What happens is that the upper spar is put in tension while the lower spar is put in compression, the resulting forces between the upper and lower spars is a shear force at a 45 degree angle to the spars where the upper spar is trying to elongate while the lower spar goes into compression (assuming a positive G load).
Are you sure about this? I would assume that with positive G load, the upper spar would be in compression. What am I missing?