Towertalk
[Top] [All Lists]

Re: [TowerTalk] Guyed + self supporting /2 ??

To: "Roger \(K8RI\) on TT" <K8RI-on-TowerTalk@tm.net>, <towertalk@contesting.com>
Subject: Re: [TowerTalk] Guyed + self supporting /2 ??
From: "Patrick Greenlee" <patrick_g@windstream.net>
Date: Thu, 16 Oct 2014 08:04:24 -0500
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Roger, Thank you so much for your succinct reply that eschewed the terms compression, tension, shear, and column.

Stan, your call says it well "GO!"

I have a two section crank-up tower of unknown origin (20 ft per section.) I set a 5 inch OD 1/2 inch wall thickness steel tube into a concrete base such that the top is 15 ft above the concrete. Prior to installation of the tube into the concrete I had capped the top and installed one half of a home brew piano hinge. The other half of the piano hinge was welded to a steel plate that spans two legs of the tower and is welded to same at the 7 ft point above the bottom of the triangular tower base section. Voila, a tilt over/crank-up tower. I installed a handle (lever arm) to the tower extending downward from the bottom of the base section about 6'11" to get some purchase. I can easily tilt over and re-erect the tower manually with it fully retracted being careful to not let the top of the tower get lower than the pivot point as the upper section will go shooting out (once burned twice wary.) After adding a top mast extension and beam and control cables and coax I need to add a hand winch to control the tilt function.

I installed a 3 point guy arrangement at the top of the lower of the two sections. I only extend the tower when needed and retract it when not in use. I have replaced the original hand winch for extending the tower with a small 120VAC unit. I replaced the bearing in the systems only pulley. I aspire to rig redundant limit switches to enable remote raising and lowering. My station power is backed up by a 17KW propane fired genset that automatically replaces grid power if voltage or frequency go out of limits so an approaching storm can't strand
the tower in the extended position due to power outage.

This tower has been up nearly a year with no problems. I do not know the max wind speed or gusts it has handled but here in the heart of "Tornado Alley" (south central Oklahoma) we do get wind.

I'll report any failures or problems should there be any.

73,

Patrick NJ5G





-----Original Message----- From: Roger (K8RI) on TT
Sent: Thursday, October 16, 2014 1:50 AM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Guyed + self supporting /2 ??

On 10/15/2014 10:31 PM, Patrick Greenlee wrote:
Can someone supply an executive summary? What about the case when you have a crank-up/tilt-over tower made to be free standing but you guy the bottom section and reduce the size/weight of the steel reinforced concrete base?

The short answer is, "It should work."

73,

Roger.

Patrick NJ5G

-----Original Message----- From: Richard (Rick) Karlquist
Sent: Wednesday, October 15, 2014 9:42 AM
To: Roger (K8RI) on TT ; towertalk@contesting.com
Subject: Re: [TowerTalk] Guyed + self supporting /2 ??



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
_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk


--

73

Roger (K8RI)


_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk

<Prev in Thread] Current Thread [Next in Thread>