A passive wireless displacement sensor suitable for use in civil structural health monitoring applications is presented.
The sensor is based on a resonant electromagnetic cavity with one end of the cavity formed by a flexible membrane. A
rod attached to the membrane causes the dimensions of the cavity to change when the rod is displaced. The change in
dimensions causes a shift the resonant frequency of the cavity that is directly related to the displacement of the rod. In
the example shown the shift is 7 MHz per mm for a cavity with a resonant frequency of 2450 MHz. Using a pulse echo
interrogation technique resonant shifts of 100 kHz are resolvable. In the laboratory, displacements of 0.014 mm were
measurable, with the distance between the interrogator and the sensor of up to 4.5 m.
Deflections of structures, such as bridge girders, are often the most difficult to monitor. Strain measurement is relatively
simple with the use of electronic strain gauges, fiber optic sensors, or other strain measuring devices. This paper
investigates two different methods for predicting or monitoring the deflection of a simply-supported full-scale bridge
girder subjected to a partially distributed uniform load using strain measurements. A full-scale pre-stressed concrete
bridge girder was instrumented and tested under a static monotonic load in the linear elastic range. This paper highlights
the experimentally measured deflections along the length of one half of the girder and compares them to theoretically
predicted deflections and deflections predicted using numerical integration along with harmonic analysis of curvatures
determined from theoretical and observed experimental strains. Experimental test results indicate that estimating
deflections from observed strains is feasible within the linear-elastic range of such girders. The methods outlined for
predicting deflections of full-scale pre-stressed concrete bridge girders from observed strains are a valuable tool for
structural engineers and for the periodic and continuous monitoring of civil structures such as bridges.
As the design and construction of civil structures continue to evolve, it is becoming imperative that these structures be monitored for their health. In order to meet this need, the discipline of Civionics has emerged. Civionics is a new term coined from Civil-Electronics, which is derived from the application of electronics to civil structures. It is similar to the term Avionics, which is used in the aerospace industry. If structural health monitoring is to become part of civil structural engineering, it should include Civionics. It involves the application of electronics to civil structures and aims to assist engineers in realizing the full benefits of structural health monitoring (SHM). In past SHM field applications, the main reason for the failure of a sensor was not the installation of the sensor itself but the egress of the sensor cables. Often, the cables were not handled and protected correctly. For SHM to be successful, specifications must be written on the entire process, beginning with system design and concluding with data collection, interpretation, and management. Civionics specifications include the technical requirements for a SHM system which encompasses fibre optic sensors, cables, conduits, junction boxes and the control room. A specification for data collection and storage is currently being developed as well. In the spring of 2004 research engineers at the University of Manitoba constructed a full-scale second generation steel free bridge deck. The bridge deck is the first of its kind to fully incorporate a complete civionics structural health monitoring system to monitor the deck's behaviour during destructive testing. Throughout the construction of the bridge deck, the entire installation of the civionics system was carried out by research engineers simulating an actual implementation of such a system in a large scale construction environment. One major concern that consulting engineers have raised is the impact that a civionics system that uses conduit, junction boxes, and other electrical ancillary protection, will have when embedded and installed externally on full-scale infrastructure. The full-scale destructive testing of a second generation steel-free bridge deck using a civionics system designed and implemented following guidelines in a civioncs specification manual at the University of Manitoba will provide engineers with the information necessary to address the constructability and structural integrity issues. Civioncs combined with structural health monitoring will provide engineers with feedback necessary to aid in optimizing design techniques and understanding our infrastructures performance, behaviour and state of condition.
The arching action in concrete deck slabs for girder bridges is utilized fully in steel-free deck slabs. These concrete slabs, requiring no tensile reinforcement, are confined longitudinally by making them composite with the girders, and transversely by external steel straps connecting the top flanges of external girders. Between 1995 and 1999, five steel-free deck slabs without any tensile reinforcement were cast on Canadian bridges. All these slabs developed fairly wide full-depth cracks roughly midway between the girders. While extensive fatigue testing done in the past three years has confirmed that the presence of even wide cracks does not pose any danger to the safety of the structures, wide cracks are generally not acceptable to bridge engineers. The developers of the steel-free deck slabs have now conceded that these slabs should be reinforced with a crack-control mesh of nominal glass fibre reinforced polymer (GFRP) bars. Steel-free deck slabs with crack-control meshes are being referred as the second generation slabs. With the help of testing on full-scale models, it has been found that deck slabs with GFRP bars have the best fatigue resistance and those with steel bars the worst.