"Buckling of the web of steel beams may occur near the supports due to the compressive component of the shear.  The lower flange of a cantilevered wide-flange beam, being thin and compressed in bending by vertical loads, may also buckle.  This buckling occurs in a lateral direction, [...], and twists the beam, besides bending it." —from Mario Salvadori, in Why Buildings Stand Up: The Strength of Architecture, W. W. Norton & Company, 1980, p.88