Local web buckling in tapered composite beams

Abstract

Recent demand for very heavily serviced widespan office floors has led, as a natural development of the concept of haunched and tapered frames, to the use of composite flooring systems supported by tapered (varying web depth) beams. These systems are very attractive from an economic point of view, combining wide spans, rapid erection and easy access to services between the shallow parts of the beam and the ceiling below. One of the most pressing needs is for a better understanding of the local buckling behaviour of the relatively deep slender web at changes of slope in the lower flange, in particular at the slope change which occurs at the centre of a beam whose depth increases linearly from each support. In the past, with plate girders, this failure mechanism was less relevant than lateral torsional buckling. With the substantial top flange of a composite section, lateral buckling is less likely, and local buckling more likely to govern the design. The tapered beam sections are fabricated from plates by welding and are susceptible to imperfection effects and residual stresses. These phenomena may interact with the localized compressive stress field which is generated in the web at any slope change in the flange to cause local web buckling. The provision of local stiffening is technically feasible, but very expensive. The beam itself is produced fully automatically and the subsequent hand installation of stiffeners consumes man-hours out of all proportion to the weight of steel involved. The prime objective of the present study was to iucntify the relative importance of several parameters that influence the phenomenon, such as angle of taper, web thickness, flange area and others. For practical reasons, the experiments were executed at half-scale and the models simulated the concrete slab of the composite beam by a heavy steel top flange. Three tapered models have been tested with different web thicknesses i.e, 3, 5, and 7 millimetres. Three more specimens with different plastic neutral axis positions were also tested. An extensive parametric study has been made using a non-linear elastoplastic finite element program. This study covered practical ranges of the important parameters including the area of the tension flange and the position of the plastic neutral axis. Moment-rotation relations, peak moments and failure mechanisms have been predicted. Very good correlation was obtained between the results of the parametric study and the experiments. It has become apparent that in the great majority of practical cases it is possible to avoid the use of the expensive hand welded stiffeners. A design formulation has been developed to produce a very simple test of the need for stiffeners.