I think what halls120 is getting at is that even at or past the certifiied G-load design of the airframe and its wing/wing box, it was tested well beyond that to show a) it (and others built like it) will at least survive to the certified G-load specification (assuming type test theory) and b) there's a whole bunch of margin built into the overall structural design of the aircraft given uncertainties in structural models and bulk material properties, variations in manufacturing builds and tolerances, as well as lot-to-lot variations in the materials used to build the aircraft.

David