Owen Duffy wrote:
Jim Lux wrote in
:

...

that would be what's known as a "pier pin" base, and, as noted, it
greatly reduces the torsional load on the tower. Whether the force
causing the tower to fail is torsional, I'm not sure. Guyed towers
fail by buckling from the compressional force exerted by the guys.


The vertical elements of a tower framework should take their load in line
with the element. They are slender elements which are tied in by bracing
to prevent buckling.

It seems to me that in a typical construction where the end of that
vertical element in each section is not free to hinge, that twisting of
the section deforms the vertical element and would assist buckling if the
downwards force in the element is very large... as it is on very large
structures.

Actually, you'd look at the diagonal braces, too. On several theatrical
truss designs, the bending load limit is set by the compression buckling
of the diagonal braces, not the tension or compression of the main
tubes. A torsional load will put a bending load on the vertical main
tubes, but a longitudinal load on the diagonal braces (compression or
tension depending on which direction they go). probably also a bending
moment on the diagonal struts because they're welded joints.

Jim Lux wrote in
:

Actually, you'd look at the diagonal braces, too. On several
theatrical truss designs, the bending load limit is set by the
compression buckling of the diagonal braces, not the tension or
compression of the main tubes. A torsional load will put a bending
load on the vertical main tubes, but a longitudinal load on the
diagonal braces (compression or tension depending on which direction
they go). probably also a bending moment on the diagonal struts
because they're welded joints.

Jim, thinking about this a bit more...

There are two broad ways to fix the bottom of the tower (if it fixed).

One is to embed the bottom section in concrete, the other is to have a
plate on the bottom tower section and fix it to threaded studs /
framework drilled or embedded in the concrete base.

The second allows for find adjustment of the plumb of the first section
(eg using nuts above and below the plate) wheras the first method does
not.

I wonder what the preload on the lower tower sections is if the first
section is fixed and not plumb (for whatever reason). Perhaps that is an
aspect that the design engineer cannot quantify, so the easy way out is
to specifiy pier pin mounting as (properly done) it should eliminate
bending moments caused by lack of plumb of a fixed base section.

Owen