It's worth a look at N7NV's FEA analysis of guyed towers to gain some
understanding of what is going on.
http://k7nv.com/notebook/towerstudy/towerstudy1.html
If the tower is bending under load (and I think it does except for a
unique pier pin & guy configuration) then one face or leg is compressing
and the other is stretching. Whether the stretching exceeds the guy
load isn't clear from the FEA data, I think Kurt only shows the maximum
compressive load. However, with enough stretch in the guys (e.g. those
who use nylon rope for guys) the tower can fail in tension in a leg as
well as buckling in the compression leg (i.e. the tower becomes a
freestanding implementation). Note that the goal is for the tower to
stay in column, no matter how much it leans, and that is a big advantage
of the pier pin base with the right guys, as the base pivot eliminates
the bending load on the bottom sections.
IMO, both leg bolts are absolutely required!
Grant KZ1W
On 5/8/2014 11:39 AM, Roger (K8RI) on TT wrote:
On 5/8/2014 12:53 PM, Drax Felton wrote:
Why even need one bolt? Just during assembly? Once the tower is
guyed the downward weight holds it together. When is there an upward
force to seperate the sections?
Put one together without bolts and you will fins that the tower gets
shorter and shorter and shorter.
There is a lot of load on the lower joints, Then you have the vertical
load and the down component of the guy tension. On a windy day the
downwind leg gets considerably more load. With no bolts, the joint
rocks at the legs, slowly swedging the two sections together, The
inside gets smaller and the outside gets larger. If you've ever taken
a tower apart where the bolts were not tight enough, you find the bolt
holes enlarged considerably and the joints are loose.
Over guying a tower can easily bell out the lower joints. I've taken
100' plus 25Gs that required a jack to get them apart. A hydraulic
jack and two heavy boards. The tower sections between the bolts had
belled out. It's only a couple inches, but the holes were egg shaped
and the tower legs were noticeably larger.
I always drove the taper pins into the small hole. although it often
took both large and small to get the bolts to fitI'd get the bolts to
fit properly in the small holes, insert the bolts and drive the pins
in the larger holes till those bolts fit. Yoy can go large to small,
or small to large, but there's usually enough galvanizing to require
all holes to be enlarges with the taper pins. ROHN is very specific
about not drilling them out, ot screwing the bolts in.
As for two bolts, quite possibly it's to prevent rocking. Although
slight, with only one bolt per leg there is much less resistance to
rocking and enlarging the bolt holes than two. Once the enlarging
starts, it just accelerates and gets worse. Just a guess, but the
little 20G did have only one bolt hole. Sll heavier had 2, or a flange.
I would expect the small hole for alignment as the sd tend to enlarge
the hole much more than the large bolt.
73
Roger (K8RI)
Sent from my iPhone
On May 8, 2014, at 12:20 PM, Tom Nicholson <Gunsrus1942@Comcast.net>
wrote:
Jim's thoughts are the same as mine. Has anyone here purchased new
tower sections & if so, what size hardware did it come with? I
suspect that the larger hole is for alignment.
Tom W1ALZ
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