Square Tubes Bent About a Diagonal Axis (05-1454)
Bo Dowswell, Structural Design Solutions
Fouad Hilmy Fouad, University of Alabama at BIrmingham

Tests have shown that square tubes bent about a diagonal axis have greater strength than is predicted by the current design methods. The higher moment capacity can be attributed to the reserve strength after the cross section has reached its theoretical yield capacity. The shape factor for a tube bent about its diagonal axis is much higher than its shape factor when bent about a geometric axis. The local buckling capacity is also increased for tubes bent about the diagonal axis. The strain gradient on the tube wall is triangular when the member is bent about the diagonal axis. This increases the local buckling capacity when compared to tubes bent about the geometric axis, which have a uniformly compressed tube wall. A design method was developed to determine the allowable bending capacity of square and rectangular tubes bent about the diagonal axis. Compared to the allowable capacity in the 2001 Supports Specifications, the proposed design method was shown to greatly increase the allowable moment for compact tubes.