[TowerTalk] PIPE ANCHORS

[Dorn Hetzel]

An I-beam probably beats both of those regarding the amount of steel at the
best distance from the center of bending,
as long as it is properly aligned with the load through the vertical column
of the "I" so that the two flat plates at the edges
get to use their full cross section to resist the tension (on the far side)
and compression (on the near side) resulting
from the right angle bending force exerted by the guy wires :)

On Mon, Oct 31, 2011 at 4:44 PM, Grant Saviers <grants2 at pacbell.net> wrote:

> You are correct that a larger tube wins for same amount of steel.  My
> comparison was same size/same moment and showed the extra steel needed
> for round in a 4" shape.  The #/ft measure was a bad choice on my part.
> It should have included "for a given size" since otherwise a very large
> diameter thin tube has a higher moment but is useless structurally
> (unless it is pressurized as in liquid fuel rockets or Coors cans).
>
> Grant KZ1W
>
> On 10/31/2011 6:11 AM, Jim Lux wrote:
> > On 10/30/11 7:59 PM, Grant Saviers wrote:
> >> Like your logic, but squares have more steel "further out" from the
> >> neutral axis so they win (or at least moment calculators say so)
> >>
> >> see  http://www.engineersedge.com/section_properties_menu.shtml
> >>
> >> Compare a 4" od x 3" id pipe with a 4" sq od x 3.5" id  sq tube.  Many
> >> shapes are there to play with.
> >>
> >> About the same moments and the pipe is much heavier.  5.5 sq in cross
> >> section of steel for the pipe and 3.75 sq in for sq tube, a 50% increase
> >> in weight per foot for tube.
> >>
> >> And there is a diagonal moment calculator for squares (squares win in
> >> all cases).
> >>
> >> What am I missing?  (of course there is a lot more to structures than
> >> this one number)
> >>
> > You're comparing a 1/2" wall round tube with a 1/4" wall square tube.
> > Since the moment is Router^4-Rinner^4 the added wall thickness adds
> > weight without much strength (tube is better than rod, as it were).
> >
> >
> > If you make the weight/ft (cross sectional area) the same (I did it by
> > making the tube bigger), you see the expected advantage for round.
> >
> >
> > Try 4" square tube, 3" inside (1/2" walls)
> > area (weight per length) = 7
> > moment = 14.6
> >
> > Compare to
> > 5" ID round tube, 4.01" id (1/2" walls)
> > area = 7.01
> > moment = 18.0
> >
> > If your constraint were "what's the strongest that will fit in a 4x4"
> > square, then square tube will win, since even a solid 4" diameter round
> > bar won't be as strong in bending:  that r^4 term out to the corners.
> >
> >
> > Large diameter thin walls is the strongest in bending and torsion, etc.
> BUT
> > the practical problem is that as it gets thinner, small defects become a
> > bigger problem.  If you column load it, it's even worse (the standing on
> > an empty aluminum can thing).. a tiny asymmetry in loading or the metal
> > will lead to buckling.
> >
> >
> > Yes, if you know the direction of the load, and it's always the same,
> > then some other shape will be better (a long thin sheet, for instance)
> >
> >
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