A Minnesota Department of Transportation research study has developed a new method for estimating vertical displacements on bridges using accelerometers installed on the Interstate 35W St. Anthony Falls Bridge in Minneapolis. The dual-model approach shows potential for using these sensors to measure vertical displacement on steel, cable-stayed and other less-stiff bridges where traffic generates higher vibration frequencies. The method expands the industry’s knowledge of how to use smart sensors in new ways.

What Was the Need?

Since September 2008, the I-35W St. Anthony Falls Bridge has carried traffic over the Mississippi River in Minneapolis and funneled sensor data to researchers and MnDOT bridge engineers. This smart bridge features over 500 sensors that monitor strain, load distribution, temperature, bridge movement, and other forces and functions.

Sensors help designers and bridge managers learn more about how bridges shift and flex over time. Concrete expands and contracts, and bearings shift; sensor systems continuously gather data about these minute changes, offering an alternative to time-consuming inspection.

Researchers continue to identify potential uses for sensor data and new ways to use such information to analyze bridge properties and performance. In a 2017 study about monitoring bridge health, researchers learned to distinguish and associate specific vibration frequencies with structural damage, weather conditions and other factors. These frequencies were gathered by accelerometers, which measure structural vibrations triggered by traffic and environmental conditions.

Decks, piers and other structural elements displace vertically under loads and environmental conditions. Researchers and bridge managers wanted to know if accelerometers could be used to measure vertical displacements and help monitor bridge health.

What Was Our Goal?

MnDOT needed a procedure for measuring and monitoring vertical displacement on bridges under traffic and environmental forces. Investigators would use the sensor systems on the I-35W St. Anthony Falls Bridge to design and analyze this procedure.

“We need to learn more about sensors because we don’t have a lot of experience with them. This study gave us valuable information about accelerometers and the information they provide,” said Benjamin Jilk, Complex Analysis and Modeling Design Leader, MnDOT Bridge Office.

What Did We Do?

Indirect analysis and measurement of vertical displacements rely on estimations obtained through modeling. Investigators evaluated the most well-developed approach for measuring vibration frequencies like those tracked by accelerometers and refined the method. The team developed a dual-model approach: One model estimates loads and the other estimates displacements.

In a laboratory, investigators evaluated the impact of loading on displacement and vibration frequencies on a girder with contact sensors and accelerometers under moving and stationary loads. Researchers applied the dual-model analysis to laboratory displacement readings to compare the effectiveness of the model with contact sensor responses to loading.

Using laboratory data, investigators tuned the dual-model approach to accelerometer data available from the I-35W St. Anthony Falls Bridge. The research team then applied its identified tuning approach to the data from the bridge’s 26 accelerometers to determine the procedure’s suitability for estimating vertical displacement from vibration response on this bridge and its potential for other structures in the MnDOT bridge system.