March 13, 2008 By Chandler Harris
When the Minneapolis Interstate 35W bridge collapsed on Aug. 1, 2007, sending 88 vehicles and hundreds of people into the Mississippi River - 13 were killed and 100 injured - nobody, including state bridge inspectors, foresaw such a catastrophic event.
The collapse was a sad reminder of the aging U.S. infrastructure, which needs a $1.6 trillion overhaul over the next five years, according to the American Society of Civil Engineers. Its report graded the U.S. infrastructure as a D in 2005, down from a D+ in 2001. Bridges earned a cumulative C.
U.S. bridges are a pressing concern because stress loads have increased substantially due to a spike in traffic congestion. Since the Minneapolis I-35W bridge was built in 1967, the number of vehicles using the bridge had tripled, according to state documents. Also, truck weight limits have increased nationally, according to the U.S. Department of Transportation (DOT).
The I-35W bridge had been diligently inspected since 1993, and it always passed. Although state officials knew the bridge needed repairs, they had no idea the bridge was in danger of collapse. In January, the National Transportation Safety Board (NTSB) announced the bridge failed due to a design flaw in the gusset plates that connected steel beams; the gusset plates were only half the thickness they should have been. The NTSB investigation found no evidence that cracking or corrosion played a role in the collapse. However, the tragedy had already drawn much-needed attention to the problem of corrosion in aging bridges.
The Minnesota Department of Transportation (Mn/DOT) knew of some risks beforehand: Prior to the accident, Mn/DOT was so concerned about the bridge's structural deficiencies that officials considered replacing it. Other proposed fixes included bolting steel plates to the bridge to prevent stressed areas from cracking. Mn/DOT officials, though, ultimately decided that only more frequent visual inspections were required.
Today, U.S. bridges are almost entirely visually inspected, and if corrosion or cracks are spotted, tests using eddy currents, ultrasound or penetrating dyes are often used. However, detecting weak spots visually is an imperfect and faulty science.
Bridging the Monitoring Gap
A new engineering technology, structural health monitoring (SHM), is being developed to address critical infrastructure needs. SHM uses sensors embedded in structures to alert crews of defects in critical structures before catastrophic failures occur. The process uses wireless technology to monitor a structure's physical properties such as loads, stresses, strains and cracks, as well as changes in chemical and electrical properties related to deterioration - corrosion, fatigue, chlorides and humidity. Experts see wireless sensors as a cheaper, more reliable way to monitor bridges and other critical infrastructure.
New York is one of many states re-evaluating bridge inspection methods after the Minneapolis bridge collapse by turning to SHM technologies. The state department of transportation's pilot program uses wireless sensors placed on bridges to transmit data on stress and vibration, and should warn if a bridge is weak or needs repairs. The system, designed by Clarkson University associate professor Kerop Janoyan, is expected to help engineers monitor the state's 17,000 bridges.
"Providing more information is the first step to a feedback system," Janoyan said. "Without information, you can't have any feedback when ultimately you're trying to control potential damage."
A New York bridge between Canton and Potsdam is serving as a test site - 40 wireless channel sensors affixed to the bridge log in real-time data to a base station. Each sensor is about the size of a few decks of playing cards, and cost about $200. The battery-powered sensors are connected to a computer that aggregates sensor data and determines whether to alert inspectors.
Information collected by the sensors is stored in a central unit attached to the bridge that transmits the data either to a laptop nearby or an office