technologies and guidelines to provide
for safe bridges and structures during earthquakes.
Flooding and scour in the U.S. cause more bridge collapses
than all other causes combined. Approximately 85% of the structures contained
within FHWA's National Bridge Inventory are over water. Since the late 1980s,
state highway agencies have undertaken a nationwide effort to evaluate these
bridges to identify those that are scour critical.
FHWA 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 Hydraulics Laboratory and an
ongoing training course offered by the National Highway Institute, FHWA helps
states and consultants evaluate the effects of scour. Despite the efforts of
FHWA and the National Cooperative Highway Research Program (sponsored by the
American Association of State Highway & Transportation Officials), scour
continues to undermine many of the nation's bridges.
A critical evaluation of the philosophy of including
designed countermeasures as an integral part of the foundation design for new
bridges needs to be conducted. Researchers need improved techniques for
physically and numerically modeling unique scour problems. Rational techniques
are needed for evaluating scour in rock subject to erosion and determining
scour rates for cohesive, fine-grain bed materials. Also needed are hydrologic
techniques that account for the expected time of exposure to various flood
levels over the life of a bridge. Researchers also need to develop 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 be used to improve predictions of flood
runoff. Portable instrumentation is critical to assess foundation damage before
reopening a bridge after a major flood.
The dramatic collapse of the original Tacoma Narrows Bridge
near Tacoma, Wash., in 1940 alerted the engineering profession to the
significant effect that wind can have on the design, safety and performance of
structures. Since that milestone event, the field of wind engineering has
evolved steadily and matured, addressing many of the issues associated with the
interaction of wind and the built environment. Although engineers have learned
much about the problem, and the list of tools available to designers continues
to grow, much work still remains.
Wind-induced problems cause significant concern, as
evidenced by recent problems with the large-amplitude oscillation of cable
stays under conditions of light rain and wind (as in the Fred Hartman Bridge in
Houston, Texas, Veterans Memorial Bridge in Beaumont, Texas, Burlington Bridge
in Burlington, Iowa, and Cochrane Bridge in Mobile, Ala.).
"The aerodynamics re-search program at FHWA is studying
this and similar problems with new bridges to develop appropriate
countermeasures," said Harold Bosch, director of the FHWA Aerodynamics
FHWA plans to confront wind-induced natural hazards by
developing comprehensive guidelines for the design of long-span bridges,
specifications for assessing the aerodynamics of new designs, a rational method
for wind-climate analyses and guidelines to retrofit bridges that have
Researchers will conduct more extensive experimental and
analytical work relative to vortex-induced vibration of bridge decks, study the
wind- and rain-induced vibration of bridge cables and how to mitigate the
problem, explore new damper technologies and aerodynamic surface treatments and
encourage national and international benchmarking activities.
They also will address the design needs of other highway
support structures (such as traffic signals, cantilevered message signs and
Further, FHWA intends to develop a suite of software tools
to analyze the effects of wind and vehicle gusts on transportation and
The tools also will explore the aerodynamic ramifications of
innovative design concepts and optimize (or aerodynamically tailor) commonly
used bridge deck sections.
Bridges vs. man
Shipping by truck is convenient and efficient as evidenced
by the fact that trucking now accounts for 80% of expenditures on freight
transportation in the U.S. Truck size and weight have a significant impact on
maintenance and construction costs for both highway pavements and bridges.
Federal laws and regulations govern axle weight limits, gross weight limits and
the dimensions of trucks, buses and trailers. Because limitations on truck size
and weight influence trucking costs, increasing the allowable loads that can be
carried on highways can benefit the motor carrier industry.
In May 2002, the Transportation Research Broad (TRB)
completed a study on federal truck size and weight regulations. Regulation of
Weights, Lengths, and Widths of Commercial Motor Vehicles-Special Report 267
recommended improving the efficiency of the highway system by reforming size
and weight regulations to allow larger trucks to operate. Acknowledging the
lack of information on costs and benefits of truck transportation and the
impacts of size and weight regulations, TRB argued that a program of basic
research should be established to determine fact-based regulations for truck
size and weight. Further, the study recommends allowing longer combination
vehicles on roadways, conducting a study on the routes and roads to which
federal standards should apply, and establishing pilot studies involving
temporary exemptions from federal size and weight regulations for the purpose
of evaluating the consequences of changes in regulations.
The issue of overweight and oversize trucks not only affects
the condition of highway pavements but also has a large impact on the bridge
population. With the large number of bridges that are structurally deficient,
research is necessary to assess the ability of those bridges to carry heavier
loads. The large percentage of bridges identified as functionally obsolete also
presents a challenge if trucks are oversize or exceed height limits. The ability
of the highway infrastructure to carry heavier loads must be studied and should
be an essential element in the decision-making process before changing rules
The events of Sept. 11, 2001, re-emphasized the nation's vulnerability
to terrorism. Nearly two years later, transportation agencies continue to
define strategies and solutions to protect the nation's highways from terrorist
threats. Although the civilian highway community has little experience with
designing transportation infrastructure for security, the military is familiar
with these issues. FHWA is partnering with the defense community to draw on
that body of knowledge and experience, synthesizing and transferring applicable
Terrorists could attack a bridge or tunnel in a variety of
To protect the infrastructure, researchers need a more
complete understanding of the threats, ways to identify specific
vulnerabilities, methodologies to eliminate or protect against these
vulnerabilities and a framework for translating this knowledge into standards
and specifications for new and existing bridges and tunnels.
FHWA envisions a multiyear program that will lead to the
next generation of bridges and structures that will be more resistant to terrorist
To develop a resilient physical infra-structure that can
withstand acts of terror, FHWA proposes developing new design systems, analysis
techniques, materials and science (such as nanotechnology).
Engineers need improved ways to prevent incidents, better
methodologies for assessing the safety and residual capacity of structures
after an incident and new techniques for repairing and restoring bridge
FHWA anticipates that the R&T program will encompass
topics ranging from systems analysis and design to improved materials,
post-event assessments, repair and restoration, evaluation and training, and
prevention, detection and surveillance.
On Aug. 11, 2002, a gasoline tanker truck loaded with 8,300 gal
of gasoline overturned on the Rte. 528 ramp leading onto I-4 just west of
Orlando, Fla. An explosion and fire resulted that caused a piece of concrete to
fall onto I-4. Several vehicular crashes resulted as drivers maneuvered to
avoid the falling debris. In the end, the crash caused two fatalities, one
serious injury and six minor injuries.
The intensity of the fire concerned state highway
authorities. Unsure of the structural integrity of the Rte. 528 overpass after
the crash, authorities closed the overpass for approximately 30 days for
repairs. With approximately 590,000 bridges and culverts on the highway system,
incidents like this are not uncommon.
For bridges over navigable waterways, the piers are the most
vulnerable to damage. Design codes need to provide more effective provisions
for collisions with ships and barges and other commercial craft or enemy
vessels. In heavily trafficked rivers with ships and large barges, bridge piers
should be protected and monitors should be installed to track movement caused
by vessel collisions.
Developing crashworthy barriers is an ongoing effort at
FHWA, but more research is needed to improve analytical capabilities to predict
the actual performance of barriers during impact. "We are in the process
of developing better mathematical formulas to describe the outcome of actual
crash testing so we can reduce the number of full-scale crash tests," said
FHWA Research Safety Engineer Marty Hargrave. "And more work in this area
needs to done. Finite element analysis currently is being used to predict the
crashworthiness of various concrete barrier shapes, which could lead to new
shapes even more effective than the standard Jersey barrier."
Commitment to safety
FHWA's proposal for a refocused and revitalized R&T
program sets a strategic direction for developing and deploying breakthrough
The initiative for stewardship and management specifically
calls for reliable and timely data and information, improved decision-support
tools and the development of quantitative, relevant and useful measures of
performance. The initiative for the bridge of the future specifically meets the
need for enhanced materials, structural systems, technologies and
specifications for improved structural performance. And to prevent bridge
failures due to natural and man-made hazards, the third and final focus of the
R&T program will produce the knowledge and technologies required to ensure
that the nation's bridges are safe and will continue to function reliably
during extreme or infrequent events.
For more information on bridge research at FHWA, visit