Buckling of Prestressed Steel Girders

Prestressing of steel plate girders may lead to substantial
economies of material. Densford et al. quote savings of 30%
in steel tonnage and 27% in concrete tonnage gained by
prestressing a short composite steel-concrete bridge that was
designed by the Oklahoma Department of Transportation.
Similar savings may be obtained for steel girders. Although
the use of prestressed steel girders is relatively new in the
United States, the technology is well-established in eastern
Europe.Perhaps the easiest way to prestress a steel girder is to
use straight high-strength rods, which are anchored at the
ends of the beam as shown in Fig. 1, and stressed in a manner
analogous to that for prestressing concrete beams. These
"hard" anchorages may have a number of "soft" anchorages
between them, allowing relative movement of the tendons.
Other methods of prestressing steel girders are discussed in
Ref. 1. Stressing the high-strength rods induces substantial
compressive stresses in the bottom flange of the beam before
the external loads are applied, and raises the question of the
stability of the girder under this loading. If the attachment of
the tendon to the web is at large spacing intervals, then the
girder may buckle between the points of attachment in an
overall or lateral mode.
The designer must be certain that the
prestressing force is not large enough to cause buckling of the
girder. The use of LRFD design methods and the method in
this paper may be used to calculate the buckling strength.
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