[TowerTalk] Guyed + self supporting /2 ??

Patrick Greenlee patrick_g at windstream.net
Thu Oct 16 09:04:24 EDT 2014 

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 at 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 at 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
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-- 

73

Roger (K8RI)


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