Impact of Bottom Flange Confinement Reinforcement on Performance of Prestressed Concrete Bridge Girders

For many years AASHTO provided no recommendation to state DOT’s on bottom flange confinement reinforcement for their bridge superstructures. The 1996 edition of AASHTO Standard Specification for Highway Bridges stated that nominal reinforcement be placed to enclose the prestressing steel from the end of the girder for at least a distance equal to the girder’s height. A few years later the 2004 AASHTO LRFD Bridge Design Specification changed the distance over which the confinement was to be distributed from 1.0h to 1.5h, and gave minimum requirements for the amount of steel to be used, No.3 bars, and their maximum spacing, not to exceed 6”. Research was undertaken to study what impact, if any, confinement reinforcement has on the performance of prestressed concrete bridge girders. Of particular interest was the effect confinement had on the transfer length, development length, and vertical shear capacity of the fore mentioned members. First, an analytical investigation was performed on the subject, and then an experimental investigation followed which consisted of designing, fabricating, and testing eight tee-girders and three NU1100 girders with particular attention paid to the amount and distribution of confinement reinforcement placed at the end of each girder. The results of the study show: 1) neither the amount or distribution of confinement reinforcement had a significant effect on the initial or final transfer length of the prestress strands; 2) at the AASHTO calculated development length, no significant impact from confinement was found on either the nominal flexural capacity of bridge girders or bond capacity of the prestressing steel; 3) the effects from varied confinement reinforcement on the shear resistance of girders tested was negligible, however, distribution of confinement did show to have an impact on the prestressed strands’ bond capacity; 4) confinement distribution across the entire girder did increase ductility and reduced cracking under extreme loading conditions.

Constructability Testing of Folded Plate Girders

A new steel girder bridge system was developed at the University of Nebraska. The innovative girder design is a box girder folded from a single steel plate that has a trapezoid shape with an opening on the bottom. The girder has application in short span bridges and accelerated construction situations. The structural performance of the girder requires investigation in all stages of a bridge’s lifecycle. This thesis contains descriptions and results from the first two tests from a series of tests developed to evaluate this new girder shape. The objective of these two tests was to investigate the constructability of the girders. During construction a bridge is in its least stable condition and it is important that the bridge components exhibit both adequate strength and stability during this critical stage. To this end, two girders were tested in flexure over a simple span as a non-composite beam simulating the loading the girders would be subjected to during construction. The results of the two tests indicate that the folded girder as a whole, and its components, provide adequate strength and stability at construction load levels. Failure occurred at loads that were above normal construction load levels and resulted in a ductile failure mode, which is a well documented benefit of steel components. The girders remained stable through all phases of loading including failure. The top flange was the weakest component of the beam during construction due to its role as a compression element that has a slender and un-braced form. The compression in the top flange caused local buckling in the top flange even at elastic load levels. This was the cause for loss of stiffness and failure in both specimens. Incorporation of a ridge at the center of the top flange of specimens, results of which are not reported in this thesis, proved to resolve this very early buckling issue.