Browsing by Author "Hovell, Catherine Grace, 1983-"
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Item Evaluation of redundancy in trapezoidal box-girder bridges using finite element analysis(2007-08) Hovell, Catherine Grace, 1983-; Williamson, Eric B., 1968-The AASHTO Bridge Design specifications define a fracture-critical member as a component in tension whose failure is expected to result in the collapse of a bridge. The tension flanges of twin box-girder bridges are thus labeled as fracture-critical. In order to avoid the catastrophic collapse suggested by the AASHTO specifications, fracture-critical bridges, constituting 11% of all steel bridges in the country, are subjected to frequent and stringent evaluation and inspection. The Texas Department of Transportation, interested in reducing the cost of an otherwise attractive bridge design, is now questioning the validity of the original statement by AASHTO. In particular, it is not clear whether or not a single localized fracture can lead to the collapse of a bridge. Contrary to this belief, there have been multiple instances of fracture-critical bridges with two tension flanges that have experienced fracture without collapse. This project was designed to determine the level of redundancy that can be found in twin box-girder bridges. To achieve this goal, a full-scale test specimen of a box-girder bridge was built at the Ferguson Structural Engineering Laboratory in Austin, Texas. In unison, a finite element model of the bridge was built using ABAQUS/Standard. A fracture was initiated in one bottom flange of the test specimen. The data gathered during the test were compared to the calculated response from the model to verify the predictive capabilities of the model. If able to predict response accurately, a computer model could be used during design to indicate the presence of redundancy and the decreased need for frequent inspection of a bridge. The computer model was used to simulate a full-depth web fracture event in the exterior girder of a twin-girder bridge with a very large horizontal radius of curvature. The model was then modified to consider the influence of several parameters, including radius of curvature, structural redundancy through continuous spans, and external bracing. Results obtained from the finite element model indicate that adequate redundancy exists in the bridge design to maintain stability after the fracture of one girder. The most significant design change is to add continuity through spans, as adding structural redundancy greatly reduced the expected deflections and stresses that would be induced in the system. Further study using the modeling techniques presented in this thesis should begin by verifying or improving upon the assumptions that were made. Specifically, the concrete material model and the shear stud modeling method should be examined in more detail and should be used to predict the response of smaller-scale laboratory tests. With further refinement, this model could be utilized during the design phase to verify the presence of redundant load paths and thus reduce the necessity for frequent inspections.Item Structural performance of Texas U-beams at prestress transfer and under shear-critical loads(2011-08) Hovell, Catherine Grace, 1983-; Wood, Sharon L.; Bayrak, Oguzhan, 1969-; Jirsa, James O.; Williamson, Eric B.; Ezekoye, Ofodike A.The Texas U-Beam standard designs were released in the 1990’s and have been used increasingly in bridges across the state since. While prototypes of the 54-in. deep prestressed concrete beam were built during the design phase, no full-scale load tests were performed. This study of the U-Beam had five goals: (i) determine the magnitude and location of stresses induced in reinforcing bars in the end region of the beam at prestress transfer, (ii) measure concrete curing temperatures in square and skewed end blocks, (iii) establish the vertical shear capacity of the standard section, (iv) evaluate interaction between behavior at prestress transfer and performance under shear-critical loads, and (v) identify design and detailing improvements and make recommendations. Eight full-scale Texas U54 prestressed concrete beams were fabricated to achieve these goals. Load testing of the first four of these beams revealed a critical weakness along the bottom flange-to-web interface of the beam. The weakness caused failures that occurred at loads well below the calculated shear capacity. Given the horizontal sliding observed, the failure mode was called horizontal shear. The next two beams were fabricated to test three modifications to the end-region design, two of which were deemed successful. The final two beam sections tested contained the recommended new standard reinforcement and concrete geometry. A method to evaluate the horizontal shear demand on and capacity of the bottom flange-to-web interface of prestressed concrete beams was developed. The calculations were formulated using the theories of beam bending and shear friction. This method was calibrated and verified using the U-Beam test data, a series of small-scale specimens, and results of shear tests in the literature. Stresses induced in reinforcing bars at prestress transfer met expectations set by existing codified equations. No modifications to the current U-Beam standard design are needed to manage these stresses. The induced stresses did not influence vertical shear behavior, and no interaction between the two is believed to exist for U-Beams. This dissertation contains the specifics of the beams tested and the data collected, and provides the details of recommended changes to the Texas U-Beam standard drawings.