UNRESTRAINED BEAM DESIGN-I 11 UNRESTRAINED BEAM DESIGN – I 1.0 INTRODUCTION Generally, a beam resists transverse loads by bending action. In a typical building frame, main beams are employed to span between adjacent columns; secondary beams when used – transmit the floor loading on to the main beams. In general, it is necessary to consider only the bending effects in such cases, any torsional loading effects being relatively insignificant. The main forms of response to uni-axial bending of beams are listed in Table 1. Under increasing transverse loads, beams of category 1 [Table1] would attain their full plastic moment capacity. This type of behaviour has been covered in an earlier chapter. Two important assumptions have been made therein to achieve this ideal beam behaviour. They are: ♦ The compression flange of the beam is restrained from moving laterally, and ♦ Any form of local buckling is prevented. If the laterally unrestrained length of the compression flange of the beam is relatively long as in category 2 of Table 1, then a phenomenon, known as lateral buckling or lateral torsional buckling of the beam may take place. The beam would fail well before it could attain its full moment capacity. This phenomenon has a close similarity to the Euler buckling of columns, triggering collapse before attaining its squash load (full compressive yield load). Lateral buckling of beams has to be accounted for at all stages of construction, to eliminate the possibility of premature collapse of the structure or component. For example, in the construction of steel-concrete composite buildings, steel beams are designed to attain their full moment capacity based on the assumption that the flooring would provide the necessary lateral restraint to the beams. However, during the erection stage of the structure, beams may not receive as much lateral support from the floors as they get after the concrete hardens. Hence, at this stage, they are prone to lateral buckling, which has to be consciously prevented. Beams of category 3 and 4 given in Table 1 fail by local buckling, which should be prevented by adequate design measures, in order to achieve their capacities. The method of accounting for the effects of local buckling on bending strength was discussed in an earlier chapter. In this chapter, the conceptual behaviour of laterally unrestrained beams is described in detail. Various factors that influence the lateral buckling behaviour of a beam are explained. The design procedure for laterally unrestrained beams is also included. © Copyright reserved Version II 11-1
UNRESTRAINED BEAM DESIGN-I Table 1 Main failure modes of hot-rolled beams Category Mode Comments 1 Excessive bending triggering collapse This is the basic failure mode provided (1) the beam is prevented from buckling laterally,(2) the component elements are at least compact, so that they do not buckle locally. Such “stocky” beams will collapse by plastic hinge formation.
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