Bill Aycock wrote:
> I have a real problem with this. I have asked for a description of the way a
> force is applied to the tower, and how the tower section is mounted, to show
>
Shear strength is perpendicular to the tower (or brace) and even if we
know how it was measured it doesn't give us a lot of information as a
load is seldom applied in a direct shear mode. (It would be if tied to a
horizontal brace.) There is almost always some bending moment. I think
it was mentioned earlier that shear is in the weakest direction. It also
has very little to do with normal antenna loading unless the antenna is
located right at a guy point, or as in the case of a TIC Ring. Maximum
antenna wind load is a small fraction of the shear strength.
What scares me is watching some one climb one of the Aluminum towers
while bending the braces from their weight.
I wouldn't call it silly, but I agree, "shear strength" in most cases
tells us little in the practical sense.
73
Roger (K8RI)
> how this "strength" is defined. So far I have no luck.
> Tossing phrases like "Shear Strength" around without knowing what they mean
> is silly.
> Bill--W4BSG
>
> ----- Original Message -----
> From: "Bill Coleman" <aa4lr@arrl.net>
> To: "Richard Elizondo" <relizondo@ionoscom.com>
> Cc: <towertalk@contesting.com>
> Sent: Thursday, December 04, 2008 7:07 AM
> Subject: Re: [TowerTalk] Climbing and working on Rohn 25g/45g towers -
>
>
>
>> On Nov 23, 2008, at 11:43 PM, Richard Elizondo wrote:
>>
>>
>>> Here is the shocking sad truth:
>>>
>>> Rohn 25g section shearing strength - 2500lbs (this is the strength
>>> of the
>>> entire section, all three legs and lattice, not just that one leg
>>> you may be
>>> attaching to.
>>>
>>> So a Rohn 25g section could fail if a climber falls and the lanyard
>>> shock
>>> pack does not activate, or the climber is using a lanyard without a
>>> shock
>>> pack.
>>>
>>> Sadly enough the Rohn 45g section is not that much stronger.
>>>
>> However, in a fall arrest situation, it is unlikely that all of the
>> arrest force occurs in the shear direction. 25G has considerably more
>> vertical support strength. On a 200 foot 25G tower, the static load on
>> the base likely exceeds 2500 lbs to start.
>>
>>
>>> Solutions for the Climber:
>>>
>>> First and foremost - Never attach to the Z lattice of these towers
>>> for any
>>> reason.
>>>
>> If a single weld fails, then the Z bracing will deform -- that
>> deformation will absorb some of the fall energy. The remaining energy
>> will be directed at the remaining upper and lower welds. It seems
>> unlikely that a 6 foot fall would create so much energy as to undo
>> every weld on a section.
>>
>> It does seem seriously unwise to attach to the top or bottom "flat"
>> part of the bracing. There, a single weld failure would likely allow
>> the fall arrest lanyard to detach from the tower entirely.
>>
>> Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@arrl.net
>> Quote: "Not within a thousand years will man ever fly!"
>> -- Wilbur Wright, 1901
>>
>> _______________________________________________
>>
>>
>>
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>
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