[TowerTalk] Rohn 25 Max Overturning Moment?

Thu Aug 29 01:04:51 EDT 2013

Well, duuhhh.  You think you're confused.  As you point out, for a guyed 
tower I don't need this number.

Yes, I understand your other points.

Oh well, it was an interesting exercise.  I would still like to know 
what the max overturning moment is so I can see whether or not my 
calculations were correct.

73, Jim	VE7FO

On 2013-08-28 19:11, Roger (K8RI) on TT wrote:
> On 8/28/2013 9:27 PM, Jim Smith wrote:
>> Hi folks,
>
> I'm a bit confused here.  You are looking at guying the tower, but
> working on a self supporting base figures.  The over turning moment
> isn't normally a consideration for a guyed tower, or is it? I've never
> used it. For a guyed tower the base only has to be held in place. IE:
> keeb the
> base from sliding sideways.  The roof has to be strong enough to
> support the dead load Including the load produced by the guy tension
> Plus the dynamic load (Horizontal wind force converted to downward
> force due to the guys.
>
> Then what ever you tie the guys into has to be strong enough to take
> the tension plus wind load
>
> 73
>
> Roger (K8RI)
>
>>
>> I want to put some Rohn 25 up on my flat roof (50 ft if it will work).
>> Will have to go to 4 point guying because 3 point would mean that at
>> least one of the guy anchors would be way too close to the tower base.
>>
>> I'm setting up a spreadsheet so that I can check out the feasibility
>> of various heights and antenna loads with the 4 way guy constraint.
>> Once I've come up with something that looks like it will work I'll run
>> it past a PE.
>>
>> I've studied Leeson's "Physical Design of Yagi Antennas" (and what a
>> wonderful resource this is - thank you David) and believe I've got a
>> pretty good handle on calculating wind loading but I'm missing a
>> critical piece of info - the max specified Bending Moment for 25G.
>>
>> I've got a copy of an old post to TT from Leeson in which he states
>> that the value for 25G is 5130 ft-lb.  (Lots of other good wind info
>> in here too)
>>
>> http://lists.contesting.com/_cq-contest/1995-09/msg00107.html
>>
>> I've got Rohn's latest 25G brochure off their web site.  Info doesn't
>> seem to be there.
>>
>> I've got a 2004 Radian/Rohn catalog.  On the foundation details
>> drawing B870725 2-19-88 there is a table showing the size of the
>> concrete base for 25G, 45G, 55G and 65G along with an overturning
>> moment value for each.  For 25G it's 6800 ft-lbs.  However, I don't
>> know if this is the value for the tower or the overturning moment the
>> block will withstand.
>>
>> Well, what the heck, I'll just take their data for the 40 ft
>> freestanding and work it backwards to find the overturning moment on
>> which it is based.
>>
>> Hmm..  9936 ft-lb  This is WAY more than what I was expecting
>>
>> HELP!!!
>>
>> This has certainly shaken my confidence in my ability to do this
>> stuff.  Here's how I came up with the number.  It is based on EIA
>> 222F.  The catalogue info is based on EIA 222E but I don't recall
>> there being much difference.
>>
>> ROHN 25G - 40 ft - 70 mph - no ice - tower base at ground level
>> one 5/8" line on each face (total of 3) - round antenna members
>>
>> I couldn't find a spec for the projected area of a 25G section so I
>> calculated it to be 4 sq ft.  If looking at a tower face this includes
>> the areas of the 2 legs and the bracing (true length) for that face.
>> It also includes the third leg and the projected (not true length)
>> areas of the bracing on the other two faces.
>>
>> 3 legs            3.13 sq ft
>> Horizontal braces 0.35 sq ft
>> Diagonal braces   0.57 sq ft
>> Total for 1 sect  4.05 sq ft
>>
>>
>> TOWER
>> Projected area = 16 sq ft  (4 sections @ 4 sq ft each)
>> Cd = 1.20 (because the tower members are round)
>> Proj Area X Cd = 19.2 sq ft
>> Kz = 1 (Calculated as 0.87 but allowable range is 1.0 to 2.58)
>> Qz = 12.5 lb/sf
>> Gh = 1.23
>> Wind Force = 297 lbs
>> Moment arm = 20 ft (because the wind force is uniformly distributed
>> over the full height of the tower)
>> Overturning Moment = 5943 ft-lb
>>
>>
>> COAX
>> Projected area = 6.25 sq ft
>> Cd = 1.20 (because the coax is round)
>> Proj Area X Cd = 7.5 sq ft
>> Kz = 1
>> Qz = 12.5 lb/sf
>> Gh = 1.23
>> Wind Force = 116 lbs
>> Moment arm = 20 ft (because the wind force is uniformly distributed
>> over the full height of the tower)
>> Overturning Moment = 2321 ft-lb
>>
>>
>> ALLOWABLE ANTENNA
>> Specified Area = 1.5 sq ft
>> Projected area = 2.25 sq ft
>> Cd = 1.20 (because the antenna members are round)
>> Proj Area X Cd = 2.7 sq ft
>> Kz = 1
>> Qz = 12.5 lb/sf
>> Gh = 1.23
>> Wind Force = 41.8 lbs
>> Moment arm = 40 ft (because the antenna is at the top of the tower)
>> Overturning Moment = 1671 ft-lb
>>
>> Total overturning moment = 5943 + 2321 + 1671 = 9935 ft-lb
>>
>> I would be very grateful if someone could point out where the
>> discrepancy is between my calculated value and the 5130/6800 values I
>> quoted.
>>
>> 73, Jim VE7FO
>> _______________________________________________
>>
>>
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