----- Original Message -----
From: "Jerry K3BZ" <k3bz@arrl.net>
To: <towertalk@contesting.com>
Sent: Saturday, April 09, 2005 5:03 PM
Subject: Re: [TowerTalk] Guying a self-supporting tower
> Doug wrote: "...leg compression will be increased with properly tensioned
> guys." Well, of course it will.
>
> But why do guys on a self-supporting tower have to be "properly
tensioned"?
> I thought they would be just for "insurance", a little extra support when
> there's a little extra wind, or as a last stop-gap against a catastrophic
> failure, or with aluminum towers, to limit the motion that "eggs out" the
> bolt-holes. Why not make such guys "snug" but not "tensioned"?
>
> 73, Jerry K3BZ
Well.. if the problem is loading a flexible structure as a column, then
"extra insurance from a guy" is the wrong approach. At the very time when
you want the tower to flex (because it's loaded to the max), you introduce
an extra axial load which aggravates the buckling tendency.
If you want to constrain where the crumpled wreckage winds up, then the
"loose guy" approach would work admirably. This "safety cable" kind of
scheme is very, very common in lots of applications. Look up in any
(reputable) theatrical situation, and you (should) see steel cable loops
over the supporting pipe or truss and around the lighting instrument, so
that if the clamp fails (or the lighting tech drops the instrument) the
instrument doesn't go crashing to the ground (or on the talent's head).
Same thing in some seismic restraint setups (for instance you'll see it in
suspended lights in factories).
If you've got motion that deforms the holes, you've got a fundamental design
problem that should be addressed another way (as in, don't try to carry the
load in shear on a steel bolt in an aluminum hole?). Something like jack
screws or wedges would be a better solution. Lap joints bolted with shear
bolts should be constrained from motion for just this reason. In my
recollection, you'd put enough tension in the bolt so that the clamping
force is high enough that the friction between the two plates is higher than
the applied loads. The bolt is there as a clamp, not as a shear bearing
member. You could design the bolt so that it could take the full shear load
as a safety measure (unless there's some other way you wanted to do it).
You can also design so that the bolt shank is a very snug (as in
interference) fit in the hole (certainly not if the threads are in the
hole!!). Doing this in highly dissimilar metals (i.e. a steel bolt in
aluminum plate) is a bit dicey, since the hole will probably deform. (this
is the classic problem of distributing loads among members with different
elasticity)
You'd have to do some calculations to make sure that the required clamping
force isn't more than the yield strength of the metals being clamped (the
bolt pulling through the wood framing member problem).
Maybe accepting the fact that the holes will "egg" is the best overall
solution. The holes being an egg (if it's not too big) will not reduce the
strength of the system too much, so it won't fail catastrophically in a high
wind. There's some rough calculations below that show that the required
clamping force is pretty high (as in impractical).
Here's a sample rough calculation...
Say you wanted to withstand a force of 10000 pounds tending to cause two
aluminum plates to slide past each other ( a 100 pound load at the top of a
100 ft tower, with a 1 foot base). Assuming the friction coefficient is
around 0.15 (I have no idea if this is reasonable, but that's what static
friction for steel on steel is)... you'd need 10k/.15, or a clamping force
of 67k pounds. That's a bunch...
If you wanted to develop that load with a steel bolt, assuming you want to
keep the load to less than 50kpsi on the bolt, you'd need a cross sectional
area of 1.3 square inches. Maybe a 1.5" diameter bolt would do? (and boy,
are you going to have to really crank on that wrench!)
You'd also need to keep the clamping area fairly large, so the aluminum
doesn't fail.. 6061-T6 has a yield strength of around 40ksi, so you probably
want a clamping area of 3 square inches. A 2.5" diameter steel washer
that's fairly thick (so it doesn't deform too much) might do.
By the way, this gets kind of tricky if there's more than one bolt through
the plates you're clamping, because the loads don't necessarily distribute
like you think, and the clamping forces might deform the plates reducing the
clamping area.
There's a whole lot of engineering that goes into clamping that aluminum
head on the aluminum block with steel head bolts.
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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