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
a few miles away. The information is more reliable than a worker in the field who performs visual inspections, Janoyan said.
The $575,000 project is funded by the Federal Highway Administration Innovative Bridge Research and Construction Program and the New York State Energy Research and Development Authority (NYSERDA). Janoyan also has begun working with TransTech Systems Inc. to create a commercial version of his bridge monitoring system.
Besides monitoring bridges' health, it's hoped that the sensor system will help the state's overall transportation infrastructure since it will give real-time updates of bridge infrastructure, longevity, maintenance and repairs, said Paul D. Tonko, president and CEO of NYSERDA.
"If we can streamline and have a better analytic approach to bridge life, maintenance, repair and construction by having this monitoring system, we can avoid the energy and costly expense of construction congestion," Tonko said.
The Department of Civil and Materials Engineering at the University of Illinois at Chicago is developing a similar bridge monitoring approach, using fiber-optic wires instead of wireless sensors. A research program at the university called - Smart Sensors and NDT Laboratory - develops fiber-optic sensors for monitoring structures during construction and throughout their service lives.
The Illinois Department of Transportation awarded a $55,000 contract in September 2007 to the laboratory - to devise a system that can detect bridge scour - when sediment is washed from the bottom of the river and weakens a bridge's support. The project involves embedding a fiber-optic sensor in a rod and driving the rod into a river base near the piers.
Fiber-optic sensors assess a bridge's overall health by measuring microfractures and vibration frequency. Fiber optics promise many innovations for bridge monitoring since they are smaller and more flexible than electrical wires, are immune to interference from wireless devices, and aren't prone to fires or explosions, said Farhad Ansari, professor and head of the Civil Engineering and Materials Department at the University of Illinois at Chicago.
The sensors have already been used on a pedestrian bridge in Turin, Italy, for the 2006 Winter Olympics and in some bridges in China, but haven't yet been implemented in the United States. The cost of installing the fiber-optic sensors and monitoring a 400-foot bridge span is about $250,000 the first year. Subsequent costs are estimated to be 10 percent of the original cost, a relatively cheap way of monitoring bridges, Ansari said.
"I think any bridge, like the bridge that collapsed in Minneapolis, that is a relatively large, older bridge in such a strategic situation should have monitoring devices on it - whether they are electrical sensors or my [fiber-optic] sensors - where they could give you real-time data in addition to what they do by visual inspections," Ansari said.
Many SHM projects have sprung up nationwide, spurred by the Minneapolis incident and reports of an estimated 73,000 structurally deficient bridges nationwide, according to the U.S. DOT.
The Los Alamos Engineering Institute is working on a technology using radio frequency identification sensing for "rapid, economical and reliable assessment" of damage in large concrete and steel structures such as bridges. One project includes electronic sensors powered by microwaves that gather and send data to an airborne computer in a radio-controlled helicopter.
The University of Michigan, meanwhile, has developed a "sensing paint" that can be sprayed on a surface, making that surface a sensor patch that provides 2-D images of microscopic faults. The result expected is a multifaceted solution for bridge monitoring that's cheaper and more effective than visual or cable-based bridge monitors.
"Anytime you do this sort of up-close analysis, you have state-of-the-art information on how that bridge structure is holding up," Tonko said. "We've heightened efforts to prevent bridge collapse, and have better maintenance, repair and upkeep functions. Now we're adding another layer of innovation with a state-of-the-art way to measure our bridges."
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