Most commercial airliners are rated to take +2.5g and -1.0g. Why is this? Is it designed that way on purpose? To clarify, this would mean that the cost difference between -1g and -2.5 g is minuscule, so price (and thus engineering difficulty, which drives price) isn't really a factor.
Or, are there in fact significant difficulties with creating a frame that can handle a lot of negative Gs? (And if there are, what are they?)