[TowerTalk] Rohn 25 Max Overturning Moment?

[Grant Saviers]

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
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>