wootens third law article discussion

huangyhg

wooten's third law article discussion
i just read on article by jim wooten titled
mr. wooten's very entertaining piece brings to mind two things.  one is my favorite engineering quote which i have posted elsewhere, but will do so again here:
"engineering is the art of modeling materials we do not wholly understand, into shapes we cannot precisely analyze, so as to withstand forces we cannot properly assess, in such a way that the public has no reason to suspect the extent of our ignorance."
- dr. a. r. dykes, from desktop engineering magazine
the other is a fun thought experiment that illustrates how caught up we get in the formulas.  nothing has hit me so hard as when i found out how ethereal the euler buckling equation is.  consider:
assume we have two steel pipe columns that, when loaded axially, will fail in an elastic buckling mode.  on one column, weld a plate to the top and load that plate axially while providing no moment resistance.  fill the other column with hydraulic fluid and insert a sealed plug in the end of the pipe that is allowed to move vertically, but does not allow the fluid to leak out.  load the top of the plug in the same manner that you loaded the first column.  now, let's look at the stresses in the columns at mid-height.  in the first column, you'll have an axial stress equal to the load plus half of the pipe's weight divided by the pipe's area.  on the second column, you'd have an axial stress from only the pipe's self weight, plus a hoop stress from the liquid pressure inside.  now, comes the key question:  which pipe buckles first as you raise the load on top?  the answer is neither - they buckle at the same time.  the euler buckling stress works in either case, as long as you use the total load divided by the total area of the pipe, although in one of the pipes, you would never measure that stress...
just read wooten's article, and i really don't know what the point was.  i do know that i get paid to rationally justify all those design assumptions, and in the end produce a structure that is safe according to the latest engineering principles.  and that paycheck i get produces a steady flow of happiness in the stuff my wife buys.  and her happiness produces a steady flow of whatever wooten's first two laws are regarding!
wooten's article is very interesting and thought provoking.  are the residual stresses from fabricating insignificant compared to the magnitude of stresses caused by loads imposed by dead, live and lateral loads?  apparently the equations used to calculate a structural capacity account for the residual stresses.  trial and error.  that guy did it successfully, so use it.  i tried that and it didn't work well, avoid it.