Representative Technologies Developed at the Smart Sensors & NDT Laboratory
1. Distributed Detection of Damage in Cables of Cable-Stayed Bridges
The objective for this work was to develop a cost effective method for detecting damage in stay cables of the cable-stayed bridges. The method is cost effective because the sensor is installed along the bridge deck and eliminates the cost for attachment of sensors to individual cables,. It will drastically reduce the wiring costs, sensor installation time, and bridge down times. The technology developed is capable of detecting the location and percent loss in cable forces due to corrosion and fatigue.
The method uses the distributed measurement of strains along the bridge deck to detect the cables that have totally or partially lost their tensile force carrying capabilities. The fundamental principle employed in formulating the method is the interrelationship between the individual cable forces and the bending moment along the bridge span. The proposed method was evaluated through an experimental program that involved fabrication and testing of a reduced scale model of a single plane cable-stayed bridge. Distributed strain along the span length was monitored by a Brillouin Optical Time Domain Analysis fiber optic sensor system. Strain gauges, Fiber Bragg Grating sensors, and a finite element model of the bridge were employed for evaluating the efficiency of the method. Several different damage cases were considered in the experiments, including single and multiple cable tension loss scenarios. Experimental results revealed that it was possible to detect the cables that had experienced tension losses of 30% or more.
2. Remote Monitoring of Double Masonry Vault Cracks of the Brooklyn Bridge
Brick masonry vaults for the approach structures of the iconic Brooklyn Bridge in New York City were instrumented for remote monitoring of their health. The objective of the project was to monitor and quantify safety of the structure due to appearance of cracks at the crowns of the double span vaults. A fiber optic sensor system was installed in the Brooklyn Bridge to monitor the crack opening displacements, wall rotations, temperature fluctuations, and vibrations. To quantify safety, a scaled model of the masonry vault was also tested in the laboratory in order to determine the failure crack opening displacement limits of the vaulted structure. The structural health monitoring activities were continued remotely from our laboratory in Chicago for a period of twelve months. Examination of the temperature and crack opening displacements, as well as tilting of adjoining support vaults indicated that the cause of cracks were due to seasonal movements of the structure. With this information, there was no reason for costly replacement of the vaults. The cracks were repaired accordingly in order to accommodate seasonal movements due to temperature effects.
3. Distributed Detection of Cracks in a Five Span Box Girder Bridge
A distributed Brillouin Scattering based sensor system (BOTDA) was employed for detection of the location of microcracks in a five span post tensioned concrete box girder bridge in Northern Illinois. The bridge has a total length of 1090 feet. A distributed optical fiber sensor was installed within the bridge along the entire 1090 feet length of the five spans. Calibrated trucks were employed on each one of the five spans of the bridge and the distributed strains acquired were able to pinpoint the location of the cracks.