Stability of precast prestressed concrete bridge girders considering imperfections and thermal effects
Hurff, Jonathan B.
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The spans of precast prestressed concrete bridge girders have become longer to provide more economical and safer transportation structures. As the spans have increased, so has the depth of the girders which in turn have increased the slenderness of the girders. Slenderness in a beam or girder would increase the likelihood that a stability failure would occur. Stability failures could pose a danger to construction personnel due to the sudden nature in which a stability failure would occur. Furthermore, stability failures of prestressed concrete girders during construction would cause a detrimental economic impact due to the costs associated with the failure of the girder, the ensuing construction delays, damage to construction equipment and potential closures to highways over which the bridge was being constructed. An experimental and analytical study was performed to determine the stability behavior of prestressed concrete beams. Two stability phenomenons were investigated: (1) lateral-torsional buckling and (2) global stability. An emphasis was placed on the effects of initial imperfections on the stability behavior; the effect elastomeric bearing pads and support rotational stiffness was investigated. The experimental study involved testing six rectangular prestressed concrete beams for lateral-torsional buckling, a PCI BT-54 for thermal deformations and the same PCI BT-54 for global stability. The 32-ft. long rectangular beams were 4-in. wide and 40-in. deep. The PCI BT-54 had a 100-ft. long span. A material and geometric nonlinear, incremental load analysis was performed on the six rectangular beams. The nonlinear analyses matched the experimental load versus lateral displacement and load versus rotation behavior, and the analysis predicted the experimental maximum load within an error of 2%. The nonlinear analysis was extrapolated to several different initial imperfection conditions to parametrically study the effect of initial lateral displacement and initial rotation on the inelastic lateral-torsional buckling load. A simplified expression for lateral-torsional stability of beams with initial imperfections was developed. The data from the parametric study were used to develop reduction parameters for both initial sweep and initial rotation. The rollover stability behavior of the PCI BT-54 was investigated experimentally, and it was found that support end rotations and the elastomeric bearing pads had an adverse effect on the global stability. The nonlinear analysis was employed with the addition of a bearing pad model. It was found that the behavior was sensitive to the bearing pad stiffness properties and the assumption of uniform bearing. From the research, it was apparent that rollover stability was the controlling stability phenomenon for precast prestressed concrete bridge girders, not lateral-torsional buckling.