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 the behavior of the bridge. The research outlined in this dissertation is part of a large study focused on the development of a wireless system for use in long-term monitoring of bridges. The wireless monitoring system had a target maintenance-free life of ten years, and independent from the power grid. Thus, the feasibility to harvest energy for the monitoring system is an important step in the development of the system. In addition, the reliability of the sensors in the bridge is very important upon the success of the system. The focus of this dissertation is on two primary aspects of the wireless monitoring system. First, the feasibility to harvest energy from vehicular-induced vibrations is evaluated through analytical models of highway bridges under truck loads. Acceleration results from simple line-element models and detailed finite element models of five steel bridges in Texas and Oregon are compared with actual field data from the same bridges. Second, the dissertation also highlights studies on the identification of strain gages and installation procedures that result in long lives. In addition, the effect of temperature fluctuations and other environmental factors on the sensor drift and noise is also considered. In long-term monitoring applications, slight sensor drift and noise can build up over time to produce misleading results. This dissertation presents the results of transient dynamic analyses of bridges under moving truck loading and laboratory tests on gage durability that were conducted as part of a research project sponsored by the National Institute of Standards and Technology (NIST). / text

Cases of brittle fractures in major bridges prompted AASHTO to publish its first fracture control plan in 1978. It focused on material and fabrication standards, and required periodic 24-month hands-on inspection of bridges with fracture critical members. The practical result of this plan was to significantly increase the life cycle cost of these bridges, rendering them uneconomical. Apart from the Point Pleasant Bridge that failed in 1967, no other bridge has collapsed in the USA following a fracture, even though large fractures have been observed in many other bridges. All these bridges showed some degree of redundancy and therefore could be reclassified as non-fracture critical if detailed analyses are carried out. The goal of this study is to provide guidance on redundancy evaluation of fracture critical bridges, specifically three girder bridges and twin box-girder bridges. The effect of various loading, analysis and geometric parameters on the post fracture response and the remaining load carrying capacity of the damaged bridge is evaluated through nonlinear finite element analysis of two well-documented structures: the Hoan Bridge and the twin box-girder bridge. Parameters such as damping definition, modelling of composite action, modelling of secondary elements, boundary conditions, and rate dependent material properties are found to be crucial in capturing the bridge response. A two-step methodology for system redundancy analysis of fracture critical bridges is proposed, leading to a reclassification of these elements as non-fracture critical for in-service inspection. The first step evaluates bridge capacity to withstand collapse following fracture based on whether the residual deformation is perceivable to people on or off the bridge. If the bridge satisfies the first step requirements, then the reserve load carrying capacity of the damaged bridge is evaluated in the second step. The Hoan Bridge failed to satisfy the proposed requirements in the first step and therefore its girders could not be reclassified as non-fracture critical. The twin box-girder bridge successfully resisted the collapse in two out three loading scenarios and displayed reserve load carrying capacity following full depth fracture in the exterior girder, and therefore can be reclassified as non-fracture critical for in-service inspection. / Ph. D.

In current bridge practice, all tension members in a truss bridge are identified as fracture critical members which implies that a collapse is expected to occur once a member of this type fails. However, there are several examples which show that bridges have remained standing and shown little distress even after a fracture critical member was completely damaged. Due to the high inspection cost for fracture critical members, it would be beneficial to remove fracture critical designation from some tension members. This could be achieved via considering system redundancy. Since there is no clear guidance in existing codified provisions for assessing system redundancy, this research is undertaken to develop simplified analysis techniques to evaluate system redundancy in truss bridges. The proposed system redundancy analysis in this research starts with the identification of the most critical main truss members whose failure may significantly affect the system redundancy. The system redundancy is then measured by the remaining load capacity of a damaged bridge after losing one of the critical members. The bridge load capacity is checked using 3D models with nonlinear features that can capture the progression of yielding and buckling in a bridge system. The modeling techniques are validated through the case studies of the I-35W Bridge and one test span of the Milton-Madison Bridge. Reasonable correlations are demonstrated between the models and the measured data for these two bridges both in an undamaged and in a damaged state. The feasibility of the proposed methodology for system redundancy evaluation is examined by applying the methodology blindly to two other simple truss bridges. The application shows that the proposed methodology can efficiently measure the system redundancy. To improve the system redundancy, this research also proposes sample retrofit strategies for the four example bridges. / Ph. D.

Based on the definition given in the AASHTO LRFD Bridge Design Specifications, twin steel box-girder bridges are classified as bridges with fracture critical members (FCMs), in which a failure of a tension member is expected to lead to a collapse of the bridge. However, a number of such bridges with either a partial or full-depth crack in one girder have been reported and are still providing service without collapsing. The main objective of this research project is to understand the behavior of twin steel box-girder bridges and to develop methods for evaluating their redundancy level in the event of the fracture of one tension member. The research project included an experimental investigation on a small-scale steel twin box-girder bridge, field testing of a full-scale twin box-girder, analysis of existing research and design data, and an extensive amount of numerical analyses carried out on calibrated 3-D nonlinear finite element models. The results from this study provide in-depth understanding of twin steel box-girder bridge behavior before and after a fracture in the tension member occurs. In addition to the experimentally verified finite element method, the report also proposes simplified methods for evaluating the load-carrying capacity of twin steel box-girder bridges under vii concentrated loads and provides a list of important factors that could control the reserve capacity of the damaged bridge. The main conclusion of this research is that the redundancy exists in twin steel boxgirder bridges in an event that a fracture of a tension member(s) takes place. This research project also provides a comprehensive roadmap for assessing the redundancy of twin steel box-girder bridges in which the elements of the roadmap are identified, and solutions for several of the steps are provided. The development of solutions for remaining steps of the roadmap is proposed for a future research.

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Bridge redundancy can be described as the capacity that a bridge has to continue carrying loads after suffering the failure of one or more main structural components without undergoing significant deformations. In the current AASHTO LRFD Bridge Design Specification, two-girder bridges are classified as fracture critical, which implies that these bridges are not inherently redundant. Therefore, two-girder bridges require more frequent and detailed inspections than other types of bridges, resulting in greater costs for their operation. Despite the fracture-critical classification of two-girder bridges, several historical events involving the failure of main load-carrying members in two-girder bridges constructed of steel plate girders have demonstrated their ability to have significant reserve load carrying capacity. Relative to the steel plate girder bridges, steel box-girder bridges have higher torsional stiffness and more structural elements that might contribute to load redistribution in the event of a fracture of one or more bridge main members. These observations initiated questions on the inherent redundancy that twin box-girder bridges might possess. Given the high costs associated with the maintenance and the inspection of these bridges, there is interest in accurately characterizing the redundancy of bridge systems. In this study, twin steel box-girder bridges, which have become popular in recent years due to their aesthetics and high torsional resistance, were investigated to characterize and to define redundancy sources that could exist in this type of bridge. For this purpose, detailed finite element bridge models were developed with various modeling techniques to capture critical aspects of response of bridges suffering severe levels of damage. The finite element models included inelastic material behavior and nonlinear geometry, and they also accounted for the complex interaction of the shear studs with the concrete deck under progressing levels of damage. In conjunction with the computational analysis approach, three full-scale bridge fracture tests were carried out during this research project, and data collected from these tests were utilized to validate the results obtained from the finite element models. / text

Energy harvesting is a promising and evolving field of research capable of supplying power to systems in a broad range of applications. In particular, the ability to gather energy directly from the environment without human intervention makes energy harvesting an excellent option for powering autonomous sensors in remote or hazardous locations. This dissertation examines the possibility of using energy harvesting in new and innovative ways to power wireless sensor nodes placed in the substructures of highway bridges for structural health monitoring. Estimates for power requirements are established, using a wireless sensor node from National Instruments as an example system. Available power in a bridge environment is calculated for different energy sources, including solar radiation, wind, and vibration from traffic. Feasibility of using energy harvesting in such an application is addressed for both power availability and cost as compared with grid power or primary batteries. An in-depth functional analysis of existing energy-harvesting systems is also presented, with insights into where innovation would be most beneficial in future systems. Finally, the development of a suite of complementary energy-harvesting devices is described. Because conditions on bridges may vary, multiple solutions involving different energy domains are desired, with the end user able to select the harvester most appropriate for the specific installation. Concept generation techniques such as mind-mapping and 6-3-5 (C-Sketch) are used to produce a wide variety of concepts, from which several promising concept variants are selected. The continued development for one concept, which harvests vibration using piezoelectric materials, is described. Analytical modeling is presented for static and dynamic loading, as well as predicted power generation. Two proof-of-concept prototypes are built and tested in laboratory conditions. Through the development of this prototype, it is shown that the example wireless sensor node can successfully be powered through energy harvesting, and insights are shared concerning the situations where this and other energy harvesters would be most appropriate. / text