Browsing by Subject "Prestressed concrete"
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Item Discrepancies in Shear Strength of Prestressed Beams with Different Specifications(Minnesota Department of Transportation Research Services Section, 2010-01) Dereli, Ozer; Shield, Carol; French, CatherineAlthough Mn/DOT inspection reports indicate that prestressed concrete bridge girders in service do not show signs of shear distress, girders rated with the Virtis-BRASS rating tool and Load Factor Rating (LFR) have indicated that a number of the girders have capacities lower than design level capacities. One of the reasons for the discrepancy was suspected to be conservatism of the rating methods (i.e., LFR). Other suspected reasons included potential flaws in the rating tools used by Mn/DOT (i.e., Virtis-BRASS software) including neglecting possible additional shear capacity parameters (e.g., end blocks). As a consequence, the rating methods have made it difficult to discern the cases for which shear capacity may be a real concern. In order to identify the reasons for the discrepancies and inconsistency in rating results relative to observed performance of the prestressed bridge girders, an analytical research program was conducted. The report provides a brief description of the models that provide the basis for the AASHTO shear design provisions and descriptions of the provisions through the 2002 AASHTO Standard specifications. This is followed by a description of the Virtis-BRASS rating tool, which was verified with example bridges provided by Mn/DOT. To investigate prestressed bridge girders within the inventory that might be most at risk for being undercapacity for shear, 54 girders were selected from the inventory for further evaluation. Some of the 54 girders were found to have larger stirrup spacings than required at the time of design. These girders were subsequently rated and evaluated per the 2002 AASHTO Standard Specifications to determine the adequacy of the designs based on the LFR inventory and operating rating methods. Potential sources for increased shear capacity were identified and reviewed.Item Effects of compressive and tensile fields on the load carrying capacity of headed anchors.(2011-02) Piccinin, RobertoThe results of research initiated in the early 1980s led to the replacement of plasticity-based design guidelines for the load-carrying capacity (concrete breakout) of headed anchors embedded in concrete with those developed using fracture mechanics. While provisions are available in the design codes that account for the presence of tensile fields causing concrete cracking, no provisions are available for anchors embedded in prestressed concrete. This thesis presents the results of linear and nonlinear elastic fracture mechanics analyses of the progressive failure of headed anchors embedded in a concrete matrix under compressive or tensile prestress. In addition, and because of the complete lack of experimental evidence, the results of a relatively large experimental investigation of the behavior of headed anchors embedded in compressively prestressed concrete are presented. Discrete crack finite element models and experiments predict an increase (decrease) in load-carrying capacity and post-peak dissipated energy with increasing compressive (tensile) prestress for all the embedment depths investigated. For extremely shallow cases, in which the embedment depth is less than the (typical) maximum aggregate size of concrete, it is shown that deterministic continuum-based models are not applicable. Overall, the results show that there is very little difference between the linear elastic and nonlinear elastic fracture mechanics approaches, this implying that the concrete breakout strength is governed by the strongest possible size effect. In addition to providing analytical support to the existing design approaches for the capacity of headed anchors embedded in cracked concrete (under tension), the present work provides an experimentally and analytically based preliminary easy-to-use design formula for the concrete breakout capacity of headed anchors in compressively prestressed concrete.Item Modeling and behavior of prestressed concrete spandrel beams.(2011-06) Mercan, BulentSpans for precast and prestressed concrete spandrel beams (or spandrels) are typically as long as 30 to 48 ft. In the near future, precasters might begin to produce spandrels with longer spans (up to 60 ft) and thinner webs (as little as 8 in.), which facilitate more cost-effective parking garages and advance the competitiveness of the precast concrete industry. However, there are likely to be spandrel dimensions beyond which excessive deformations make such designs unfeasible. In particular, lateral deformations developing at the top and bottom of a slender spandrel beam need to be investigated for short-term and long-term loading effects. Spandrels in parking garages typically support double-tee beams (or double tees) on ledges, spot corbels, or pockets, all of which introduce lateral eccentricity as the double tee loads are transferred to the spandrels. Upon connecting the spandrels to the double tees using deck ties, these connections serve as lateral restraints for the spandrels due to the axial stiffness of the deck ties. Thus, lateral deflection of the spandrel is prevented at the connection points. However, given that the deck ties are thin steel plates, they are unable to eliminate twisting of the spandrel section. Thus, subsequent deformation of the spandrel will include both vertical and lateral deflections. Excessive lateral deflections might cause undesirable serviceability problems in the structure or even collapse of the concrete deck. For this reason the study of lateral deflections is the focus of the research presented in this dissertation. Two approaches are pursued to study the lateral deflection response of the slender spandrels under static loads: analytical and computational. First, an approximate analytical solution for the lateral deflections of spandrels is presented. Next, the effects of various parameters on the results from finite element analyses are examined. This is followed by a detailed discussion of modeling suggestions under which computational results can be sufficiently reliable. The best possible modeling approach, deduced from such parametric studies, is used for a three-dimensional nonlinear finite element analysis of spandrels. Finally, time-dependent (long-term) lateral deflections in spandrels having various span lengths and cross sections are investigated.Item Reusability and Impact Damage Repair of Twenty-Year-Old AASHTO Type III Girders(1992-01) Olson, Steven A.; French, Catherine E.; Leon, Roberto T.Prestressed concrete has been used as a bridge construction method in the United States since 1949. Presently, there are thousands of pretensioned prestressed concrete bridges in service in North America. Each year, approximately 200 girders are damaged as a result of impact damage (primarily overheight vehicles striking a bridge from below). This thesis describes the results of a four girder test series which evaluated impact damage and repairs. The girders used for the study were fabricated in 1967 and placed in service. They were removed from service in 1984 as a result of a road realignment project. The objectives of the research project were: 1) to determine the effective prestress in the strands after 20 years, 2) to determine the influence of impact damage on girder performance, 3) to evaluate the performance of two impact damage repair schemes under static, fatigue, and ultimate loadings, and 4) to develop a model to estimate the strand stress ranges in damaged girders.