It should be noted that cupping is not unique to glulam; the conditions which cause glulam beams to cup will have similar effects on other wood beams, including structural composite lumber products (LVL, PSL, LSL). Cupping of glued laminated beams is primarily caused by a moisture gradient across the cross-section of the member, where one face is significantly wetter than the other. This is generally caused by storage conditions that allow one face of the beam to dry, while the other face is exposed to moisture (i.e. stored on damp ground in the sun during construction or stored off the ground with the top surface exposed to rain and the bottom protected). The cupping will generally flatten back out as the moisture gradient lessens. Cupping should be minimal when the beam reaches moisture equilibrium. Increased checking may occur on the wetter face as it re-dries.
In and of itself, cupping will not reduce the strength or stiffness of the glulam beam, however, cupping can cause an eccentric loading, inducing torsion in the section. Severe cupping may also interfere with the installation of the beam. Generally speaking, a small amount of cupping will not reduce the design capacity of a beam. However, if there is a concern, a structural engineer should be consulted to evaluate the effects of any cupping-induced torsion in the beam.