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ABSTRACT This paper describes analytical structural models created for various components of the Tobin Memorial Bridge, and a program of instrumentation and non-destructive testing for the Little Mystic Span. A detailed global finite element model of the Little Mystic Span, one of the two truss spans, was developed along with supporting special studies of continuity and boundary conditions. Special studies include modeling the rotational stiffness of the truss connections and consideration of the piers and bridge shoes. 61752
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An instrumentation plan was developed and deployed for the Little Mystic Span to capture structural responses of key truss members. Non-destructive testing using loaded trucks was carried out for the purpose of model verification and calibration. Preliminary comparisons between experimental and analytical strains for key truss members show reasonable agreement. Special study results will be used to fine-tune the global finite element model. The verified models may be used as a condition assessment program and structural health monitoring system for the management of the Tobin Memorial Bridge. INTRODUCTION Visual inspection is the primary means to evaluate the condition of virtually all highway bridges in the United States. Structural deterioration is assigned a numerical rating from zero to nine, with zero being a “failed” condition and nine being in “excellent” condition. The ratings are subjective to an extent, based on detailed guidelines and in part on the experience and approach of the inspectors. Furthermore, visual inspections are, by definition, limited to what can be seen. Hidden structural members, or conditions that are difficult to directly view, may not receive the same level of inspection treatment (Farhey 2005). The inspector’s expertise is required to evaluate the adequacy of the overall structural condition of the bridge. In addition to visual inspections, load ratings are performed using the AASHTO allowable stress, load factor, or load and resistance factor rating methods. The purpose of load ratings is to determine the load capacity of an existing bridge. Development of a bridge’s load capacity indicates if a load limit should be posted on the bridge, if special freight policies should be implemented, and to develop truck routes around congested urban areas. Bridge testing for load ratings is a complementary tool to visual inspections in assessing the structural condition of the bridge and determining the appropriate maintenance strategy. Software is available to aid in the load rating process, including the AASHTO Bridge Analysis and Rating System that has been in use throughout the country since the early 1970's. Advances in computing power and availability have provided a platform for consistently improving structural analysis and load rating software, including AASHTO's new bridge load rating system named Virtis/Opis (Thompson et al. 2000). Model calibration using measured data. Computer modeling can be an objective approach for creating a guided visual inspection, leading to more efficient bridge condition assessment. However, while the analytical model seems to be more objective, errors in computer modeling of structural components significantly impact the value of the analytical responses for condition assessment.
Errors include oversimplification of assumptions, uncertain boundary and continuity conditions, and unknown loading conditions. It is possible to calibrate a structural bridge model by evaluating its performance under a known loading and comparing the predicted response to the measured response. Instrumentation of the bridge with an array of sensors designed to capture structural responses of the bridge can be used to help validate the computer model. The combination of a calibrated analytical model along with sensors to measure the structure’s response can be used to develop a structural health monitoring (SHM) system. The system can provide more objective, supplemental data that aids in decisions for apportionment of limited maintenance funds. The SHM system may detect unusual structural behavior at an early stage, thereby alerting bridge owners that a comprehensive investigation is required. Through application of the system and more detailed modeling, components appearing to have damage may still have sufficient capacity to carry design loads. Therefore, objective assessment, in combination with subjective visual assessment, helps to direct limited maintenance funds to components in need of repair. Also, a calibrated structural model can be used for load rating and permitting. Effective monitoring requires the development of a computer model that accurately characterizes the entire structure, including system performance, continuity and boundary conditions. Such a model will be more detailed and elaborate than a design model, which has factors of safety and conservative assumptions appropriate for design, but not necessarily accurate enough for modeling the structures system response under loading. Connections and boundary conditions. Special studies may be required to develop an analytical model that accurately characterizes a real structure. Examples of factors influential to computed structural responses include connection stiffness and boundary conditions.