35W bridge will be closely monitoredby Tim Post, Minnesota Public Radio
When it opens Thursday morning the new 35W bridge will be one of most monitored bridges in the country. That's because the span will house hundreds of sensors, some placed inside the bridge's concrete during construction. Information from those sensors will be sent to a team of experts at the University of Minnesota, who will interpret the data for the Minnesota Department of Transportation.
Minneapolis, Minn. — It won't be long before University of Minnesota professor Catherine French will receive real time data from the new 35W bridge on her office computer in the school's civil engineering building.
Right now the data can only be seen on computers at the site of the bridge. But when all the final connections are in place French will be one of about five U of M employees able to monitor the bridge's statistics in real time at their desks.
The data will come from 300 sensors placed all over the bridge, and even inside the bridge.
Most of the sensors are what's known as strain gauges.
"Strain gauges measure a change in length over time. For example when the bridge deflects and curves, you would see an elongation of the bottom fibers shortening of the top fibers and that gauge would collect that reading," French says.
Other gauges, called accelerometers, will measure how much and how fast the bridge moves when vehicles drive over it. Some sensors are set up to detect corrosion causing moisture that might seep into the bridge's infrastructure. And special sensors will track temperatures inside the bridge. That's first of its kind data made possible because the sensors were put in place during construction.
The amount of data being collected from the new 35W bridge rivals the information coming from one of America's best-known spans, the Golden Gate Bridge.
"There are dozens of sensors out on the Golden Gate Bridge in various locations," according to Mary Currie, spokeswoman for the Golden Gate Bridge District.
The Golden Gate Bridge has many of the same types of sensors as the new 35W bridge, but the sensors in San Francisco are being used for different reasons, Currie says.
"We're basically looking for any kind of movement that's detected through an earthquake and it's motion...and so all the instrumentation helps for us to understand how the bridge will respond and does respond during an earthquake."
The Golden Gate Bridge is 1.7 miles long and contains fewer than half the sensors on the Minneapolis bridge, which is far shorter at 504 feet. The new bridge is a smart one, says University of Minnesota professor Catherine French.
"With current technology this bridge is one of the best instrumented bridges probably in the country in terms of numbers of sensors, in terms of the forethought put in during construction of the bridge of what sensors should be placed as well."
The first job of the sensors on the 35W bridge will be to monitor the structure of the bridge, and catch any problems early on. But French says the data she and her colleagues collect in coming years will provide valuable information to help engineers build better bridges.
- All Things Considered, 09/17/2008, 5:21 p.m.