A nation's transportation infrastructure is crucial to its economic growth and public safety. The function and condition of roads, rails, and ports determine the efficiency of commerce. Bridges are part of the critical transportation infrastructure that can be considered the backbone of a nation's healthy economy. The condition of bridges affects many facets of society, including the efficient movement of people and goods, critical access to communities during emergencies, and a mechanism to overcome physically-challenging environments (e.g., large rivers or valleys). Society often overlooks the importance of maintaining these structures in a manner sufficient to allow them to continue in service for many years into the future.
The importance of bridge safety was brought to the fore when the I-35W Bridge in Minnesota suddenly collapsed in August 2007. Bridges in the United States are facing a crisis of high deterioration rates coupled with a scarcity of maintenance and new construction funding. More than 70 percent of in-service bridges in the United States were built before 1935. For the most heavily used bridges, which are on the interstate highway system, 17 percent were constructed during the 1950s, 44 percent were built during the 1960s, and 20 percent were built during the 1970s. A report published by the American Society of Civil Engineers (ASCE) rated the current state of repair of all infrastructure systems, including highways and bridges, a “D” (ASCE 2009). The report stated that more than a quarter of all bridges are considered either structurally deficient or functionally obsolete.
Federal funds are issued for public bridge maintenance and rehabilitation. Federal funding for surface transportation comes mainly from the Highway Trust Fund (HTF). With the increasing investment needs for national infrastructure improvement, the HTF is facing the problem of a financial deficit. The cumulative gap between federal transportation revenues and investment needs will be roughly $400 billion from 2010-2015. This deficit amount may increase to about $2.3 trillion through 2035. Efficiently allocating the available highway and bridge maintenance and improvement funds will be critical to the future reliability of the nation's transportation infrastructure.
All public bridges in the United States are required to be inspected once every two years. There are presently a number of techniques and procedures available for collecting information and data on a bridge's physical condition. Currently, visual-based inspection represents the primary method for bridge inspection in the United States. Errors caused by visual-based inspection are high, and ratings generated by different inspectors for the same bridge can vary considerably. Furthermore, visual-based inspections tend to be time consuming as the inspector must visit the bridge site, assemble photographic and text-based information, and make a determination regarding the integrity of the structure. There are other types of bridge inspection techniques, such as using nondestructive testing techniques (NDT) for evaluation. All of these techniques, however, are localized, labor intensive, and relatively expensive.
Advanced structural health monitoring (SHM) techniques provide accurate assessment of infrastructure condition and, through the promotion of proper maintenance, can reduce the cost of unnecessary structure replacement. Sensors, such as electromagnetic acoustic transducers, magnetic sensors, laser ultrasonics, infrared or thermal cameras, guided waves, field measurement probes, and strain gauges have been adopted to measure structural information, including static and dynamic displacement, strain and stress, acceleration, surface and interior damage, and corrosion. Due to the sheer size of most bridge structures, however, these SHM techniques may be cost prohibitive.
Remote sensing is a sensing technique that collects information about an object, area, or phenomenon from a distance without physically contacting it. Typically, remote sensing refers to imagery and image information taken by airborne and satellite systems, but ground-based solutions are available as well. For the past fifty years, several Commercial Remote Sensing (CRS) and Spatial Information (SI) technologies for wide-bandwidth spectral information sensing and imaging have been developed integrally with satellite, airborne, and ground-based surveillance platforms, such as IKONOS, Quickbird, OrbView-3, orthotropic and small-format aerial photography, and LiDAR scans. The obvious advantage of using remote sensing for SHM is that it does not require a time-consuming and labor-intensive visit to the site of the structure. CRS-SI applications to structural health monitoring have been extremely limited, however. Among the factors contributing to the limited use of such technologies are lack of guidelines for the use of CRS-SI technologies for bridge management, limited experience with the technology among bridge managers, and difficulties with integrating different kinds of inspection data into a manageable form.
Accordingly, there is a need for a bridge-inspection method based on CRS-SI technology that can have a wide national impact by being practical, scalable, cost-effective, and capable of integration into system-wide implementation.