Ultimate Strength of Square Tubes Under Bending Loads

Buckling behavior of square tubes under bending load depends on the direction of the bending moment. This paper considers two directions. The first has an axis which crosses at right angles to a side of a cross section ; the second has an axis which crosses at right angles to a diagonal line of a cross section. Four-point bending tests are conducted at room temperature and at 300 C, using square tubes of three thicknesses (2.0, 2.5, and 3.0 mm), but having the same inside width (115 mm, excluding corner radius) and length (4000 mm). Local deformations and ultimate bending loads are measured. An empirical formula of the relationship between buckling wave growth and bending load is obtained by analogy with an equation derived from the results of bending theory for plates with an initial curvature. An empirical formula of the ultimate bending moment is also obtained by applying Karman's effective width theorem for critical loads in plates to square tubes. These formulas are proven to represent the behavior of test and analysis values very well.