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In 2009, the San Francisco-Oakland Bay Bridge made national headlines when fatigue cracking caused damage to the very same eye-bars that had received emergency repairs a month prior. The damage sent pieces of steel falling onto the roadway during peak-hour traffic.
The incident – and countless others involving bridges across the country in recent years – happened without any warning and jeopardized the safety of drivers, including many who were simply following their normal day-to-day commute.
Structural engineers throughout the country are nevertheless faced with a hard truth: fatigue-induced cracking is an all-too-common failure mode in many steel bridges reaching their original design life. After all, many of today’s aging bridge structures have endured increasing traffic volume and weight, which contribute to deterioration of bridge components.
The U.S. invests approximately $20 trillion in infrastructure, yet the magnitude of the societal challenges associated with aging facilities – particularly, bridges – is daunting. Despite years of research in overcoming common infrastructure challenges, failures still occur. Funding challenges and increased freight transport make it difficult for local agencies to keep up with maintenance demands. Even factors such as climate change – and resulting extreme weather events – can place added burden on roadways and bridges.
Recognizing this, a joint team of University of Maryland (UMD) and North Carolina State University (NCSU) researchers partnered with the URS Corporation – now AECOM – to develop a smart bridge condition monitoring system known as the Integrated Structural Health Monitoring (ISHM) system. The ISHM system features a number of technology innovations including remote sensing capability particularly suited for fatigue condition assessment of highway steel bridges. The project researchers also hope to extend this technology for use in evaluating other types of bridge damage, such as breaks and corrosion of steel strands of pre-stressed concrete bridges.
Led by Department of Civil and Environmental Engineering (CEE) Research Professor and Director of the Bridge Engineering Software and Technology (BEST) Center Chung C. Fu, along with CEE Professor Yunfeng Zhang, the research team developed the ISHM system based on wireless sensor networks with self-sustained power supplies, which make the system scalable for remote monitoring of large complex highway infrastructure. In addition to Fu and Zhang, NCSU Department of Mechanical and Aerospace Engineering Professor Fuh-Gwo Yuan, and Y.E. Zhou of the URS Corporation are co-principal investigators on the project.
Backed by funding from the U.S. Department of Transportation’s Office of the Assistant Secretary for Research and Technology, the research team worked to integrate recent advancements in civil, aerospace, and electrical disciplines to produce a transformational system that quantifies damage and degradation at an early stage, thereby reducing operating and maintenance costs in the long run.
After an ISHM system is installed on a structure, researchers can use its sensor data to update the deterioration models for future condition forecasting, Fu said.
“Condition rating data based on periodic inspection of bridge components to provide an overall characterization of the general condition of a bridge has been routinely used by inspection engineers,” he said. “Combining visual inspection and ISHM system data, failure conditions can be detected earlier, even in some hard-to-detect areas, and catastrophic bridge failures can be prevented.”
The research team’s overarching mission is a crucial one: enhance the sustainability of U.S. civil infrastructures by reducing the life cycle costs of bridges.
In the state of Maryland alone, 317 bridges are considered structurally deficient, according to a recent U.S. Department of Transportation National Bridge Inventory database report. While this number may be jarring to the countless drivers who commute through the Free State each day, it represents just under 6 percent of Maryland bridges and pales in comparison to states like Rhode Island and Pennsylvania, where more than 20 percent of bridges are considered structurally deficient.
But, it is important to note that structural deficiency does not automatically deem a bridge unfit for travel, Fu noted.
“Structural deficiencies are determined by poor condition ratings or from low load ratings,” he said. “Bridges are considered structurally deficient if significant load-carrying elements are found to be in poor or worse condition due to deterioration or damage.”
As common practice to keep structurally deficient bridges in service, the State Highway Administration often posts weight limits to restrict the gross weight of vehicles using the bridges to less than the maximum weight typically allowed by statute, Fu said.
Still, all structurally deficient bridges eventually require rehabilitation or replacement.
“With hands-on inspection and field testing, unsafe conditions may be identified and, if a bridge is determined to be unsafe, the structure must be closed,” Fu said.
That is where Fu’s research team can make a world of difference. The ISHM system incorporates recent innovations that could transform remote sensing and management of highway infrastructures in two ways. First, ISHM uses wideband acoustic emission (AE) signals to evaluate localized damage, even in areas where traditional sensors cannot be placed due to geometry and structural constraints. Second, ISHM incorporates smart wireless sensor networks that can self-power using wind or solar power, self-calibrate, and automatically scan and diagnose problem sites.
This enables early detection of structural damage, which is critical given that, in states such as Maryland, most structurally compromised bridges are on the Interstate Highway System – and among the most heavily traveled bridges in the state, according to American Road and Transportation Builders Association (ARTBA) analysis of the DOT bridge report. The ARTBA analysis underscores the challenges many states face with federal highway and transit funding.
Are Fuel-Efficient Vehicles Posing Challenges for Bridge, Roadway Maintenance?
Adding another layer to the problem of bridge and roadway maintenance is the current rate of the federal gas tax – a major funding mechanism for the Highway Trust Fund, which holds the purse strings for highway and bridge maintenance. For more than two decades, the federal gas tax rate has held steady, even as environmentally driven consumers are opting for eco-friendly cars or other modes of transportation, and are thereby purchasing less and less gas.
“One of the challenges to the funding issue is the fact that modern cars are more fuel-efficient than older cars,” said Mark Franz, Assistant Director of Outreach and Technology Transfer for the National Transportation Center at the University of Maryland. “From an environmental perspective, fuel-efficient cars are obviously a great thing. But, if people are not buying as much gas nowadays, they might not be contributing their share of the cost toward the system they’re using. If more and more drivers are going home and plugging in their cars overnight, those drivers are essentially using roads without contributing direct funding for those roads.”
But, while everyday consumers may purchase less fuel than in years past, consumer behavior remains a factor behind the deterioration of some of the nation’s most heavily traveled roadways and bridges.
As Americans continue to take on lengthy commutes to work, the burden on highways, bridges and even local roads increases. Additionally, while the popularity of online shopping has reduced the need for consumers to drive to and from local stores, it has, in turn, increased demand for the transport of goods by freight vehicles. And, according to a recent DOT report titled, “Beyond Traffic 2045: Trends and Choices,” in 2012, trucks moved 13.2 billion tons of freight throughout the U.S. – compared to second-place rail transport, which accounted for just 2 billion tons of freight the same year.
“When you think about what is contributing most to wear and tear on the roads, freight vehicles prove costly,” Franz said. “The irony is, in many ways, the Highway Trust Fund counts on everyday travelers to use those same roadways so that more dollars will funnel in via the state and federal gas taxes to help fund both road and bridge maintenance.”
Incidents like the February case of fallen concrete from the I-495 Suitland Bridge in Prince George’s County, Md., have directed a spotlight onto highway maintenance issues, and many directly impacted by the bridges or roadways in question hope that such incidents will drive government to direct more funding to support road and bridge maintenance and repair projects.
Highway and bridge construction funds are a mix of state and federal dollars, Fu noted. At the federal level, the Highway Trust Fund is the main source of support; however, there has not been an increase in the tax on gasoline since 1994.
In fact, the federal Highway Trust Fund ran dry in 2008 and again in 2015 and had to be subsidized by Congress, Fu said.
“The difference between the funds available and the system’s needs represents the funding gap,” Fu said. “The only other source available is the state funding. Lack of state resources to fill any gap in federal funding is a cause for concern because it would cripple road and bridge construction in Maryland.
“Bridges are aging year by year,” he continued. “The average age of Maryland state bridges is about 50 years. As bridges age, their abilities to handle the loads they are designed for are gradually reduced. As a result, some bridges have to be posted with weight restrictions and then more bridges are put into the deficient bridge category. With an aging infrastructure system, shortage of funding will only increase needs in the future.”
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September 2, 2015
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