Vertical column calculation for shelves

First post, hope I'm it's not in the wrong subforum. :B



I'm curious to know how to calculate the width and depth for the vertical column that will hold the shelves. Material i would use is pine wood. Most of us got used to measure by experience (or simply ask at the shop), but i'm curious about the math that's going on behind. Can anyone point me into actual equations of this particular problem?



The goal is to have vertical column that will not bend or swing under full capacity load, particularly not when new things are inserted on shelves i.e. to have a sturdy feel all the time.



Each shelf would carry roughly 30lbs of weight (this includes its own weight). Total of 13 shelves would be used, so that's 390 lbs under total load, or 97,5 lbs per column. Column height is 79". Each shelf is tightened by 8 screws. Shop owner told me to use 2x4" for columns. When i asked him how did he calculated that, he couldn't answer. Well, i'm curious to know...



Thanks!