In a message dated 09/29/2001 1:30:41 PM Eastern Daylight Time,

<< Sorry for coming into this a bit late, but I started to wonder as I read
through all the discussion about this if there might be an alternative that
hadn't been mentioned or thought of.. What about using 4" box steel tube,
welding 4 pieces together to form a larger box? The inner "+" pattern of the
tube walls would add a significant amount of rigidity to avoid flexure and
possibly eliminate any harmonic resonance. A center notch-cut in each piece
would allow the welding of a piece of threaded rod to the tubes which could
pass through a top plate and interface directly into a wedge base.

Thoughts and feedback from all appreciated!

-Doug >>

Having the additional places to bolt or weld the end plates to would be
convenient. But other than that, it's a losing deal compared to a simple pipe
or box section.

A square box section isn't much worse than a cylindrical pipe pound for
pound, but your solution puts half the mass inside the outer perimeter, which
is much worse than either a pipe or square box section. Both of those have
all the mass in the outer perimeter where it does the most good.

So all the mechanical properties of the pier itself are not as good as
putting that extra mass out at the edge instead of near the center. That is
true even if the whole shape were extruded as a continuous, single cross
section. Having to fabricate it by welding makes things even worse, as it's
not practical to weld the entire surfaces together, and any rounding of the
corners of the individual boxes cuts into the critical outer perimeter
thickness regarding torsional stiffness, only partly made up for if welded
well.

So it was a good try, but putting 50% of the mass in the interior of the
hollow section hurts rather than helps both the torsional stiffness and
bending stiffness. In bending, it's the equivalent of adding wings in the
center of an I beam, like typing a "-" on top of an "I" or making it a
tri-plane instead of a bi-plane. Unfortunately, the center of an I beam does
absolutely nothing to resist bending, which is why the shape of an I is used
to begin with, with the center of the I just thick enough to resist shear and
buckling.

John

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