The wind standards include a wind profile formula, i.e. the wind
velocity at ground level is zero and increases with height. I doubt
that Rohn or any other tower maker wouldn't take advantage of the "free"
exposed area of the tower with essentially little or no wind load. That
makes the calc a little more difficult, but it's easy to segment the
tower into 10' lengths and use the average wind speed value for each one
in the moment calc. You could linearize the wind speed for each 10'
segment as the formula is of form k*h^x IIRC. Conversely, you could take
the Rohn max antenna load spec for 10, step 10 to 40' high and figure
out what average wind speed they were using for each segment. Kind of
tedious though.
Then there are some considerations about the turbulence in a lattice
structure, but that is above my pay grade.
Otherwise, your thinking makes sense to me. Thanks for finding that
Leeson post, some good data there.
Grant KZ1W
On 8/28/2013 6:27 PM, Jim Smith wrote:
Hi folks,
I want to put some Rohn 25 up on my flat roof (50 ft if it will work).
Will have to go to 4 point guying because 3 point would mean that at
least one of the guy anchors would be way too close to the tower base.
I'm setting up a spreadsheet so that I can check out the feasibility
of various heights and antenna loads with the 4 way guy constraint.
Once I've come up with something that looks like it will work I'll run
it past a PE.
I've studied Leeson's "Physical Design of Yagi Antennas" (and what a
wonderful resource this is - thank you David) and believe I've got a
pretty good handle on calculating wind loading but I'm missing a
critical piece of info - the max specified Bending Moment for 25G.
I've got a copy of an old post to TT from Leeson in which he states
that the value for 25G is 5130 ft-lb. (Lots of other good wind info
in here too)
http://lists.contesting.com/_cq-contest/1995-09/msg00107.html
I've got Rohn's latest 25G brochure off their web site. Info doesn't
seem to be there.
I've got a 2004 Radian/Rohn catalog. On the foundation details
drawing B870725 2-19-88 there is a table showing the size of the
concrete base for 25G, 45G, 55G and 65G along with an overturning
moment value for each. For 25G it's 6800 ft-lbs. However, I don't
know if this is the value for the tower or the overturning moment the
block will withstand.
Well, what the heck, I'll just take their data for the 40 ft
freestanding and work it backwards to find the overturning moment on
which it is based.
Hmm.. 9936 ft-lb This is WAY more than what I was expecting
HELP!!!
This has certainly shaken my confidence in my ability to do this
stuff. Here's how I came up with the number. It is based on EIA
222F. The catalogue info is based on EIA 222E but I don't recall
there being much difference.
ROHN 25G - 40 ft - 70 mph - no ice - tower base at ground level
one 5/8" line on each face (total of 3) - round antenna members
I couldn't find a spec for the projected area of a 25G section so I
calculated it to be 4 sq ft. If looking at a tower face this includes
the areas of the 2 legs and the bracing (true length) for that face.
It also includes the third leg and the projected (not true length)
areas of the bracing on the other two faces.
3 legs 3.13 sq ft
Horizontal braces 0.35 sq ft
Diagonal braces 0.57 sq ft
Total for 1 sect 4.05 sq ft
TOWER
Projected area = 16 sq ft (4 sections @ 4 sq ft each)
Cd = 1.20 (because the tower members are round)
Proj Area X Cd = 19.2 sq ft
Kz = 1 (Calculated as 0.87 but allowable range is 1.0 to 2.58)
Qz = 12.5 lb/sf
Gh = 1.23
Wind Force = 297 lbs
Moment arm = 20 ft (because the wind force is uniformly distributed
over the full height of the tower)
Overturning Moment = 5943 ft-lb
COAX
Projected area = 6.25 sq ft
Cd = 1.20 (because the coax is round)
Proj Area X Cd = 7.5 sq ft
Kz = 1
Qz = 12.5 lb/sf
Gh = 1.23
Wind Force = 116 lbs
Moment arm = 20 ft (because the wind force is uniformly distributed
over the full height of the tower)
Overturning Moment = 2321 ft-lb
ALLOWABLE ANTENNA
Specified Area = 1.5 sq ft
Projected area = 2.25 sq ft
Cd = 1.20 (because the antenna members are round)
Proj Area X Cd = 2.7 sq ft
Kz = 1
Qz = 12.5 lb/sf
Gh = 1.23
Wind Force = 41.8 lbs
Moment arm = 40 ft (because the antenna is at the top of the tower)
Overturning Moment = 1671 ft-lb
Total overturning moment = 5943 + 2321 + 1671 = 9935 ft-lb
I would be very grateful if someone could point out where the
discrepancy is between my calculated value and the 5130/6800 values I
quoted.
73, Jim VE7FO
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