Economical constraints on the design of bridges usually necessitate the use of as few girders as possible across the bridge width. The girders are typically uniformly spaced transversely with the deck extending past the fascia girders, thereby resulting in an overhang. While designers commonly employ rules of thumb with regard to the geometry of the overhang, these rules of thumb generally lack research justification and the actual girder behavior is not well understood. Overhang construction often produces torsinally unbalanced loading on the girder system, which can lead to problems in steel and concrete girder bridges during construction. The main issue with concrete girder bridges is excessive lateral rotation in the fascia girder, which can cause potential problems of construction safety and maintenance. Field problems on concrete bridges have been reported in the state of Texas where the fascia girders experienced excessive rotation during construction. For steel girder bridges, the unbalanced overhang loading can lead to both local and global instability. Locally, the overhang brackets often exert a large force on the web plate that can distort the web and increase the magnitude of the plate imperfection. Global stability problems have also occurred primarily on bridge widening projects when a few girders are added to an existing bridge system. The girders in the widening are usually isolated from the existing bridge and the unbalanced load from the overhang can cause excessive twist that intensifies the global stability of the girder system. The objective of this study was to improve the understanding of the bridge behavior due to the unbalanced loading from the overhangs and to identify critical factors affecting the girder behavior. The study was also aimed at developing simple design methodologies and design recommendations for overhang construction. The research included field monitoring, laboratory tests, and parametric finite element analyses. The data from the field monitoring and laboratory tests were used to validate finite element models for both concrete and steel girder bridges. Based on the validated models, detailed parametric studies were conducted to investigate the effects of the unbalanced loading. Results from the parametric studies were used to identify the geometries of girder systems that are prone to problems with the overhangs as well as to provide design suggestions. In addition, a closed-form solution for lateral rotation in the fascia girder in a concrete girder bridge was derived using a rigid-body model, and was used to develop design methodology and design recommendations for overhang construction. / text

Lateral bracing systems consisting of cross frames and their connections play a significant role in the elastic buckling strength of steel girder bridges. By providing lateral and torsional stability, they prevent lateral torsional buckling of the girder during bridge construction prior to the concrete bridge deck curing. To perform this function, the bracing system must possess adequate strength and stiffness. And since each component of the bracing system acts in series, the overall stiffness of the system is less than the least stiff component. In skewed bridges, cross frames at the ends of the girders are installed parallel to the bridge skew angle, and their connection to the girder requires that the cross frames be at an angle that prohibits welding a stiffener from the cross frame directly to the girder web. To make this connection, many states use a bent plate to span the angle between the web stiffener and cross frame. While this bent plate connection is now being widely used, it has never been rationally designed to account for its strength or stiffness in the bracing system. Results from field studies show that the bent plate connection may be limiting the cross frame stiffness thereby hampering its ability to provide stability to the girder during construction. The result is significant girder end rotations. The purpose of this research is to classify the impact of the bent plate connection on the end cross frame stiffness in skewed straight steel girder bridges and propose methods to improve the end cross frame’s structural efficiency. This research uses laboratory testing, finite element modeling, and parametric studies to recommend design guidance and construction practices related to the end cross frames of skewed steel girder bridges. In addition to recommending methods to stiffen the existing bent plate connection, an alternative pipe stiffener connection is evaluated. The pipe stiffener not only offers the possibility of a stiffer connection, but can also provide warping restraint to the end of the girder which may significantly increase the girder elastic buckling capacity. / text

No description available.

Optimal pick point location is discovered in order to have the full buckling capacity of a suspended, non-prismatic, singly symmetric plate girders. The results show that a significant increase in stability occurs at the pick point location of 0.25L, where L is the span length. This recommendation can be utilized in the field to save construction cost and time because of how simplistic it is to use. A finite element software, ABAQUS 6.14, is used to determine the buckling capacities at different pick point locations and of different cross sections. The dimension of the cross sections and span lengths for the research is based upon a case study.

Built-up steel I-girders are very commonly used in bridge construction. Their spans are typically very long, and they are susceptible to lateral torsional buckling if not enough lateral support is provided. This thesis includes guidelines for preventing lateral torsional buckling of steel I-girders under dead and wind load, accompanied with finite element analysis of double girder systems. The first portion includes capacity envelopes for single girders with single and double symmetric cross sections under various loading conditions and boundary conditions for double and single symmetric cross sections with double girders subjected to dead loads only. The second portion is dedicated to finite element analysis of double girders. Buckling analyses have been conducted on single symmetric double girders to verify their capacity equations and investigate the behavior of double girders subjected to wind load. The analyses focus on the weak axis bending of the double girder system as a whole and an evaluation of whether buckling of cross-bracing is an issue when lateral loads are present.

Steel bridges and structures often need strengthening due to increased live loads, or repair due to corrosion or fatigue cracking. This thesis explores the use of adhesively bonded prestressed carbon fibre reinforced polymers (CFRP) strips in retrofitting intact steel girders, through experimental and analytical investigations. The first part of the research program investigates the behaviour of CFRP-strengthened steel beams comprised of W Structural Sections (W ) with cover plates welded to the tension flange. Six beams, 2000 mm long, were tested under cyclic loads to examine the effects of CFRP strip strengthening on the fatigue life. The CFRP strip prestressing process, type of CFRP strip, level of prestressing, and the location of the CFRP strips were the main parameters examined in this study. Debonding at the end of strip was a significant problem that can be controlled by applying a proper end clamp. The maximum increase in fatigue life observed in the experiments was 125 percent, for a specimen strengthened using high modulus CFRP strips bonded onto the cover plates with the highest level of prestressing. An analytical model and a finite element model were developed for analyzing the strengthened beams. A fracture mechanic analysis was performed to investigate the effects of prestressing on the crack growth rates at the critical weld toe. The models were verified using experimental results, and then used to perform parametric studies. It is shown that the effectiveness of reinforcement is greatest for beams with strips on the cover plate, higher CFRP elastic modulus, and higher prestressing level. In general, this study demonstrates that steel beams can indeed be successfully strengthened or repaired using prestressed CFRP materials.

Thesis (Ph. D.)--University of Texas at Austin, 2002. / Supervisors: Joseph A. Yura and Karl H. Frank. Vita. Includes bibliographical references. Available also from UMI Company.

This 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.

Steel bridges and structures often need strengthening due to increased live loads, or repair due to corrosion or fatigue cracking. This thesis explores the use of adhesively bonded prestressed carbon fibre reinforced polymers (CFRP) strips in retrofitting intact steel girders, through experimental and analytical investigations. The first part of the research program investigates the behaviour of CFRP-strengthened steel beams comprised of W Structural Sections (W ) with cover plates welded to the tension flange. Six beams, 2000 mm long, were tested under cyclic loads to examine the effects of CFRP strip strengthening on the fatigue life. The CFRP strip prestressing process, type of CFRP strip, level of prestressing, and the location of the CFRP strips were the main parameters examined in this study. Debonding at the end of strip was a significant problem that can be controlled by applying a proper end clamp. The maximum increase in fatigue life observed in the experiments was 125 percent, for a specimen strengthened using high modulus CFRP strips bonded onto the cover plates with the highest level of prestressing. An analytical model and a finite element model were developed for analyzing the strengthened beams. A fracture mechanic analysis was performed to investigate the effects of prestressing on the crack growth rates at the critical weld toe. The models were verified using experimental results, and then used to perform parametric studies. It is shown that the effectiveness of reinforcement is greatest for beams with strips on the cover plate, higher CFRP elastic modulus, and higher prestressing level. In general, this study demonstrates that steel beams can indeed be successfully strengthened or repaired using prestressed CFRP materials.

No description available.