外文翻译钢结构.doc

4-6 Inadequate Lateral SupportAs the distance between points of lateral support on the compression flange (l) becomes larger, there is a tendency for the compression flange to buckle laterally. There is no upper limit for l. To guard against the buckling tendency as l becomes larger, however, the ASDS provides that be reduced. This, in effect, reserves some of the beam strength to resist the lateral buckling. Figure 4-10b shows a beam that has deflected vertically with a compression flange that has buckled laterally. The result is a twisting of the member. This is called lateral-torsional buckling. For simplicity we refer to this buckling mode of the beam as lateral buckling. Two general resistances are available to counteract lateral buckling: torsional resistance of the cross section and lateral bending resistance of the compression flange. The total resistance to lateral buckling is the sum of the two. The ASDS conservatively considers only the larger of the two in the determination of a reduced Fb . The ASDS, Section F1.3, establishes empirical expressions for Fb for the inadequate lateral support situation. Tension and compression allowable bending stresses are treated separately. The tension Fb is always 0.60 Fy . Only the compressionFbis reduced. Fortypical rolled shapes, this is of no consequence because the shapes are symmetrical and the lower Fb of the two values will control. Note that the provisions of this section pertain to members having an axis of symmetryin, and loaded in, the plane of their web. They also apply to compression on extreme fibers of channels bent about their mayjor axis. The ASDS provides three empirical equations for the reduced compression Fb . The mathematical expressions that give an exact prediction of the buckling strength of beams are too complex for general use. Therefore, the ASDS equations only approximate this strength for purposes of determining a reasonable Fb . The Fb that is finally used is the larger of the va