-----Original Message-----
>From: Doug Faunt N6TQS +1-510-655-8604 <faunt@panix.com>
>Sent: Nov 14, 2007 2:45 PM
>To: jimlux@earthlink.net
>Cc: TOWERTALK@contesting.com
>Subject: Re: [TowerTalk] Unguyed, with house bracket?
>
>
> Date: Wed, 14 Nov 2007 13:52:30 -0800 (GMT-08:00)
> From: Jim Lux <jimlux@earthlink.net>
>
> Are you thinking of having a post next to the tower that you'd bolt to?
>
>Vertical post set in concrete. Attached top and bottom.
>
> Post or beam being vertical? Or at an angle?
>
> Don't forget that the house bracket essentially provides bracing in
> all directions (i.e. in the plane of the wall, the side of a house
> is pretty stiff, and if the wall you're bolted to is well attached
> to the adjacent walls, those walls provide the stiffness. ) The
> house is like a big fairly rigid box sitting on the ground.
>
> Consider it like you had a tower with a base that was 20x20 feet.
>
>OTOH, the couple I've seen (and one I disassembled) have been out on
>the eaves, and there's not a lot of structure out there.
The eaves aren't very strong in the "pulling it off" direction, but are fairly
good in the side to side or pushing back towards the house (since it's pushing
on the rafters). Kind of depends on how the fascia board is attached, or if
you bolt the bracket to the rafters sticking out (assuming they aren't just
decorative). These questions are why it's worth hiring an engineer to take a
look.
And the
>bolts to the tower from the house brackets didn't appear very robust,
>either. Clearly there is a spec for the "house" structure
>(somewhere).
Not really... it's not like there's necessarily a spec that "the eaves shall
withstand a force of X thousand pounds without permanent deformation". For
houses and similar things, you get a spec that sort of backs into it: The roof
shall be supported by rafters of such and such a size on no more than such and
such center to center distance, and shall be attached to the wall header with
so many nails of such and such size. More construction requirements than
engineering specs.
Granted, there are engineering specs (like seismic loads) that get checked
against the building codes, but still, it's a sort of roundabout process that
is more like codifying what people have learned will work over the decades.
It's not like wood frame construction houses are an unusual thing, so
empiricism works pretty well.
The engineering specs come in when you're doing something that isn't a
straightforward "stick built house on cast slab"... Like if you cantilever out
over hillside and need to sink a piling into the soil beneath the deck. Then
you get the loads analysis and the soils analysis, etc.
There are some structural engineers on the list who might weigh in on what the
"design loads" would be for the eaves or for a typical residential wall.
And that should be able to be turned into a force at the
>top of a vertical pole. For theings like I-bemas and pipes there are
>tables or formulas for allowable forces, I just don't have them or
>even know where they are kept.
Sure.. basic mechanics.. you know the section moment and the material
properties and can calculate the loads at the base with a given point load at
the end of the beam, etc. Before I just checked, I would have thought such a
summary table would be googleable, but apparently not. I use formulas from
the little handbook by Gieck and Gieck, and there are the standard textbooks
out there (like Roark), as well as handy laminated reference cards, but oddly,
no online version.
You might try this:
http://www.ncees.org/exams/study_materials/fe_handbook/index.php
specifically sections like:
http://www.ncees.org/exams/study_materials/fe_handbook/4_mechanics_of_materials.pdf
>
>For an example, would a railway rail of common dimensions do?
Not as good as a piece of pipe of the same material that weighs the same.
Diameter counts. (That's why your tower is a latticework truss, not a single
rod with the same mass)
I suspect, though, that to make this work, you'd need an amazingly large piece
of pipe or Ibeam, so that it would be easier and cheaper to just buy a
free-standing tower.
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