Browsing by Subject "Prestressed"
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Item Analytical and Experimental Assessment of an AASHTO I-girder Type I Prestressed with AFRP Tendons(2014-12-12) Cummings, Wesley DavidCorrosion induced deterioration is one of the main reason for repair and rehabilitation programs in conventional steel reinforced concrete bridge decks. Of all bridges in the United States, over 50 percent are constructed of conventional reinforced or prestressed concrete (NACE, 2013), where one in three bridges are considered structurally deficient or functionally obsolete due to corrosion of the steel reinforcement. According to NACE International (2013) the annual cost of corrosion-related maintenance for highway bridges in the U.S. is estimated at $13.6 billion. Over the past couple of decades, fiber reinforced polymer (FRP) bars have been noted by researchers and engineers as a corrosion-resistant alternative for either conventional reinforcing steel or prestressing strands. High strength-to-weight ratio, corrosion resistance, ease in placement of the bars and accelerated implementation due to light weight are the special characteristics that make these bars an appealing alternative. Up to this end, extensive research has been conducted on the structural performance of FRP reinforced concrete beams and slabs; however, less attention has been paid to FRP reinforced concrete bridge girders in composite action with the bridge deck. Accounting for the effect of composite action between the bridge girder and deck can significantly impact the structural performance of the girder including the load and deformation capacities as well as the failure mode. Therefore, separate tests of the FRP concrete beams and slabs may not be sufficient to study the structural behavior and to provide design guidelines for engineers. This thesis presents the experimental and analytical investigations on structural performance of a full-scale AASHTO I-girder Type I, reinforced and prestressed with aramid fiber reinforced polymer (AFRP) bars, where the bridge girder is composite with the deck. The major objectives of this research were to develop a reliable prestressing anchorage system, examine the constructability of the full-scale specimen, study the load and deformation capacities, determine whether or not the design criteria per AASHTO LRFD were met, and improve the performance of the specimen by adjusting the prestressing layout. The specimen was constructed at a prestressing plant in San Marcos, Texas and tested at the High Bay Structural and Material Testing Laboratory on the campus of Texas A&M University. The cross-section of the bridge girder was composed of self-consolidating concrete with a total of 24 prestressed and 8 non-prestressed AFRP bars. The bridge deck consisted of a 203 mm (8 in.) conventional steel reinforced concrete slab. A flexure test was conducted to determine the moment-curvature relationship, flexure load capacity, and failure mode. The test was conducted as a simply supported, four point bending test in order to create a region of constant moment at the center of the beam. Two shear tests were conducted to determine the shear capacity, failure mode, maximum strain in the web, and moment-curvature relationship. The shear tests were conducted as a simply supported, three point bending test with varying load placement. The results of these tests were compared to a similar study which investigated the structural performance of a conventional steel reinforced AASHTO I-girder Type I with topping deck (Trejo et al. 2008). The specimen was also analyzed analytically to determine the effect on performance of varying the prestressing ratio of the separate layers in the bottom flange of the girder. The goal of this analysis was to determine an optimal prestressing layout to improve the performance at the ultimate state, while still satisfying serviceability limits. The prestressing ratio of the layers were varied from 0 to 50 percent in 5 percent increments to study the moment and curvature at both the cracking and ultimate states, along with the available compressive stress due to prestressing at the bottom of the girder. The results of this research confirms that the experimental specimen showed adequate strength and deformation capacities, satisfying the AASHTO LRFD design criteria. Additionally, the experimental specimen showed significantly greater cracking when compared to the conventional steel reinforced specimen, which is an early warning of impending failure. It was also determined that reducing the prestressing ratio of the AFRP bars in the lower layers improves the ductility of the specimen. The moment capacity can also be improved depending on the prestressing layout. However, reducing the prestressing ratio of the bottom layers causes the cracking moment and available compressive stress at the bottom of the girder to diminish. In order to compensate for this loss, the non-prestressed bars in the web can be prestressed. The optimal prestressing layout features the bottom three layers of the specimen prestressed to 35, 40, and 45 percent of their ultimate capacity, and two of the three layers of middle bars prestressed to 50 percent of their ultimate capacity.Item Effects of prestress on strains and deflections in pretensioned beams(2013-12) Koutrouvelis, Stergios; Tassoulas, John LambrosIn this research, nonlinear structural analysis along with finite element analysis were carried out for a pretensioned concrete beam at different levels of pretension in order to examine the effect of the change in the tendon force on the geometric stiffness of the beam. Several results were obtained for deflection, horizontal displacement and surface strains to investigate how they are affected by the level of pretension under the application of the same load in each case. These computations were compared with the tendon force to conclude whether they can be used to estimate the pretension level by means of simple measurements. The purpose was to develop a methodology for quantifying the prestress losses by taking advantage of the dependence of the prestressed concrete beam stiffness on the tendon force.Item Evaluation of high strength concrete prestressed bridge girder design(Texas A&M University, 2004-09-30) Cuadros Olave, GladysThis research study focuses on evaluating the design of HSC prestressed bridge girders. Specifically there were three major objectives. First, to determine the current state of practice for the design of HSC prestressed bridge girders. Second, to evaluate the controlling limit states for the design of HSC prestressed bridge girders and identify areas where some economy in design may be gained. Third, to conduct a preliminary assessment of the impact of raising critical design criteria with an objective of increasing the economy and potential span length of HSC prestressed girders. The first objective was accomplished through a literature search and survey. The literature search included review of design criteria for both the AASHTO Standard and LRFD Specifications. Review of relevant case studies of the performance of HSC prestressed bridge girders, as well all as of important design parameters for HSC were carried out. In addition, a survey was conducted to gather information and document critical aspect of current design practices for HSC prestressed bridges The second objective was accomplished by conducting a parametric study for single span HSC prestressed bridge girders to mainly investigate the controlling limit states for both the AASHTO Standard (2002) and LRFD (2002) Specifications. AASHTO Type IV and Texas U54 girder sections were considered. The effects of changes in concrete strength, strand diameter, girder spacing and span length were evaluated. Based on the results from the parametric study, the limiting design criteria for HSC prestressed U54 and Type IV girders using both the AASHTO Standard and LRFD Specifications for Highway Bridges were evaluated. Critical areas where some economy in design may be gained were identified. The third research objective was accomplished by evaluating the impact of raising the allowable tensile stress for service conditions. This stress limit was selected for further study based on the current limit for uncracked sections provided by the ACI 318 code (2002) and the limit used for a specific case study bridge (Ralls 1995). Recommendations for improving some critical areas of current bridge designs, as well as for increasing bridge span lengths, are given.Item Impact of AASHTO LRFD specifications on the design of precast, pretensioned u-beam bridges(Texas A&M University, 2006-04-12) Adnan, MohsinTexas Department of Transportation (TxDOT) is currently designing its highway bridge structures using the AASHTO Standard Specifications for Highway Bridges, and it is expected that TxDOT will make transition to the use of the AASHTO LRFD Bridge Design Specifications before 2007. The objectives of this portion of the study are to evaluate the current LRFD Specifications to assess the calibration of the code with respect to typical Texas U54 bridge girders, to perform a critical review of the major changes when transitioning to LRFD design, and to recommend guidelines to assist TxDOT in implementing the LRFD Specifications. This study focused only on the service and ultimate limit states and additional limit states were not evaluated. The available literature was reviewed to document the background research relevant to the development of the LRFD Specifications, such that it can aid in meeting the research objectives. Two detailed design examples, for Texas U54 beams using the LRFD and Standard Specifications, were developed as a reference for TxDOT bridge design engineers. A parametric study was conducted for Texas U54 beams to perform an in-depth analysis of the differences between designs using both specifications. Major parameters considered in the parametric study included span length, girder spacing, strand diameter and skew angle. Based on the parametric study supplemented by the literature review, several conclusions were drawn and recommendations were made. The most crucial design issues were significantly restrictive debonding percentages and the limitations of approximate method of load distribution.The current LRFD provisions of debonding percentage of 25 percent per section and 40 percent per row will pose serious restrictions on the design of Texas U54 bridges. This will limit the span capability for the designs incorporating normal strength concretes. Based on previous research and successful past practice by TxDOT, it was recommended that up to 75% of the strands may be debonded, if certain conditions are met. The provisions given in the LRFD Specifications for the approximate load distribution are subject to certain limitations of span length, edge distance parameter (de) and number of beams. If these limitations are violated, the actual load distribution should be determined by refined analysis methods. During the parametric study, several of these limitations were found to be restrictive for typical Texas U54 beam bridges. Two cases with span lengths of 140 ft. and 150 ft., and a 60 degree skew were investigated by grillage analysis method.Item Shear performance of ASR/DEF damaged prestressed concrete trapezoidal box bridge girders(2010-08) Wang, Tz-Wei; Jirsa, J. O. (James Otis); Bayrak, Oguzhan; Ghannoum, Wassim M.; Wheat, Harovel G.; Zhu, JinyingConcrete bridges in Texas have developed large cracks in bent caps and pretensioned trapezoidal bridge girders. The bridges show premature concrete deterioration due to alkali-silica reaction (ASR) and delayed ettringite formation (DEF). There is concern that deterioration due to ASR/DEF may lead to a loss of structural capacity. However, there are no quantitative guidelines to relate the level of concrete deterioration due to ASR/DEF to structural performance. Using such guidelines, the need for rehabilitation of beams with ASR/DEF cracking can be assessed. The goal of this research was to determine the shear capacity of pretensioned trapezoidal box girder specimens exhibiting varying degrees of ASR and/or DEF cracking and to use the shear testing results to evaluate the severity of the problem that may exist in Texas bridge structures. To achieve this goal, beams that were severely deteriorated due to ASR/DEF over a period of more than ten years were transported to the University of Texas for testing to failure. Both severely deteriorated and uncracked beams were tested in shear. The test results were used to evaluate the shear performance of trapezoidal box beams affected by ASR/DEF. In addition, three different types of forensic analyses were conducted on the beams to understand the nature of the ASR/DEF cracks and severity of the deterioration. After testing, it is found that the shear capacity of the test specimens was not significantly reduced even with heavy ASR/DEF cracking. Assessment using current US design provisions for bridges or buildings (ACI 318-08 and AASHTO LRFD 2008) and the proposed provision from an earlier project (TxDOT Project 5253) yielded conservative estimates of strength. Results from forensic analyses provided a qualitative indication of ASR/DEF damage but did not correlate with the observed levels of ASR/DEF deterioration.