Most every gantry crane I see LOOKS like it would rack easily from fairly light side loading (parallel with the main beam.) It must not be true, because there are lots of them out there with similar designs, and most of them have probably been tested by ham-fisted fools, and came back for more. I understand that they are not usually more than lightly side loaded. Still, the more they get out of line, the worse the joint loadings get. It would not be a slow failure...