That kind of analysis was already done back in Hal Debolt's day, no need to reinvent the wheel. The properties of wood, especially balsa are too variable for elaborate mathematic analysis to be of much value. The simple answer to the original question is that in most typical applications, horizontal webs are a waste of wood, might as well just go with a continuous full depth spar from a single piece of wood. Vertical grain stationed against each rib keeps the spars in their original, as designed positioms and also gives the ribs more structural value.