Post Seismic Structural Health Monitoring of Bridges
Seyed. A. Bassam, PhD Candidate, Advisor: Dr. Farhad Ansari
The focus of this study is on a quarter-scale test model of a four-span bridge frame supported on three shake tables. The experiments were performed at the University of Nevada, Reno shaketable test facility (Fig. 1). The bridge was subjected to the Northridge ground motion. The study consists of preliminary analytical studies of the system, construction of the model, instrumentation, data collection, and data processing. The design and installation of this system was prompted by need to design an automatic structural health monitoring system in a vast number of highway bridges and the need to present a quantitative assessment of structural damage.
The bridge was instrumented with FBG sensors specially designed to be mounted on the concrete surface (Fig. 2). Information regarding the change of structural characteristics was then obtained through visual observations, moment curvature analysis, spectral and also modal analysis from the acquired data. Depending on the location and severity of damage, modal parameters, including natural frequencies, damping ratios and mode shapes change in the structure.
The results provide a tool to predict the time and location of the formation of plastic hinge in the column.
Following general conclusions were drawn from the results:
1- The fiber optic sensors used in this study could survive even following the full scale seismic loads experienced by the bridge. Considering their high resolutions and high resistance in harsh environmental conditions, using these types of sensors is recommended for the health monitoring applications where high reliability of the measuring sensor is required.
2- Moment curvature diagrams obtained from the surface mounted FBG sensors could precisely detect the cracking of concrete, yielding of the steel and the formation of the plastic hinge.
3- Fiber optic sensors could obtain natural frequency of the column with great precision for use as a tool to measure the extent of the damage in the structure.
4- Strain Modal analysis provides a powerful tool to automatically determine the damage localization and its severity. Also, fewer sensors are required with respect to more traditional modal analysis methods.
Current Bridge Monitoring Research at the UIC
Iman Talebinejad and Chad Fisher, PhD Candidates, Advisor: Dr. Farhad Ansari
Sensors for monitoring bridges have received a great deal of media coverage following the recent bridge collapse in Minnesota. Such sensors are being researched and designed to monitor the performance of bridges and detect damaged locations. Sensors are a tremendous supplement to visual bridge inspections as they can detect damage in areas difficult to access by inspectors and can continuously monitor a bridge between periodic visual inspections. The Smart Sensors and NDT Laboratory, headed by Dr. Farhad Ansari, has developed fiber optic sensors that have been used to monitor the “health” of bridges and is continuing to research practical applications for their use. One such recent project was the monitoring of a cable supported pedestrian bridge.
A long span pedestrian bridge constructed to link the athlete village with the rest of the city was constructed in Torino, Italy for the 2006 Olympic Winter Games. The bridge design was quite complex and included a number of different structural elements and materials. For example, parts of bridge deck were supported by cables that were attached to an elliptical arch pylon that was set on a diagonal with cables at various levels. The bridge construction consisted of a steel framed superstructure, concrete deck, high tension steel cables and an arched pylon.
As a result of the complex bridge geometry along with performance and construction issues following its erection, access to the bridge was limited during the Olympic Games. Following the Games, it was decided to monitor the performance of the bridge cables and monitor the health of the bridge. The bridge cables were outfitted with long gauge fiber optic deformation sensors. The fiber optic sensors possess the capability for making dynamic real-time measurements. Aspects of the structural health monitoring involving dynamic modeling of the entire structure, the design and placement of the sensors and the analysis schemes for assessment of damage were researched. The bridge has since been re-opened for full use.
Mr. Talebinejad and Mr. Fischer are continuing their PhD research on upcoming bridge monitoring projects which include building a small scale bridge for monitoring and fatigue crack monitoring of a bridge in New York. They are working under the direction of Dr. Farhad Ansari. Mr. Talebinejad graduated from University of Tehran, Iran with his masters and bachelors degrees. Mr. Fischer attended the University of Illinois at Urbana-Champaign for his undergraduate and masters degrees and is also currently employed as a structural engineer with Engineering Systems Inc. in Aurora, Illinois.