82-5 The Behavior of Beams Subjected to Concentrated Loads

Tests on composite steel and concrete beams, continuous steel beams and simply supported plexiglass model beams, subjected to a concentrated load at midspan have demonstrated a buckling phenomenon where the bottom flange, which is primarily in tension, moves laterally. A complete parametric study was undertaken using a linear elastic finite element buckling analysis progaram to gain insight into this behavior.

Results are presented for beams under a single midpsan concentrated load with barying and restraint in-plane. The phenomenon was found to be a combination of local web buckling and lateraltorsional buckling, the former effect being dominant in simply supported beams and the latter in fixed ended beams. For beams with end restraint in between these cases (includin most continuous beams), an interaction of the two effects occurs.

It was found that bending stresses significantly influence the load at which there is a local web buckle between the top and bottom flange braces at the load point. Also observed was the fact that when the bottom flange was unbraced, the addition of a brace there did little to increase the buckling load if the beam was simply supported, but dramatically increased it when the beam was fixed ended.

Design recommendations for beams subjected to concentrated loads are provided along with a design example.