Jeremiah Fasl

Use of Strain Data to Estimate Remaining Fatigue Life of a Fracture-Critical Bridge

A fracture-critical steel I-girder bridge was instrumented with strain gauges to estimate the remaining design fatigue life. The two girders on the bridge had extensive fatigue cracking. Continuous, dynamic strain data were collected for nearly 2 months to determine an effective stress range and cycle count according to Palmgren–Miners rule. A simplified rainflow counting algorithm was developed and used to calculate the amplitude of each fatigue cycle. The effective stress range and cycle count were combined with AASHTOs S (stress range)-N (number of cycles to failure) curves to estimate the remaining design fatigue life of certain bridge details. The data revealed that the estimated design fatigue life was exceeded in the east girder (right lane), whereas some life remained in the west girder (left lane). The distribution of observed cracks in the girders was closely correlated with the calculated fatigue life. A method is presented in this paper to index the effective stress range so that strain measurements can be compared over extended periods.

Long-term gage reliability for structural health monitoring of steel bridges

Real-time monitoring of fracture critical steel bridges can potentially enhance inspection practices by tracking the behavior of the bridge. Significant advances have occurred in recent years on the development of robust hardware for field monitoring applications. These systems can monitor, process, and store data from a variety of sensors (e.g. strain gages, crack propagation gages etc.) to track changes in the behavior of the bridge. Thus, for a long-term monitoring system to be successful, the reliability of gages that are to be monitored for several years is very important. This paper focuses on the results of a research study focused on developing a wireless monitoring system with a useful life of more than 10 years. An important aspect of the study is to identify strain gages and installation procedures that result in long lives as well as characterizing the effect of temperature fluctuations and other environmental factors on the sensor drift and noise. In long-term monitoring applications, slight sensor drift and noise can build up over time to produce misleading results. Thus, a wide variety of gages that can be used to monitor bridges have been tested for over a year through environmental tests. The environmental tests were developed to determine the durability of the gages and their protective coatings (e.g. zinc-based spray, wax and silicon, etc.) against humidity, sun exposure and other environmental effects that are expected in long-term bridge monitoring applications. Moreover, fatigue tests were performed to determine the fatigue category of the weldable gages and to reveal any debonding issues of the bondable gages. This paper focuses on the results of laboratory tests on gage durability that were conducted as part of a research project sponsored by the National Institute of Standards and Technology (NIST).

Development of a wireless monitoring system for fracture-critical bridges

This paper provides a summary of ongoing research sponsored by the National Institute of Standards and Technology (NIST) that seeks to improve inspection practices for steel bridges by providing the technology and methodology for real-time monitoring. In order to reduce the time and cost of installing a monitoring system, the research team elected to use wireless communications within the sensor network. The investigation considered both IEEE 802.11 and IEEE 802.15.4 communications protocols and identified the latter as more practical for bridge monitoring applications. Studies were conducted to investigate possible improvements in the network performance using high-gain antennas. Results from experiments conducted outside and on bridges with different antennas are presented in this paper. Although some benefits were observed using high-gain antennas, the inconsistent performance and higher cost relative to the current stock, omni-directional antennas does not justify their use.

Evaluation of Wireless Devices for Monitoring Fracture-Critical Steel Bridges

Highway bridges are vital links in the U.S. transportation network, providing the public with routes for daily commutes and businesses with the infrastructure needed to supply goods and services. Identifying possible safety problems in the approximately 600,000 bridges across the country generally is accomplished through labor-intensive visual inspections. The National Bridge Inspection Program requires that bridges be inspected at least once every 2 years. Pending legislation spurred by the collapse of the I-35W steel truss bridge in Minneapolis, Minnesota, would increase the inspections for fracture-critical bridges to once a year and likely strain department of transportation resources that are already stretched to the limit. This paper outlines ongoing research sponsored by the National Institute of Standards and Technology that seeks to improve inspection practices by providing the technology and methodology for real-time monitoring of steel bridges. The first generation of two wireless systems (one based on IEEE 802.11 and another on IEEE 802.15.4) from National Instruments has been evaluated in field conditions. This paper presents results of a study to characterize wireless communication in steel box girders and I-girders. In addition, enhancements to be made in the second generation are discussed.

Instrumentation of a Horizontally Curved Steel I-Girder Bridge during Construction

Horizontally curved, steel I-girder bridges can present unique challenges for engineers and contractors because the curved geometry can result in a complicated torsional response. The most complicated stages for predicting behavior of the girders usually occur during erection and construction when the loads and support conditions are the most unpredictable. Although laboratory experiments can provide valuable insight into the behavior, the high cost of the specimens often precludes meaningful experiments, whereas field monitoring of bridges during construction provides invaluable opportunities to understand the behavior and gather data for validating computational models. A horizontally curved, steel I-girder bridge was instrumented to monitor the bridge during erection and concrete deck placement. Stresses were monitored as girders were lifted into position, followed by measurements of vertical deflections, rotations, and stresses during the concrete deck placement. The stresses during the erection process were relatively low owing to the proper use of lifting and placing methods; however, high stresses can be induced after girders are placed when the cross frames are ratcheted into position. As expected, higher stresses, compared with the steel erection process, were recorded during the concrete deck placement. Nonetheless, the monitored bridge did not have stability problems because the bridge utilized a relatively stocky flange width-to-depth ratio. For bridges more susceptible to stability challenges, such as tightly curved bridges, highly skewed bridges, narrow bridges, bridges with odd span arrangement, or some combination of these attributes, it is recommended that the designer consider the implication of slender girders and explicitly design for the possibility of construction-related stability challenges. In addition, resulting from the limited availability of field measurements of horizontally curved girders throughout the construction process, the data represent a valuable resource researchers can use to validate computational models for conducting parametric investigations. This paper outlines the methods used during the field monitoring and summarizes the results from the field measurements.

Development of a wireless strain node and the software to monitor fracture-critical bridges

Real-time, monitoring systems can enhance the bridge inspection process by providing data for estimating the health of the bridge and potentially notifying bridge owners of problems between inspection visits. A low-power, wireless, strain data acquisition device has recently been developed to acquire dynamic strain data. Strain gages can be used to monitor the number and size of stress cycles in fatigue-sensitive members. From the cycle count, Palmgren-Miners rule can be used to determine an effective stress range. The remaining fatigue life can then be calculated and compared to existing conditions and the age of the bridge. Because damage is expected to escalate over time, more frequent inspections may be needed when a bridge approaches its fatigue life. The strain node can be programmed in LabVIEW WSN to detect critical events or perform a rainflow analysis. To aid in system interaction, a software interface will be designed to allow for automated processing and transmission of data to a cloud server, thereby allowing engineers and bridge owners to access the data from anywhere so as to make informed decisions when prioritizing inspections. This paper will present the development of the strain node and the software interface.

Evaluating vehicular-induced bridge vibrations for energy harvesting applications

Highway bridges are vital links in the transportation network in the United States. Identifying possible safety problems in the approximately 600,000 bridges across the country is generally accomplished through labor-intensive, visual inspections. Ongoing research sponsored by NIST seeks to improve inspection practices by providing real-time, continuous monitoring technology for steel bridges. A wireless sensor network with a service life of ten years that is powered by an integrated energy harvester is targeted. In order to achieve the target ten-year life for the monitoring system, novel approaches to energy harvesting for use in recharging batteries are investigated. Three main sources of energy are evaluated: (a) vibrational energy, (b) solar energy, and (c) wind energy. Assessing the energy produced from vehicular-induced vibrations and converted through electromagnetic induction is the focus of this paper. The goal of the study is to process acceleration data and analyze the vibrational response of steel bridges to moving truck loads. Through spectral analysis and harvester modeling, the feasibility of vibration-based energy harvesting for longterm monitoring can be assessed. The effects of bridge conditions, ambient temperature, truck traffic patterns, and harvester position on the power content of the vibrations are investigated. With sensor nodes continually recharged, the proposed real-time monitoring system will operate off the power grid, thus reducing life cycle costs and enhancing inspection practices for state DOTs. This paper will present the results of estimating the vibration energy of a steel bridge in Texas.