Taking a look at previous posts about the tower damage I sustained back in Tropical Storm Ian at the end of last September, it appears the last mention of what I was working on was on January 22nd, three days before my surgery. I had said I expected to have the short term fix in place before the surgery (Jan. 26).
Knowing the surgery was coming and that I'd be restricted by that in my ability to work for the heart of the best time of year to work on towers and such hardware, I came up with a workaround: a short term and a long term fix. The short term fix is up there now, but I need to improve that a bit and that should be done before the mandatory time off. The long term fix is to change my whole method of cranking the tower over.
My attempt to improve that hit a dumb mistake and I had to go back to original short term fix, and that ended up barfed up. It stayed barfed up until after my two week followup checkup and then I created a temporary fix that was better. It involved putting two bar clamps on the tower to position the halves of the wooden tower clamp. I wasn’t supposed to be doing lifting or anything like what I did, but these clamps went up on February 12. At least the tower was more secure.
With those clamps in place, I was able to re-cut a 2x4 to replace the one you see there (painted white) that those clamps held in position, (well, the clamps along with a couple of lag screws). It also replaced the one I barfed up the day before the surgery by turning it over one too many times.
That improved pressure treated 2x4 went up on March 9th, the first day I was cleared by the surgeon’s office to lift more than 25 pounds.
(yes, that does mean Left and Front)
With the temporary fix in place, it was time to work on the permanent fix for cranking over and supporting the tower a whole new way. As described in that January post, it started out based on designs I had seen researching the Aluma Towers MP-2 (mounting pole).
First step was building things in the CAD world and I started in the direction mentioned in that January post of moving the tower to other side of the pipe I crank the tower over against, and changing it so the tower cranks over from the top of the pipe (around 7-1/2 feet) instead of the hinge on the base. After a few weeks of very little progress, I took the approach of emulating the house bracket, but about a foot lower, and instead of mounting the bracket to the house, attaching the mounting bracket to that 4” schedule 40 aluminum pipe.
The tower is on the right, of course. Everything else is structural aluminum and stainless steel hardware. There are two main arms made of 3” aluminum channels with 1-3/4” tall sides. The very end of the arm is a piece of solid 1x2.5” aluminum bar 10” long. Those are the last major design change; when I started out there were going to be pieces of channel attached to the tower with U bolts, but then I had to connect two pieces of that channel meeting at a 30 (or 150) degree angle. How to do that just evaded me until I had the idea to rotate the channel stock pieces a few degrees and just grab the tower legs with straight, solid pieces of metal.
That 1x2-1/2 required some machining; first, the inside width of that channel is a bit under 2.5” and the bar was a bit over, so it needed to be reduced in width by around 0.040” (just over 1/32 inch). Second, the inside corners of the channel have about a 1/8” radius, and I opted to just cut off the corners, leaving a flat. It’s hard to read the dimensions in this view, but it says those two arms are each 4 degrees away from the centerline of the pipe to tower.
You’ll notice there’s no obvious hardware in the drawing joining the channels and the solid bars. That was done with a couple of 5/16 bolts. The U-bolts around the tower legs are 3/8”. It would be rather trivial to enlarge the 5/16” holes, but they seem pretty stable.
The completed assembly.
This has been in place for one week (since April 7, to be exact). Last week was the windiest in months, probably the windiest since the two tropical storms. My plan is to approach storms the way I always had until Ian. Before, I removed two lag bolts to free the tower from the house bracket and the one mounting screw in the concrete slab for the tower leg. The bolt that tore off a piece of that leg. Now I’ll remove that same bolt on that one leg, plus two nuts on each U-bolt around the tower legs. Then the tower gets cranked over and the antennas removed and stowed for the storm. Once those are secured, I’ll crank the tower back up and fasten it in place so that there’s no wind load on the antennas pushing on the tower. I get the feeling doing a dry run of this, or a few of them, would be a good idea.
While I feel awkward not having good mechanical analyses of various loads from varying directions on this bracket, it’s exactly as well-documented and analyzed as the original house bracket.
At least for the moment, the house bracket is still in place, but there is nothing attached to it. It will probably be taken down once I figure out what I’ll need to repair where it has been mounted since the early 1990s.
Looks good. Sadly, only one way to find out if it works right. Which all of us would prefer not to have happen.
ReplyDeleteLooking forward to Monday. Too bad they aren't doing this at the Cape, but there are enough webcams set around Starbase.
Well Done, SiG! I know all about being "On Medical Leave" these days. Finally started getting back into the basement. A soon as I'm "Cleared For Outdoor Duty" I'll get my 20M 1/4 wave ground plane moved, and my 5/8 wave vertical assembled and in the air.
ReplyDeleteI'll be streaming from the moment I'm down here on Monday. Fingers Crossed, Guys!
I'm not an electrical engineer as you are, but am a retired mechanical engineer
ReplyDeleteNot trying to be a "Debbie Downer" but the security of that fix would concern me
Your previous installation of being bolted to the side of the house provided a very solid base against tower movement in all directions -- and that house mounting point is usually quite strong, given it is tied to the roof trusses and the top of the house wall.
I looked up the Aluma pole you are using, and it specifies that it be put into the ground to a depth of 73" -- with no requirements for concrete backfill.
That's the first concern because Florida is usually a sandy soil and has a high water table--both of which limit side to side integrity
Added to that, anchoring the antenna structure the pole as you did-- the strongest retention of the tower is in the direction to/from the antenna pole-to tower- as that is a straight-line push-pull action
Other directional movement of the tower versus the pole is variably constrained by the anti-rotation strength of single u-bolt holding the fabricated brackets to the pole, and the amount of angle from the straight line from pole to tower.
And with a high wind blowing at a right angle to the side of the house -- restraint is basically depending on the anti-rotation of that single u-bolt
Anti-rotation of a u-bolt isn't a strong point of u-bolts,
It depends on the clamping force that can be achieved. If the pole was schedule 80 steel pipe-- it might be possible to achieve enough clamp that would last thru time-- but an aluminum pole will, over time, "reconfigure" itself due to the applied clamp pressure, and the clamp pressure will drop.
Perhaps unfortunately, I have been accused of being too much of a "belt and suspenders" kind of a guy when it came to designing my personal construction. At my previous home , I had a 60' self-supporting tower, and instead of following the installation directions to use a base consisting of 1'1/2 yards of concrete --I used 3 yards. Concrete was cheap vs the damage that could occur if the tower came down.
So just as a suggestion -- if it were my installation, I would not eliminate the prior house install bracket, and would add a couple "arms thar would extend from the angle bracket on the house out to the tower.
But YMMV
Thanks very much.
DeleteI know it's always a trade off between how much to include in a post vs everything I've written over the years, and if it's something that's embedded deeply in my mind, it tends to get skipped over. That means important stuff gets left out.
Important stuff: that isn't an MP-2, it's simply a schedule 40 aluminum pipe, and it's mounted in poured concrete, 24" on a side and 40 inches deep. That was put in back in '16, replacing the original pipe, which was smaller and steel. The real MP-2 is schedule 40 galvanized steel, but it's also rated to crank over and support towers many times bigger than mine. Mine is 20' tall with a 10 square foot wind load. The MP-2 is rated to 75 foot towers and the biggest towers in their catalog.
I've thought a lot about how I could distribute the force of those 4-1/2" U bolts and reduce the tendency of that pipe to bend or deform but as I'm way out of my home turf, don't really have any ideas. The system rotating to minimize the forces has me concerned.
My wife has repeatedly said there's no rush to take down the house bracket, and while I've thought about anchoring the tower to the wall, haven't come up with a way I like. It's concrete block, so punching a hole in it - or pulling a bolt out through a block - is much easier than if it was solid, poured concrete.
This is all complicated by the reality that I just turned 69 a couple of months ago, so how much longer will I be able to do the work to take down the antennas?
If you're comfortable with longer conversations on this, I'm sigraybeard at gmail.
I would never put a U bolt on an aluminum tube without a split sleeve over it to make it a "hard point" and distribute the bending load over a distance, say six inches to a foot. Just go to a bigger U bolt to fit around a tube with at least 1/8" wall thickness. Split the tube with a bandsaw. If you wanted to be really fancy, taper the tube about half the wall thickness, down from the center and in both directions.
DeleteAs you have it, wind in the right direction will crumple the tube at the bolt.
If the idea is to spread the pressure over a wider contact area, what about a semicircle cut out of aluminum?
DeleteI'm thinking of something like this, except with the wider contact area under the semicircular portion of the bolt.
https://www.mscdirect.com/product/details/08006603
I should play with the idea in CAD and see if I can make sense of it.
When the tool you have is a CNC mill every problem looks like something with intricate cuts on the mill.
Insert a heavy steel pipe (or solid round bar) whose OD is the same as the ID of the support pipe. Preferably a press fit. This will keep it round and prevent buckling.
ReplyDeleteFill the Al sched 40 pipe with concrete.
ReplyDelete