----- Original Message -----
From: "Jim Lux" <jimlux@earthlink.net>
To: "jeremy-ca" <km1h@jeremy.mv.com>; <towertalk@contesting.com>
Sent: Thursday, June 28, 2007 12:38 AM
Subject: Re: [TowerTalk] Tower ACCIDENT
> At 05:27 PM 6/27/2007, jeremy-ca wrote:
>
>>Rohn offers 3 mounts using flat steel plates. What I have done is use the
>>idea of the self supporting mount, 25GSSB, which uses mounting holes in
>>the
>>4 corners. By drilling 3 holes and going from 5/8" to 3/4" bolts it
>>increases the strength a bit. Probably not necessary, especially on short
>>towers or light wind loads. The pier pin is not used on a bracketed tower.
>
>
> Hmm.. by going to a larger bolt, assuming it's the same material,
> you've increased the shear strength of the bolt by 20%, but decreased
> the strength of the flange that's being bolted by some amount (I
> don't have the drawing in front of me). Why not just use a stronger
> 5/8" bolt if you're worried about shear failure? You can get awfully
> strong bolts (Titanium or very strong steel)
A very basic reason; a local supplier has a stock of 3/4" J bolts and
nothing for 5/8". Flange strength is inmaterial at this point if the bolts
shear.
> On a bracketed tower, you're going to see almost all shear loads,
> because the bracket will act as a fulcrum. Even with a monster
> antenna in a huge wind, you're probably only going to see a few
> hundred pounds lateral force on the bolt. That's not a big
> amount. The bolt load is probably dominated by the vertical down
> force from the weight of the tower, unless the tower weight is
> supported by the base.
Very true and by using the TVRO bracket with a wider footprint that force
can be spread over a wider area. Home construction varies widely throughout
the USA. In NH 2x6 framing studs are required for about the past 20 years;
before that 2x4's were used. Down in the warm states Ive heard that wall
strength is much less so that using a HBU bracket may cause structual
damage. Also realize that many simply use a lag bolt into the siding and
dont even connect with a stud. There is a good probability thats what
happened to the guy who fell; they simply pulled out.
With the tower Im building I'll be able to get behind the studs with
horizontal 2x6's and use Grade 8 thru bolts from the brackets, we are
overdue for a good hurricane.
Carl
KM1H
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