Byron,
All else (alloy, fabrication means, heat treat, wall thickness) being
equal, the slight increase in leg diameter will provide an small
increase in compressive and bending strength. However, a larger factor
is the steel alloy of the tube vs pipe and how it was fabricated.
Standard schedule 40 pipe (A500 48ksi yield) is welded from flat and
done. Tube might be welded or drawn over mandrel (DOM) cold extruded
from solid. Higher strength steel (A513 74ksi yield) is often used for
welded tube. 1020, 4130 (Chromoly), 4340 and many others are also
available DOM. Then how the tube was heat treated to possibly much
higher yield strength is another variable. Then when and where the
tube/pipe was made. A trip to a recycle yard will show lots of bad weld
split seams.
So lots of variables, and it is very difficult to field verify what
alloy you have. A weld seam inside is a clue, but not definitive. A
hardness tester can tell a lot about the heat treating or lack thereof.
I am not a PE, and the unknowns do create risks. Then your wind
exposure is a bigger variable, but if that is limited, your use of the
Rohn guidance seems reasonable IMO.
Grant KZ1W
On 1/15/2021 07:37, Byron Tatum wrote:
For information and a strength opinion- I posted recently about a tower in 10' sections that
appears as 65G but has some differences. On initial inspection this tower appeared identical to 65G except
that the flanges were 0.5" thick (65G has 0.625" thick flanges), the flanges were rotated 45
degrees from 65G orientation and the flanges were simply placed against bottom end of leg and welded (65G
flanges are punched out and slid over the legs, then welded at top and small weld at bottom inside of
flange). The flanges on this unknown tower have a drain hole in center, some drain holes are about 5/8"
and others are about 7/8" roughly.
The unknown tower has the same 5/8" zig-zag bracing as 65G, same square
dimensions on flanges and same bolt sizes (4 of 5/8" size bolts per flange).
Upon closer inspection I found this unknown tower has 2.0" OD tubing for legs (65G
has 1.9" OD pipe). Both towers have a 0.15" wall thickness on legs. I plan to install around
65' of this tower self-supporting using Rohn base plans as called out for in their 65G self-supporting
catalog. On the 10' section that will be concreted in ground I went ahead and cut the flanges off so
that I could insure the legs sink down in sand bottom of hole to provide good drainage. That allowed me
to examine legs and measure wall thickness.
I found out that this unknown tower has a slightly larger triangle dimension than 65G. I have a pair of 65G rotor shelves (that fit tightly in
65G) that are built similar to how the 45G/55G ones are, with a plate having a pair of U-bolts to butt up against the tower leg. These shelves when installed
inside tower and centered have a 1/8" gap between vertical plates (for U-bolt pair) and tower leg outside surface. I am making spacers to go in these
gaps. Even with larger diameter legs (2.0" on this tower versus 65G's 1.9") there are 1/8" gaps in rotor shelf fit. With
a slightly larger leg diameter (2.0") and the slightly larger triangle dimension I am curious if that gives this tower a little bit more added strength
versus if it was identical in spacings /leg size as 65G? I realize that the 65G having thicker flanges and being that they are punched out, slid over leg and
welded in two places is a stronger design. I just wanted to ask readers here that have mechanical knowledge how they feel about this unknown tower as compared
to 65G in my intended self-supporting use? I am not going to load the tower excessively in my opinion, but somewhat heavily as my plans are to put a 7 ele 6
meter LFA and a small 10/12 meter yagi on it. I am making 3 new "spider" pieces to allow my 65G top bearing plate to fit this tower; it has 3 pieces
that bolt to central plate and go out to flanges. Thanks for any evaluations/opinions on this.Byron W5FH
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|