Strength of I-Beams in Combined Bending and Torsion

In the design of a structural steel framework it is considered good practice to avoid subjecting I-beams to the combined effect of bending and torsion, because the torsional resistance of I-beams is usually low. It is often easy to determine a layout in which torsion is either entirely eliminated or greatly reduced. Sometimes, however, this is impracticable or impossible, and cases exist where beams carry eccentric loads and are therefore stressed in both bending and torsion. Laterally unsupported building spandrels offer a common · example. Also, it is often impossible to avoid eccentricity of loads on beams in industrial structures because of the extreme complexity of the equipment they are designed to support. In these, and numerous other cases, the designer is called on to investigate stresses in I-beams loaded in combined bending and torsion.