Bryan Althouse

Fiber Bragg grating interrogation and multiplexing with a 3/spl times/3 coupler and a scanning filter

We present a new technique for fiber Bragg grating (FBG) sensor interrogation and multiplexing. The technique combines a scanning bandpass filter used to multiplex by wavelength multiple gratings in a single fiber, and an unbalanced Mach-Zehnder fiber interferometer made with a 3/spl times/3 coupler to detect strain-induced wavelength shifts. A demonstration system interrogates four gratings in a single fiber at a sampling rate up to 20 kHz, with a noise floor measured at less than 10 n/spl epsiv///spl radic/(Hz) above 0.1 Hz.

Quasi-static strain monitoring during the "push" phase of a box-girder bridge using fiber Bragg grating sensors

Introduction: An interesting and increasingly popular method of building steel and concrete composite bridges involves the use of a large steel based structure called a box-girder. These massive structures, which have dimensions on the order of 5m x 5m x 150m with wall thickness sometimes exceeding 5 cm, are assembled by welding together many shorter (e.g. 5m x 5m x 5 m) box-girder pieces near a bridge building site. The massive and long box-girder structure thus created, is then ‘pushed’ from one end of the bridge pier to the other. A concrete deck is eventually built on top of the box-girder to complete the bridge structure. This bridge building approach apparently holds advantages in the sense that it requires far fewer on-site workers and also allows for rapid building of large scale bridges over difficult terrain. One of the more interesting (and potentially dangerous) phases during the construction process of a box-girder bridges is the so-called ‘push’ phase. During this phase, the long box-girder structure, which prior to the push rests on a pier closest to land, is literally pushed from one bridge pile-on to the next, requiring it to travel a distance of over 100 meters. The far pile-on typically is built in a ravine or a deep valley over which the bridge is being built. During the push, the steel box-girder structure is subjected to static strains at various locations, depending upon the position of the box-girder at any given time. For instance, the far end of the box girder would be subject to cantilever motion, which may be driven into oscillations due to winds in the ravine. Monitoring the strain at various locations in the box-girder during the push phase of the bridge construction has potential benefits in future infra-structure modeling efforts as well as in determining construction safety factors. In addition, utilizing enabling technologies (e.g. fiber Bragg grating strain sensors) for strain monitoring during an actual construction phase of a real world structure allows determination of the maturity of: (i) the instrumentation system, (ii) sensor packaging, and (iii) installation approaches [1]. This work reports the results of a 32 element fiber Bragg grating (FBG) array used to monitor strain at various locations in box-girder vaux viaduct being constructed near Lausanne, Switzerland. The entire push phase of the box-girder bridge, which took about a day, was monitored using several FBGs whose responses were interrogated using the scanning FabryPerot (FP) method [2]. Preliminary quasi-static strain sensing data shows fine as well as gross details of the push phase. The sensors are planned to operate during many phases of the boxgirder bridge construction including the push, the concrete deck pouring, and eventually traffic monitoring. We report here on the push phase only.

Development of fiber Bragg-grating-based soil pressure transducer for measuring pavement response

A soil pressure transducer by using fiber Bragg grating (FBG) sensors associated with a circular diaphragm is developed. The FBG based transducers can be used for pavement performance study and weigh-in-motion measurement. We consider three methods of bonding the FBG to the diaphragm: (1) radially, (2) radially, inside a glass capillary, and (3) circumferentially. The investigation of strain-gradient induced spectral broadening in FBG-based transducers is conducted since spectral broadening can have adverse effects on the sensor interrogations. We derive analytical closed form results for describing measurand-induced strain gradients in circular geometry transducers, which allow us to experimentally demonstrate novel FBG bonding approaches that eliminate spectral broadening. In addition, Bragg spectral broadening analysis using T-matrix calculation is also conducted to validate some of the experimental results. Two prototypes of soil pressure transducers are field tested at the Cold Region Research Engineering Laboratory (CRREL). The buried pressure transducers are impact-tested by use of a Falling-Weight- Deflectometer (FWD), and detected by NRL-developed FBG interrogation device. Lastly, we use the Boussinesq equation to verify the soil stress measured by the buried transducers.

Measurement of pavement response with buried fiber Bragg grating based soil pressure transducers

We report on results from a field test involving pavement embedded fiber Bragg grating pressure transducers. Data from the test is compared to theoretically predicted stress distribution in soil as obtained from the Boussinesq equation.