To meet the demand for a 21st century transportation network, FHWA is proposing a comprehensive program of bridge research and technology (R&T).
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 increased.
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 construction practices.
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 Lab.
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 aerodynamic problems.
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 noise walls).
Further, FHWA intends to develop a suite of software tools to analyze the effects of wind and vehicle gusts on transportation and highway-support structures.
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 and regulations.
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 technologies.
Terrorists could attack a bridge or tunnel in a variety of ways.
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 threats.
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 infrastructure quickly.
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 technologies.
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 www.tfhrc.gov/structur/structre.htm.