[TowerTalk] Guyed + self supporting /2 ??

[Roger (K8RI) on TT]

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)