On 10/15/2014 12:08 AM, Roger (K8RI) on TT wrote:
On 10/15/2014 1:28 AM, Richard (Rick) Karlquist wrote:
can cause it to try to tip the base because the part of the tower
below the guys bends in a strong wind. This is the fallacy involved
in the idea of using just enough base to keep it from sinking into
the ground.
This is not a fallacy! Originally towers were designed to be strong
enough that this would not be a problem. If properly guyed, it's still
not a problem. With the proper guys there is very little bending moment
at the base.
The issue is not the strength of the tower, it is the stiffness.
If not sufficiently stiff, the tower bows out away from the wind
and will tilt the base if it is not big enough. Maybe what you mean
by "properly guyed" is to have multiple levels of guying. Yes
that might allow you to use a minimal base. The whole advantage
of using self supporting type sections in a guyed tower is that
you don't need so many levels of guying.
That is a complicated issue, but the guys will even out the forces on
the base from wind, but they will INCREASE the load on the base. because
of their tension and wind load.
The compressive load on the base from the guys is less than what
would have been the load in the self supporting case due to
bending moment. The issue is with tapering. If the upper sections
are very lightweight, they will not be able to handle the guy
forces, even though they are less than the forces at the bottom.
Simply because the bottom sections are much stronger. The solution
is to not taper until above the guys.
In general, self supporting towers should not be guyed. Guying reduces
the design load limits of the self supporting tower. Crank up towers
should not be guyed as it can greatly increase the load on the tower
cables.
Actual modelling shows that guying can increase the load limits if
properly applied. In the crank up tower case, UST specifically
says its OK to guy the base section. The load on the base will
be less than the self supporting case and in any event, the base
is in compression and not be the weak link.
Is the tower base designed to support the additional weight? Guying the
base is the same as adding additional weight to the tower.
When changing the installation of a tower to something different than
the manufacturers recommendations, it's wise to seek that companies
input as well as a certified engineer.
73
Roger (K8RI)
I have a certified engineer doing the modelling. I'm just reporting
here what he is telling me.
Rick N6RK
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