FHWA's Aerodynamics Laboratory is working on a number of research fronts to make bridges more resilient.

Over the years, wind-related events, such as hurricanes, tornadoes, and winter storms, have resulted in significant losses of life and property in the United States. Based upon data presented in the 2003 Office of Science and Technology Policy report, Assessing Federal Research and Development for Hazard Loss Reduction, the total annualized societal losses attributed to wind events are estimated to be $6.3 billion. The losses exceed those attributed to earthquakes and floods by more than 40 percent and 100 percent respectively.

Furthermore, direct and indirect losses from a single extreme wind event can easily surpass $26 billion, as happened in Hurricane Andrew (1992), according to the National Climatic Data Center. These events can damage highway infrastructure and disrupt operations on the Nation's transportation network. Given that highways are vital lifelines in Americans' daily lives and critical to the U.S. economy and security, impacts from wind hazards can have major consequences.

In addition, exposed sections of highways or elevated structures need to be closed periodically due to high winds to ensure the safety of the traveling public. Traffic is rerouted for long periods during the subsequent repairs or replacements, resulting in congestion and delays.

"The Federal Highway Administration's [FHWA] wind research program addresses these problems by conducting laboratory and field studies with the goal of making the Nation's highway bridges more aerodynamically resilient," says Jorge E. Pagán-Ortiz, director of FHWA's Office of Infrastructure Research and Development. "Researchers at the program's laboratory and its partners have conducted basic, applied, and exploratory advanced research leading to new test methods for aerodynamic assessments, tools for predicting structural response to wind, improved design guidance, advances in design codes, and mitigation techniques."

Background

Since the 1836 collapse of the Brighton Chain Pier in England due to oscillations, the effect of wind events on highways, and transportation structures in particular, has concerned engineers. From 1818-1889, windstorms worldwide caused 10 suspension bridges to collapse or suffer major damage, including three in the United States.

In the half century that followed, many larger and lighter structures were constructed without any major wind-related problems -- until the fateful collapse of the Tacoma Narrows Bridge in Tacoma, WA, on November 7, 1940. In the 3 years immediately preceding this event, five newly completed bridges exhibited sensitivity to winds and resulting oscillations. As a result, engineering awareness heightened about the potential sensitivity of flexible bridges to winds.

The investigation into the collapse of the Tacoma Narrows Bridge resulted in the 1949 publication of Aerodynamic Stability of Suspension Bridges by the University of Washington under the direction of the Washington Toll Bridge Authority in cooperation with FHWA's predecessor, the Bureau of Public Roads (BPR). This report documents one of the first looks into bridge aerodynamics using field observations and testing, mathematical analysis, and wind tunnel testing. Prior to this investigation, the use of wind tunnels in the study of wind effects on ground-based structures was uncommon. However, in the coming decades, they became a well-established, and in many cases essential, tool for this purpose.

Estimated Annualized Losses by Hazard

Source: Charles Meade and Megan Abbott, Assessing Federal Research and Development for Hazard Loss Reduction.

Bridges Severely Damaged or Destroyed by Wind

Source: University of Washington.

Not long after this landmark work, the BPR established the wind research program and the Aerodynamics Laboratory at the Fairbank Highway Research Station, now known as the Turner-Fairbank Highway Research Center (TFHRC), to improve understanding of the effects of wind on transportation structures. To ensure success, BPR, which by then had become FHWA, initiated ongoing collaboration with several other laboratories, such as the National Research Council Canada and Public Works Research Institute of Japan. Today, the program also has a close relationship with various universities and consultants.

Aerodynamics Laboratory Facts The FHWA Aerodynamics Laboratory features two wind tunnels of varying sizes and functions, along with an array of support equipment. Of the test facilities, the most prominent is a large, low-velocity, open-circuit wind tunnel. With its 6- by 6-foot, ft (1.8- by 1.8-meter, m) test section, this tunnel can generate extremely smooth (laminar) flow at speeds up to 44 feet per second (ft/s), 13.4 meters per second (m/s). Originally designed for testing long-span bridges, the facility can simulate prototype wind speeds up to 150 miles per hour (mi/h), 67 meters per second (m/s), and at large model scales of 1:25. At smaller scales -- for example, 1:100 -- speeds up to 300 mi/h (134 m/s) can be achieved. For experiments where turbulent flow is desired, the researchers insert an active turbulence generator into the test section between the wind tunnel and the model. To measure wind loads on structures, they place models in a large, dual force balance (a precision sensing system to measure forces, such as lift and drag, on a model) where they can vary the vertical wind attack angle by rotating the model in the flow. Because this is a dual balance system, unbalanced loads on structural components such as tapered cylinders also can be measured. An automated dual three-dimensional (3-D) robot system is available to study flow in the test section and measure flow field details around models. The second wind tunnel is a higher speed, closed-circuit facility with an open test section measuring 10 by 10 inches, in (25.4 by 25.4 centimeters, cm). Speeds up to 90 mi/h (40 m/s) can be generated. The tunnel is equipped with a fog generator and is well suited for use of particle image velocimetry to visualize flow fields around structures or structural components. This small tunnel also is equipped with a 3-D robot system for detailed flow field measurements, as well as a two-dimensional shaker system for experiments that require forced vibration tests. In the large, open-circuit wind tunnel (far left) in FHWA's Aerodynamics Laboratory at TFHRC, smooth and turbulent wind conditions are simulated during static and dynamic testing of structural models.

Past Research Projects

The FHWA Aerodynamics Laboratory has conducted a number of wind tunnel investigations to develop retrofits for problematic existing bridges and to evaluate the performance of new designs. An example of the latter is proposed designs for the Hale Boggs Memorial Bridge in Louisiana. For this major project, the TFHRC researchers performed a series of wind tunnel tests on four basic design alternatives, including a total of 15 configurations, to determine the configuration with the optimum performance in the expected wind conditions, including hurricanes. The researchers further examined the final configuration at three construction stages to ensure stability during erection.

Bridges That Have Oscillated in the Wind

Source: University of Washington.

To evaluate particularly complex problems in structural performance, FHWA sometimes performs full-scale measurements and analytical studies. At the request of the Maine Department of Transportation (MaineDOT), the TFHRC researchers conducted a comprehensive program of measurements, analytical studies, and wind tunnel simulations to evaluate the behavior of MaineDOT's Deer Isle-Sedgwick Bridge. This bridge, built about the same time as the original Tacoma Narrows Bridge and similar in configuration, had been exhibiting a number of wind response problems that resulted in periodic closures and significant damage requiring major repairs.

The researchers conducted wind tunnel tests on a 1:25 scale section model of the bridge to assess aerodynamic behavior, evaluate the influence of snow blockage of curb grates (raised open grates along the right edge of each traffic lane), and explore possible mitigation measures. Field measurements and predictions from numerical modeling confirmed observations from the wind tunnel simulations.

Through this work, the researchers developed a retrofit measure consisting of nonstructural fairings, which are streamlining plates that reduce vibrations. MaineDOT implemented the retrofit, and TFHRC researchers are monitoring the bridge's post-retrofit performance.

As part of a major rehabilitation project, a similar aerodynamic retrofit recently has been added to the Bronx-Whitestone Bridge in New York City, another bridge from the same era.

Sometimes, wind can induce vibrations in structural components such as columns, cables, tie beams, and truss members. Two examples are vibration problems with the columns of the Perrine Memorial Bridge in Idaho and truss members of the Commodore Barry Bridge, owned by the Delaware River Port Authority.

The Idaho bridge is a large, steel, deck-arch structure that has columns with unsupported lengths ranging from 7 to 160 feet, ft (2.1 to 48.8 meters, m). Moderate daily winds, traveling up and down the Snake River Canyon, were causing visible, and sometimes significant, vibrations in the longer columns. To explore this vibration problem and assist the Idaho Transportation Department, the TFHRC researchers briefly installed instrumentation on the bridge to monitor wind conditions and resulting column behavior. They also performed analytical work to develop a numerical method to assess the stability of long columns with elastic end constraints. As a result of these activities, Idaho designed and installed tuned mass dampers (a weight on a flexible beam tuned to match a structure's vibration frequency) inside the columns to mitigate the vibrations.

The Delaware River Port Authority deployed a similar solution using external tuned mass dampers on the vertical truss members of the Commodore Barry Bridge. Another type of damper solution using both internal and external damping devices on the cables was included in the original design of the Arthur Ravenel Jr. Bridge in South Carolina.

Recent and Ongoing Research

The FHWA Aerodynamics Laboratory recently conducted research on wind loading in five project areas: highway signs and lights, cable-supported structures, full-scale measurements, long-term monitoring, and large amplitude cable vibration. Some of these studies are ongoing.

Signs and lighting. The structural supports for highway signs, luminaries, traffic signals, and high mast lighting often consist of cylindrical members with multisided shapes and tapering along their length for structural efficiency. A number of these structures have collapsed due to fatigue resulting from natural wind loading or gust loading from trucks. To establish the wind loads and pressures for design of these structures, engineers need to know the lift and drag properties of the components. The literature reports considerable research on circular cylinders with uniform diameter, establishing these properties, but for multisided shapes and tapered members, detailed information is much more limited.

With this information gap in mind, researchers at TFHRC designed a series of representative models with various diameters, number of sides, and taper ratios for testing in the wind tunnel. They fabricated a total of 71 models representing circular, hexagonal, and octagonal sections with diameters ranging from 2 to 5 inches, in (5.1 to 12.7 centimeters, cm) and with taper ratios ranging from 0.0 to 0.3 in/ft (0.0 to 2.5 cm/m). Models with a greater number of sides are planned as well. So far, FHWA has conducted static tests on 53 of these models to obtain force coefficients for comparison with American Association of State Highway and Transportation Officials' code values. The researchers also are planning dynamic tests to obtain detailed information regarding wind-induced vibration problems, such as vortex shedding response (response to pressure fluctuations resulting from the shedding of vortices) and galloping (instability of slender structures with a certain shape resulting in large oscillations).

Cable-supported structures. For the most part, design codes do not provide detailed wind load criteria for large cable-supported structures or otherwise complex and unusual structures. Although some uniformity exists in the general approaches employed to arrive at design criteria for wind conditions, the assumptions, techniques, and procedures employed depend on who is performing the aerodynamic assessment and climatological study. To address the resulting variability, researchers at the FHWA laboratory launched a project to survey and review current approaches and prepare a synthesis report. The research has looked into issues such as selection of recurrence intervals for wind conditions, determination of design wind speeds, influence of directionality and terrain, climate models, and treatment of extreme events. The research will serve as the basis for later development of consistent procedures for establishing design wind loads.

Full-scale measurements. To complement the physical modeling in the Aerodynamics Laboratory, TFHRC researchers maintain a parallel program of full-scale measurements. Such measurements provide detailed information on site wind conditions and structural behavior that is not available from any other source. The measurements aid in the design of laboratory simulations and serve to validate those tests. Full-scale measurements can provide a means to calibrate the results of numerical predictions and computational simulations. They can be used to diagnose wind-related structural problems and to evaluate the effectiveness of aerodynamic retrofits to bridges.

Historically, field tests have consistently been a highly productive way to advance understanding of the interaction of loads with structures. The full-scale measurement program is ongoing and consists of short-term tests to focus on specific structural issues, as well as long-term monitoring to characterize site wind conditions and structural behavior. Currently, researchers are monitoring two long-span bridges for long-term and one bridge for short-term structural issues. The instrumentation systems developed for this task are designed to function autonomously but have remote communication capability so that TFHRC can access data and perform direct system control. To measure the wind and associated structural response, an array of wind, meteorological, vibration, and sometimes displacement sensors are placed strategically on and near the structure.

Long-term monitoring. The long-term monitoring of major bridges under adverse conditions requires robust instrumentation and sensors, sound deployment and maintenance techniques, and comprehensive data management and analysis. TFHRC researchers have developed expertise in all these areas through participation in numerous field projects spanning the past half century. As one example, Aerodynamics Laboratory staff recently assisted the Washington State Department of Transportation and Washington State University by designing a multihazard wind and earthquake monitoring system for the new Tacoma Narrows Bridge and modifications to the existing bridge. The resulting design was cost effective, merging some of the latest technologies with techniques and concepts proven reliable on previous projects. Key features were incorporated into the design, such as expandability, multiple synchronized data hubs (collection points with data acquisition equipment), a blend of wired and wireless technologies, and comprehensive remote access to the system and data via the Internet. While much could be learned from long-term monitoring of these major structures, currently there is no funding or plans for installing this system.

Large amplitude cable vibration. With increasing frequency in recent years, transportation professionals have observed large amplitude vibration of stay cables under conditions of moderate wind, sometimes in conjunction with light rain. This problem is not new; extensive study started in the mid-1980s. But with a growing inventory of cable-stayed bridges in the United States, reports of large amplitude cable vibrations and damage have increased significantly.

To mitigate these vibrations, some structures have been retrofitted. For cable-stayed bridges currently under design or construction, engineers are incorporating dampers, crossties, or aerodynamic surface treatments into the cable system. A national Transportation Pooled Fund research project led by the Missouri Department of Transportation and FHWA, SPR-3(078) Wind Induced Vibration in Cable Stayed Bridges, is underway to investigate this cable vibration problem and develop comprehensive guidelines for both retrofits and new construction. Among other things, the study involves synthesis of existing information, analysis of the mechanics of wind-induced cable vibration, wind tunnel testing to clarify dry cable galloping, and evaluation of mitigation methods.

In conjunction with this project, a number of full-scale experimental studies have established the dynamic properties of representative bridge stay cables and performance of various mitigation features. The FHWA researchers have performed tests on new bridges such as the Leonard P. Zakim Bunker Hill Memorial Bridge (Massachusetts), Bill Emerson Memorial Bridge (Missouri), and Penobscot Narrows Bridge (Maine), as well as existing bridges such as the Hale Boggs Memorial Bridge (Louisiana), Sunshine Skyway Bridge (Florida), and U.S. Senator William V. Roth, Jr. Bridge (formerly the C&D Canal Bridge) (Delaware). For the new bridges, the researchers took measurements at various stages of construction to facilitate evaluation of cables with and without grout, dampers, or crossties. They used a small and portable, but robust, instrumentation package to minimize interference with construction activities and traffic operations.

Current Research Projects

Current FHWA research projects include parametric studies, aerodynamic assessments, and measurements of stresses on structural components.

Parametric studies. Japanese researchers have conducted a number of parametric, aerodynamic studies of various deck geometries and details, and they have used the resulting information in the development of their 2008 Wind Resistant Design Manual for Highway Bridges in Japan. Building on this Japanese work, a new project is underway by the Aerodynamics Laboratory to investigate a variety of generic geometries, actual bridge sections, and bridge details. The research involves special static and dynamic studies in the laboratory to extract and catalog aerodynamic properties, and will serve as a basis for later development of draft design criteria.

Comprehensive report on aerodynamic assessments. An aerodynamic assessment, usually involving one or several wind tunnel tests, is almost always conducted on any design proposed for long-span bridges. Over the years since the dramatic collapse of the original Tacoma Narrows Bridge, hundreds of such studies have been performed in North America, largely in one of three laboratories in Canada or the FHWA Aerodynamics Laboratory, with a few in other U.S. labs. The results of these investigations represent a wealth of technical information that could advance understanding significantly and serve as an aid for design of future bridges. Unfortunately, many of the results and details remain in the archives of the laboratories conducting the work and the design consultants who commissioned the research. The results that are published are limited and scattered in the literature. The TFHRC researchers are working with other laboratories in North America to gain access to the archives to collect, catalog, and synthesize this information. They will compile the results into a comprehensive catalog suitable for use by researchers, designers, and bridge owners.

Stresses in structural components. Bridge designers estimate the expected wind pressures on a structure's vertical and horizontal surfaces and calculate the resulting stresses in structural components. For relatively simple structural shapes and configurations, the codes provide guidance for calculation of wind pressures. But for complex or flexible structures, or structures in complicated settings, establishing pressures through measurements on models in wind tunnels is quite common. Mean forces, such as uplift and drag, or forces integrated over the surfaces of the model typically are measured by installing a scale model in a force balance and placing it in a wind flow with various speeds and attack angles.

Shown here is a wind tunnel test by the laboratory's research partners at National Research Council Canada to measure wind pressures on a rigid model of an inclined cylinder representing a bridge stay cable. The researchers measured pressures using taps at several circumferential rings along the length of the model. They varied the yaw angle (horizontal angle between the approaching wind and a vertical plane containing the cable) by rotating the model on a turntable.

As part of FHWA's Exploratory Advanced Research (EAR) Program, researchers at the Aerodynamics Laboratory are investigating a new technology for measuring local pressures, using pressure-sensitive paint or surface-stress film on model surfaces. The objective is to explore, adapt, and deploy a new technology that will be well suited to the measurement of static and dynamic pressures over the surfaces of structural models with high spatial resolution while in low-speed wind flows.

New Research Activities

Two new projects will involve optimizing aerodynamic performance and addressing vibration of stay cables.

Optimization of aerodynamic performance. In the structural design of cable-stayed bridges, several road deck cross sections appear to have become favorites among design consultants throughout North America. Although designers have focused considerable effort on producing structurally efficient bridges, they have devoted much less attention to optimization of aerodynamic performance. Generally, cross sections are tested in the later stages of design to assess if they meet the specified aerodynamic design specifications or criteria. If they do not, additional tests are performed to "fix" the design.

The objective of FHWA's new study is to evaluate some of the more popular designs from an aerodynamic standpoint and focus on the significance of geometric details that are key to aerodynamic performance. The researchers will classify the most common road deck cross sections into a few generic shape categories. The Aerodynamics Laboratory will design special benchmark section models that are representative of each category and test them in the wind tunnels. The researchers will design the models so that details can be modified easily or retrofits introduced to explore ways to optimize aerodynamic performance. The researchers then will develop design guidelines.

Vibration of stay cables. Another new project is an extension of activities conducted under Transportation Pooled Fund study SPR-3(078) to address the problem of wind-induced large amplitude vibration of bridge stay cables. Although research conducted under the pooled-fund study has increased understanding and led to development of draft guidelines, a number of significant knowledge gaps remain. For example, little or no information is available in the literature regarding the performance of aerodynamic surface treatments. In addition, the limited design criteria regarding galloping was removed recently from the Post-Tensioning Institute's Recommendations for Stay Cable Design, Testing and Installation, and proposed new criteria were not approved due to insufficient supportive evidence. The objective of the FHWA project is to fill some of these gaps in order to prepare a more complete guidelines document. The TFHRC researchers will perform wind tunnel tests to provide information regarding surface treatments and specific cable instability issues. The laboratory will complement the physical modeling with numerical modeling of cable responses to various wind or wind-and-rain loading conditions and by computational fluid dynamics (CFD) simulations as appropriate.

Concluding Remarks

"The FHWA wind research program will continue to conduct applied and advanced research using physical experiments, full-scale tests, and analytical studies to assess the impacts and reduce the risks of winds, windstorms, and hurricanes on the highway infrastructure," says FHWA's Pagán-Ortiz.

In the early days of the program, building the research infrastructure was necessary before the hazards could be investigated. Now that the laboratory is well established, enhancing and extending these resources will continue through automation of facilities, implementation of new simulation techniques, development of new sensors, and establishment of new test procedures.

The program will assess and incorporate developments in instrumentation as they become available, such as wireless data hubs, self-powered sensors, and miniaturized sensors. The laboratory will continue to develop and improve numerical tools for the prediction and evaluation of structural performance in response to wind loading. Although TFHRC researchers have performed some development work and computer simulations in the past using CFD, these activities will intensify in the coming years, taking advantage of the growth in computing power. For some of the more complex problem areas involving fluid-structure interactions, researchers will explore the use of computational multiphysics mechanics.

The laboratory will continue to assess the aerodynamic stability and performance of new designs and develop retrofit measures for inservice structures. The laboratory will work with the American Association for Wind Engineering, the International Association for Wind Engineering, and others to disseminate new information and procedures and to work toward international alignment of design codes for wind resistance.

Until recently, no comprehensive or coordinated program at the national level has addressed wind hazards to the built environment. Although several Federal agencies, such as FHWA and the National Oceanic and Atmospheric Administration, National Institute of Standards and Technology, and Federal Emergency Management Agency, have conducted studies addressing wind issues, their work has represented relatively small parts of much larger missions. The result has been a fragmented approach and slow progress. In 2004, however, the National Windstorm Impact Reduction Program was signed into law to address wind hazards through a coordinated Federal effort. Researchers at TFHRC serve on the interagency working group leading this program and will continue to be involved to ensure that FHWA's research provides maximum benefit to the public and the Nation's highway infrastructure.

Harold Bosch is a wind research program manager in the FHWA Office of Infrastructure Research and Development. He coordinates wind research activities with State and local agencies, academia, and various partners and customers, and he manages the Aerodynamics Laboratory. He received his B.S. in civil engineering from The University of New Mexico and has completed numerous graduate courses in advanced fluid mechanics, wind engineering, analysis of complex structures, structural design, and vibrations. He has 40 years of experience in construction, bridge design, and structural research.

For more information, contact Harold Bosch at 202–493–3031 or harold.bosch@dot.gov.