Wind pressure on the tower varies as the square of the velocity. Going
with the max velocity and looking at really tall towers is impressive,
but why look at numbers from heights, or speeds "most" us will never
experience? This was a 70 MPH area until a single derecho caused it to
be bumped to 90, probably at the behest of the insurance companies. At
this location I've never experienced gusts over 60 MPH in 33 years. The
pressure on a tower is not linear as wind speed tends to increase as you
go up and that increase is not linear. My several thousand hours as
pilot in command verifies that is a given. Nearby objects can either
lower, or increase the speed. They also create turbulence. As we are
almost surrounded by 70 - 80 ft trees, the wind speed at 40' here is far
less than it was when I lived out on the farm which was flat land with
very few trees.
Knowing the total forces on a tower is a straight forward summation.
Unfortunately you need to know the wind speeds at the various heights as
they all add up. Calculus (the summation) can be replaced by
calculating the force at various heights and adding the result at say
every 10 feet. Yes, it takes a bit of "relatively" simple math and even
with the same math background as an engineer, I'd need to get the books
out. The wind area for the tower sections and the wind velocity at that
height
As I'd been on the tower with winds at 30 - 40 MPH and it was too windy
(well over 40) to go "up there" . The gusts were likely in, or close to
60 MPH. So the scope observations were at well over 40 MPH. I'm not
finding fault with the modeling. I finding fault with applying max
values to what most hams can expect. We should be applying the
realistic values for where we live. As for the difference in stretch, I
used two guy anchor posts a little over 115 feet apart, a loos gauge,
and a comealong. I'll leave that for someone else to confirm as I do
not have that much 6,000# Phillystran, nor can I afford to purchase it
just for an experiment.
73, Roger (K8RI)
On 9/24/2017 Sunday 6:40 PM, Grant Saviers wrote:
While I haven't built a tower FEA model and run my own Finite Element
Analysis to check Kurt's work, I've been through his work several
times and can't find fault. Not surprising, since he is much more
qualified than I am as an ME. I did have a good discussion with him
at the Visalia convention this year about his modeling assumptions and
inputs and that confirmed IMO that the modeling is well done. It is
worth reading to understand why your experience at 30mph is different
than the math at max wind (I'll go with the math).
I remember looking at the tall guyed broadcast towers east of I95
south of the Mass Pike when a hurricane was warming up. The lean was
noticeable, maybe 10's of feet at the top. Actually, there is DATA in
the NAB Engineers Handbook
https://books.google.com/books?id=K9N1TVhf82YC&pg=PA1642&lpg=PA1642&dq=broadcast+tower+wind+sway&source=bl&ots=RtqdbP9_tu&sig=7FFsqw2hXl5vVgMYpM-qk5-TWig&hl=en&sa=X&ved=0ahUKEwjoj_eK7b7WAhXLr1QKHVBJBsMQ6AEIOTAD#v=onepage&q=broadcast%20tower%20wind%20sway&f=false
shows a table 6.8.3 for 1100' tower sway at 50 mph fastest mile of
0.57deg. 100mph = 4x the force so maybe 2 degrees or 38' at the top.
Grant KZ1W
On 9/24/2017 14:15 PM, Roger (K8RI) on TT wrote:
I've been at the top of my 100' 45G when the wind became so strong I
could no longer work (probably 30-40 MPH) 40 MPH will bounce you
against the tower and difficult to breathe with the wind 90 degrees
to face on. There was definitely no slack on the downwind side, nor
was there any sense of movement. Certainly the tension on the
downwind side is reduced a bit, but it's a long way from being
slack. Whether EHS, or Phillystran the mechanics should prevent the
downwind side from going slack. My experience with the array
http://www.rogerhalstead.com/ham_files/Tower29.htm Makes it difficult
to accept the 15 to 18" deflection. Maybe with a full size 40m Yagi,
or 20m long john
With a telescope (lee side of the house, out of the wind) on a day
too windy to climb the tower, I could barely detect movement of the
tower top with the big array up there.
NOTE: scope and tripod weigh over 140#. I need to get something
lighter. A 77 year old back and Arthritis means that scope and
tripod have set in the shop for quite a few years. <:-(.
73, Roger (K8RI)
On 9/24/2017 Sunday 1:25 PM, Grant Saviers wrote:
My reference for noting that guys go slack is the K7NV FEA tower
study. While Kurt doesn't note the stress in the downwind guys the
lean of the tower at the top is pretty convincing to me that they
slack. For 100' tower the top moves 15 to 18" downwind for embedded
base and as much as 54" a for pier pin base. While the wind loads
in the study are high, nothing fails in these models.
For big commercial towers with very heavy guys/ft I have no opinion.
http://k7nv.com/notebook/towerstudy/towerstudy1.html
Grant KZ1W
On 9/23/2017 22:59 PM, Roger (K8RI) on TT wrote:
It depends on the wire (size, length, composition, and the tension.
I used to regularly climb a 200' commercial tower that had 1" wire
rope for guys. there was an 18" tall, guy attachment point (don't
remember the height) Those guys would ring like a tuning fork when
struck with with a metal tool. it was way above 440
Hz. That kind of vibration will unscrew a lot of hardware, but not
with that kind of tension. The tower was a very solid 2' on a side
in 20' sections, down onto an elevated pier pin base. The elevated
base was a good 6' on a side. The pier pin was about 7' to 8' above
ground. I have no idea as to how deep the concrete went.
Turnbuckles on 1" guys are BIG!
73, Roger (K8RI)
On 9/22/2017 Friday 8:04 PM, Steve Maki wrote:
On 9/22/2017 19:36 PM, Grant Saviers wrote:
At the specified torque values for structural bolts, the fastener
is far less likely to loosen. That isn't the static load in a
turnbuckle which is exposed to many of the conditions that cause
common fasteners to loosen.
You would think so. But guy wires (and turnbuckles) seem to be
under a different dynamic situation than nuts and bolts in a motor
for example. Vibrations are relatively slow, and tension never
nears zero in a system with proper pre-tension. I think.
-Steve K8LX
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Roger (K8RI)
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