For three flange box beams (D-cell) Perry and Bruhn always show the stringers (flanges) as geometric points with no area or moment of inertia. No neutral axis is ever mentioned and moments and all forces are usually assumed to act exactly at stringers. The stringers at the rear, vertical web are assumed to take moment equally as a force couple meaning that the neutral axis is exactly in the middle between the stringers. I can't help but assume that as soon as a stringer is assigned an actual area with an actual moment of inertia and as soon as stringers (flanges) are optimized to take higher positive than negative G's, the neutral axis (which is never indicated in an example) will shift and the flanges will receive bending forces unequally as opposed to the examples. Unfortunately, no real life design examples are worked out or flanges sized. What is the correct way to go about this? Should the moments be assumed to be acting about a neutral axis rather than at a stringer? The former makes sense to me.