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Seismic Performance of Steel Girder Bridge Superstructures with Ductile End Cross Frames and Seismic Isolation
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As the end cross frames of steel plate girder bridges are critical in the transverse seismic load path they may be designed to deform in a ductile manner to reduce the elastic base shear in a bridge. From experimental results and analytical studies using ductile single angle X-braces, and buckling restrained braces in the end cross frames, the base shear in a bridge model was reduced to as low as 40% of the elastic base shear. The buckling restrained braces resulted in 20% to 30% smaller drifts than the X-braces at a given level of base shear, a result that is attributed to better energy dissipation. However, the displacement capacity of the single angle X-braces is larger than that for the buckling restrained braces. Removing some shear studs near the supports of the girders, and allowing the shear to be transferred into the end cross frames using a top chord, allows the girders to "rock" enabling considerable transverse drifts in the girders. Reinforced elastomeric bearings allow large rotations at the base of the girders. The maximum drift measured in the girders during experiments was 7% of the girder height, with no damage observed in the girders and minimal distress to the deck slab. Despite significant reductions using the ductile end cross frames, the elastic base shear in the bridge model was reduced further using seismic isolation. The capacity of the bearings was limited by the their stability, with buckling observed in the bearings at a displacement close to the expected buckling displacement. Despite this critical state, the isolation system did not fail because dynamic inertial effects in the bridge were able to restore stability to the system.
Report No. CCEER-05-4