Neil A. Hoult

Smart bridges, smart tunnels: Transforming wireless sensor networks from research prototypes into robust engineering infrastructure

We instrumented large civil engineering infrastructure items, such as bridges and tunnels, with sensors that monitor their operational performance and deterioration. In so doing we discovered that commercial offerings of wireless sensor networks (WSNs) are still geared towards research prototypes and are currently not yet mature for deployment in practical scenarios. We distill the experience gained during this 3-year interdisciplinary project into specific advice for researchers and developers. We discuss problems and solutions in a variety of areas including sensor hardware, radio propagation, node deployment, system security and data visualization. We also point out the problems that are still open and that the community needs to address to enable widespread adoption of WSNs outside the research lab.

Long-Term Wireless Structural Health Monitoring of the Ferriby Road Bridge

As part of an effective bridge management system, sensor networks can provide data to support both inspection and assessment. Wireless sensor networks (WSNs) have the potential to offer significant advantages over traditional wired monitoring systems in terms of sensor, cabling, and installation costs as well as expandability. However, there are drawbacks with WSNs relating to power, data bandwidth, and robustness. To evaluate the potential of WSNs for use in bridge management, a network of seven sensor nodes was installed on the Ferriby Road Bridge, a three-span reinforced concrete bridge. Three displacement transducer nodes were placed across cracks on the soffit of the bridge to measure the change in crack width. Three inclinometer sensor nodes were mounted on two of the elastomeric bearing pads to measure the change in inclination of the bearing pads while a final node monitored temperature in the box that contained the gateway. The installation of the WSN is discussed and data from this network is analyzed. Finally, the use of sensor networks to support inspection and assessment is discussed.

Distributed Strain Behavior of a Reinforced Concrete Bridge: Case Study

To effectively assess and manage aging infrastructure, sensing technologies that produce accurate and useful quantitative data are required. Distributed fiber optic strain measurement systems are one potential technology that could fulfill this requirement. This case study investigates the performance of a distributed fiber optic strain measurement technology with high accuracy and spatial resolution during a load test on a RC bridge. The measurements from the fiber optic system are compared with conventional strain gauges and linear transducers, and good agreement between the sensors was found. The fiber optic measurements are also used to examine the bridge support conditions as well as the load distribution between beams. The results show that assumptions made about the support conditions during design do not match the actual bridge behavior, although the load distribution between beams is as expected.

Optimum Accuracy of Two-Dimensional Strain Measurements Using Digital Image Correlation

AbstractFoil and vibrating wire strain gauges have an optimum strain measurement accuracy of one microstrain. However, they can only provide discrete strain readings over a single fixed-gauge length. Digital image correlation (DIC) offers an alternative to conventional strain gauges because a two-dimensional (2D) surface strain field can be obtained from a single sensor (camera). However, the benefits of 2D strain measurements are only worthwhile if a similar level of measurement accuracy to conventional strain gauges can be achieved. This paper presents the results of an investigation into the optimum strain measurement accuracy that can be achieved by using the 2D technique on artificial images (which eliminate errors associated with cameras and lighting). The principle of the 2D DIC technique and its historical development will be introduced. Then, three potential techniques for taking strain measurements will be presented and compared: single readings, averaged linear readings, and an approach on the ...

Damage/Deterioration Detection for Steel Structures Using Distributed Fiber Optic Strain Sensors

Distributed fiber optic sensors have the potential to be used to detect three critical deterioration mechanisms in steel structures: (1) fatigue cracking, (2) localized damage or deterioration, and (3) distributed damage or deterioration, such as corrosion. This study investigated the strain and spatial resolution of distributed fiber optic sensors and explored the potential benefits and challenges of using distributed fiber optic strain sensors for damage/deterioration detection. The experimental program consisted of a series of axial tension tests performed on steel plate specimens with three types of simulated damage/deterioration: cracking, local cross section reduction, and distributed cross section reduction. The results indicate that similar accuracy to strain gauges can be achieved and distributed fiber optic strain sensors can provide much more detailed information about specimen behavior. The results of a finite-element analysis for each specimen were compared with the experimental measurements. There was good correlation between the two if the boundary conditions were modeled properly. However, care must be taken when selecting the sensing fiber to be used and when interpreting the results.

Curvature Monitoring of Beams Using Digital Image Correlation

A method for measuring longitudinal strains with the height at a section, and thus the curvature, using a technique based on digital image correlation (DIC), is presented. The background to this technique is introduced as well as previous work in this area. The accuracy of DIC under ideal conditions is established using artificially generated images that represent beams with various curvatures. The practical accuracy of DIC is established by comparing the strains measured using DIC to those predicted by elastic theory and measured using strain gauges for a steel beam. The correlation between these results is found to be excellent. DIC is then used to measure curvatures in RC beams and these results are compared with analytically predicted results with good agreement. The choice of an appropriate gauge length for RC is discussed and is shown to be one of the significant advantages of using DIC as opposed to strain gauges in both laboratory testing and field monitoring of bridge structures.

Effect of Deterioration on the Performance of Corrugated Steel Culverts

An experimental investigation was undertaken to compare the performance of a largely intact steel culvert with well-compacted backfill with a corroded culvert in loose backfill. The behavior of both structures is compared during backfilling and under imposed surface loads equivalent to the standard American and Canadian design trucks at two burial depths (0.6 and 0.9 m). During backfilling, the deteriorated culvert deflections were much larger than the intact culvert because of differences in both culvert and backfill stiffnesses. Under surface loading, the deteriorated culvert experienced higher deflections and bending moments, whereas the intact culvert experienced higher thrust forces. It was found that, under relatively shallow cover, the load from one wheel pad of the axle influenced the behavior of the culvert under the other wheel pad. The deteriorated culvert was then tested to failure with localized bending in the crown and buckling of the remaining steel in the corroded region serving as indicators of failure.

Distributed Sensing of Circumferential Strain Using Fiber Optics during Full-Scale Buried Pipe Experiments

AbstractAs buried infrastructure in North America and around the world reaches the end of its service life, engineers and infrastructure managers will require an improved understanding of the performance of both deteriorated pipes and repair techniques. To develop this improved understanding, sensing technologies that enable the full pipe behavior to be measured, rather than a small number of localized discrete measurements, are required. A possible solution to this problem is to use distributed fiber optic strain sensors. To this end, a series of buried pipe tests were undertaken on steel, concrete, and high-density polyethylene (HDPE) pipes instrumented with distributed fiber optic strain sensors. The distributed measurements were in agreement with conventional strain gauges, but enabled the full strain distribution around the circumference of the pipe to be measured. This allowed localized behavior that would have been missed with conventional strain gauges to be detected and quantified. In addition, t...

Modeling of an Unbonded CFRP Strap Shear Retrofitting System for Reinforced Concrete Beams

A retrofitting technique has been developed that uses carbon fiber-reinforced polymer (CFRP) straps to increase the shear capacity of reinforced concrete beams. The vertical straps are not bonded to the beam but are instead anchored against the beam, which makes this technique potentially more effective than bonded FRP retrofitting techniques. However, it also means that models for bonded FRPs are not appropriate for use with the straps. Instead, a model based on a shear friction approach has been developed where the strain in the straps is calculated based on a term that accounts for the effects of prestress and additional strain in the strap due to shear crack opening. The model can either consider the shear reinforcement to be smeared along the length of the beam or discrete elements. The “smeared” model was checked against an experimental database consisting of rectangular, T-, and deep beams, both in terms of predicted capacity and predicted strain in the straps. Overall the smeared model predicted t...

Measured Load Capacity of Buried Reinforced Concrete Pipes

Reinforced concrete pipes in North America are often designed using the Indirect Design Method, which uses bedding factors to relate in-place pipe performance to the performance obtained from three-edge bearing tests. The key performance indicator is the load at which the 0.254 mm (0.01 in.) critical crack is developed. The current bedding factors were developed using numerical modeling, and a limited amount of experimental verification has been performed to determine the accuracy of the approach. As such, a series of shallow burial tests with surface loading, a simulated deep burial test, and three-edge bearing tests were undertaken on 0.6 and 1.2 m (24 and 48 in.) diameter pipes to evaluate the current bedding factors. Both the shallow and deep burial tests indicated that the critical crack does not develop until after the specified service load has been surpassed, suggesting that current pipe designs are overly conservative.