Sounds like you have your answer already but I just want to mention a couple of observations from the details given:
1) 15 g's amounts to a 15 ft radius pullout at constant 56mph [a=(VxV/r)+g] - is that enough?
2) There will be an abrupt change in wing stiffness at the rib bay where the tubes end resulting in stress concentration if not addressed. It can be reduced by reinforcing the spars in the first bay or two outboard of the tubes to smooth the transition in stiffness.

I'm just trying to add value to your conversation guys. I don't even know if this is a trainer or a low wing pattern ship or what power you intend. Those details will make a difference. Crate.