Base requirements for a tiltable mast installation

[VK3JLS]

Hope someone can help me here.
I have recently taken delivery of a second-hand 7.5 metre mast (with a pole extension it can go to 14 metres) that is your typical triangular lattice type mast. From all accounts it looks as though it was meant to be a guyed system (eyelets at top, small pipe arrangement at bottom) however my intention is to mount this as a tiltable arrangement, through the installation of a 4-5 metre post in concrete, and some suitable hardware to enable the tilt at around the 3 metre height, with a winch type arrangement for raising and lowering.
Being second hand, its somewhat problematic getting any engineer's computations for the mast and footings, and noting other posts related to Macedon Ranges Shire approval process, I've decided to use this as a <8metre mast and avoid building permits, plans, etc. The intention is to have a hf beam and rotator on top at some stage.
However, what I am interested in is the design of the footings for such an arrangement. As it is going to be restricted to that height, I'm assuming I won't need the amount of concrete that has been suggested on other posts for 14metre and higher masts.
I'm hoping that there might be someone out there who has had a similar experience for a small mast, and can give me some advice as to what size footings they have used, or would recommend. Also any indication of the size post that they have used would be welcomed as well. At this stage my thoughts are to use a 100mm square post, with up to 2 metres in the ground.
Any suggestions or comments would be welcomed.
thanks in advance

John

[VK2KRR]

Hi John,

Another consideration is the type of ground/soil you have? If its sandy it will need more in the base than a rocky clay soil.

Any photos of the tower?

Not knowing your soil type, but if it was here, that size mast/tower 2m down wouldn't be a problem. A 500mm dia hole.

100mm square post is probably OK depending on the wall thickness, and the size of the tower face (each side of the tower).

Any more details?

[VK3JLS]

Thanks for the response.

From soil test done 6 years ago:- Soil Classification is "H" - olivine basalt, with alkaline derivatives, limburgite, scoria, minor tuff and sand. The soil type would be expected to be highly reactive with regard to soil volume change under soil moisture variation. Believe me when I say it isn't sandy!

Wind Region here is classified as A5 from Nally's chart.

Tower face on each side is 200mm. Not sure what size wall thickness is available on the proposed post, but would be looking for something around 5mm(?) thick. I'm looking for suggestions on this aspect.

Here's a pic of the mast (its been pretty dry around here!!):-

thanks again

John

[VK2KRR]

John

Are the main legs of that tower hollow ? I pressume they would be. I have a similar one here, not up in the air, its easy for them to rust from the inside out without you really knowing, so something to watch for.

Not sure how big your proposed antennas are, but I would probably be concerned using this tower without guying the top if it had a HF yagi on the top. Not sure what your winds are like, If it doesn't blow much, might be OK.
Its hard to say without physically seeing it.

100 x 5mm should be OK for the base post.

Someone else may have a different opinion?

[VK3JLS]

Thanks Leigh.

The main legs of the tower are actually solid. One part of a leg between the lattice joins was somewhat dented in transit; if it had been hollow, it certainly would have shown up in the dent (which I managed to straighten out without any visible damage)

The intention is to have a triband hf yagi on the tower, but only at the 8 metre height (well +/- a few cms!)
Again, thanks for your inputs.

John

[VK2KRR]

OK John, that sounds good then, no rust from the inside and would be more solid that I expected.

Should handle the yagi OK I would think at the 8m height without to much drama as long as the antenna is not to huge and winds aren't to savage.

[VK3AV]

Hi John,

I have a similar mast and it is also having a rest on the ground currently. Mine is a 2-stage that ought to rise to about 13m and was designed to be guyed. It is about 260mm per side.

I do not want to guy the mast and after much searching found the following site with details of a Nally style tilt over mod to a similar mast. http://vk6ysf.com/Radio_Tower.htm As I also have the ground mount with the large pin that goes through the hole in the pipe at the base of the mast I am now inclined to have it pivot from the base and place a post behind it similar to what you proposed. With a suitable brake winch I could lower it from the top of that post as well as have a bracket against which the mast can be secured when in a vertical position.

Whilst I can not imagine that the upper section would 'take off' out of the lower section, my thought is also to have some cable from the very top to high on the lower section to restrain the entire assembly. I will probably also reduce the overall raised height by re-fabricating the bracket that the upper section rests upon to a lower position thereby having more of the upper section within the lower section. My plan is to install a hex-beam on the mast and the rotator will be in a fabricated cage bolted to the top plate on the mast.

I am at 530m ASL and it does get quite windy so a 6-band hex should suit my location quite well.

Regards
Bernard
VK3AMB Kinglake West

[VK3JLS]

Thanks, Bernard.
sorry for the delay in this response, however have been in VK2 land since Sunday.
Have seen the VK6YSF site and my thoughts are along the lines of doing something similar. Not sure about the height he has used, however. While understanding that the mid point of the mast would provide the easiest lowering/lifting point, I believe a suitable brake winch mounted on the post, with a proper pulley arrangement will allow for lifting/lowering of the mast without the need for the post to be 3.7metres high: I'm still thinking somewhere around 2 to 2.5 metres high for the post, and I can source a 150 x 150 square hollow section steel post, with either 6 or 8mm wall thickness; this should provide adequate support.
I'm still looking for a suitable post hole digger to give the diameter as recommended in an earlier posting; I can readily source a 350mm diameter digger, but 500mm seems to be a bit harder to find - still looking.
Ideally, I would like to have the base of the mast "standing " on the concrete footings, rather than the weight of the mast being taken on a pivot pin at the top of the pole, however I haven't yet got a clear idea as to how I could achieve this - still thinking.
I'm at about the same height ASL in Romsey, but as I am sticking to the 8metre height restriction, I'm hopeful of not having to guy the mast as well. I'll see how it goes, and if necessary, guys could always be installed at a later date.
While it will be a work in progress for some time, I will add some photos once the project starts to "get off the ground" - pardon the pun.

thanks for the inputs

John

[VK3MEG]

I had guy from sunbury / romsey cant remember Drill my holes 450 could have been 600 for around $300 thats his 3 hour minimum. found yellow pages
cheers

[VK3JLS]

Wow, has it really been 8 months since my initial post on this topic??

Since then, I have developed some drawings for how I might tilt the mast, purchased an 8 metre 100 x 100mm post, some 6mm steel plates, a number of u-bolts and other hardware, and then started doing some fabricating of the necessary items. In more recent times, a 2metre hole 450mm diameter has been dug, and the post concreted in. A bit more welding here and there, and the addition of a brake winch to raise and lower the mast has all taken place. As of 27 October, the whole exercise was completed with a 6 band hexbeam attached to the mast pipe, and the whole assembly was winched up to its final position. The tower tilts at around 2.1metres from the ground. The weight of the tower is taken by the 3/4" pivot pin at that point, although once vertical, I have slid some metal plated between the mast base and the concrete footings.

Couldn't be happier with the end result, with all my doodlings and musings coming together with the desired result.

My thanks to all who contributed to my initial postings.
John

Pics as follows:-

[VK3QI]

John,

The way I read it, you have a 7.5 metre tower section with about 1 metre additional pipe out the top with the hex beam above that.

Your support pole is 2.1 metres out of the ground (approx.), meaning you have 7.5 + 1 - 2.1 = 6.4 metres above the pivot point.

Your tower section is 200mm per side.

I WOULD STRONGLY recommend that you guy the structure, as the bending and twisting moments on the tower at the pivot point are approaching the limits of the tower, even in moderate winds.

Even the Nally tower with 320mm per side only extends the upper tower section 5.1 metres out of the bottom section.

Cheers

Peter VK3QI

[VK3MEG]

congrats glad you got it up I would give what Peter vk3qi recomended some thought my tower is similar and has a bigger array of antenna's but the piviot point doesn't hold mine moves a bit in the wind but that is allowed for in the design specs (mine is cyclone rated) and I lower antennas when we get some serious wind just to be safe and to limit wind stress on the antenna's.
enjoy the dx
cheers

[VK3DXE]

Hi John,

Many thanks for posting this thread. I plan to get my tower up soon, and this has helped with a couple of ideas, also taking note of Peter's comments.

Could you, for the sake of others like myself, advise where/how you sourced/fabricated the various bits of hardware? I think this would be invaluable, as so often we see lots of tower tech, but the task of sourcing or fabricating the hardware becomes the most difficult part of the the project.

Thanks.

[VK3JLS]

Peter,

Thanks for the suggestion. At the outset, I would suggest that your figures are perhaps a little bit on the pessimistic side, (which in a situation like this is probably the best way to be!).

The way I read it, you have a 7.5 metre tower section with about 1 metre additional pipe out the top with the hex beam above that.
Your support pole is 2.1 metres out of the ground (approx.), meaning you have 7.5 + 1 - 2.1 = 6.4 metres above the pivot point.

I was conscious of not exceeding 8metres in height for council planning/permit issues, so the pipe extension above the mast is only 30cms, not 1 metre. Furthermore, while the pivot point might be 2.1 metres above the ground, the mast is quite secure and firm above that height, being clamped at the bottom of the mast, and again at the top of the tiltable plate; this places the top of the mast support structure at 2.5metres, so your calculations should read 7.5 + 0.3 - 2.5 = 5.3 metres above where the mast is firmly anchored. I'm guessing then that that would indicate that the bending and twisting moments on the tower at the anchored point are somewhat further away from the limits of the tower as you have suggested. Also, I am assuming that the 6.5 metre 50mm diameter galvanised pipe that extends all the way down the middle of the mast also provides some measure of strengthening of the overall mast(??) This pipe extends over 1.1 metres below the top of the clamped tiltable plate.

I was also conscious of any bending and twisting when the mast was being finally erected last Monday, noting the strength of the winds that we experienced that day. While it was erected in early afternoon when admittedly the winds had abated somewhat, once erect I noticed little perceptible movement of the antenna.

Nevertheless, I take heed of your comments, and have no desire to see my investment in this project come crashing down around my ears. I have some guy wires left over from my previous 9metre telescoping pipe "mast" and will use those to tie this sucker down.

Again, thanks for your input into this project. Much appreciated.

cheers
John

[VK3JLS]

Could you, for the sake of others like myself, advise where/how you sourced/fabricated the various bits of hardware?

Alan,

happy to oblige, but not sure how much help this will be, as the main parts were sourced locally from Romsey Engineering Services, who were a great help, and only too willing to assist with some minor work that was needed.
Main parts were 4 metres of 100 x100 x 6mm square hollow section pipe, and two 8mm steel plates. The following sketch best describes how all this was put together:-

mast plate sketch 2.pdf

(34.4 KiB) Downloaded 348 times

The right angle brackets were cut down from a 100 x 100 steel lintel left over from when I built my QTH a few years ago (must check the windows to see which one is missing a lintel!). The brackets were welded to the plates to ensure a snug fit around the post. (The result is quite snug, with little or no lateral movement).
The top of the post was rounded on one side to allow the tilting, and was then sealed by welding a thin metal plate over the top to prevent moisture getting down the post.
Welding neatly performed by my brother in law.

The winch is a Jarret Brake Winch 8000 series sourced through Blackwoods. It comes with 7.5 metres of wire cable. The winch is bolted to yet another piece of the lintel material that was welded to the post.
The 3/4" clevis pin at the pivot point was sourced from the Elmore Field Day. Blackwoods sell these also.
Other bits of hardware (U-bolts, bolts, etc) all came from Bunnings.

The one issue I came up against was that the mast pipe was 2" OD, while the support for the hexbeam required a 1 1/4 " OD pipe. Romsey Engineering had some scrap pipe that almost suited the internal diameter of the 2" pipe, and trimmed it down to size on one of their lathes, and yet another piece of scrap provided the 1 1/4" pipe for the hex beam. So I ended up with two small pieces of telescoping pipes that were welded together to provide the neat sizing from the mast pipe to the hexbeam support plate.

Hope this helps someone. If anyone needs more information, don't hesitate to ask.

John

[VK3QI]

John,

Some back of the envelope calculations:

surface are of hex beam 0.5 sq metres (as per their advertisements)

Tower above pivot point 5 metres x 200mm (assume solid as the pipe inside makes it almost solid) = 1.0 square metre (probably more like 0.75 square metres)

Maximum wind gusts in thunderstorm 120 km/hour = 33 metres per second

Wind pressure at 33 metres/second = 660 Newton /square metre


So you have 660 x 0.5 = 330 Newtons acting at about 6 metres above the pivot point (assume hex beam is approximately 1 metre high from its base plate)

and you have 660 x 0.75 = 400 newtons acting at about 2.5 metres above the pivot point.

Without doing a full analysis of the moments at the pivot, you can see that the critical issue will be the compressive strength of the bracing rods.


************

Your base support and pole are certainly more than adequate for the task - the likely failure mode would be a compressive failure of one of the diagonal braces just above the pivot point (not necessarily protected by the additional plate that is bracketed to the tower face).

You have to take the worst case scenario which would be a wind in the direction away from the plate towards the unsupported leg of the tower.

Typically this is the failure mode with Nally towers, where one of those braces fails and the tower folds at that point, due to excessive loading at the top of the tower generating too large a moment at the top of the bottom section, which is considered to be rigid, just like your support pole top.

**********

If you are concerned about the effect of the guy wires, then there is always the possible use of Kevlar ropes instead.

Over the years I have seen a number of towers (especially on the east side of the Mornington Peninsula that are exposed to the bay) that having been guyed, have survived storms that have taken out adjacent trees and roofs.


Good DXing!

Peter VK3QI

[VK7HH]

Thanks for posting.

I need to do a similar arrangement on my tower at home soon to be installed.

I believe my tower is a different "Hills" type. Each section weighs 20kg. Going to use it for VHF/UHF array and perhaps a vertical on top, but want the fold over capability.
My tower comes in 1.8m sections, and wanting to put 4 sections up.

Thinking about an arrangement similar to your's John, or perhaps use another tower section I have of the same to use as the "main support" instead of buying RHS and pivot it from the top. Probably have to cut off the top pins that mate with the next section and weld a plate with pivot arrangement in there. Not sure if anyone else has any ideas? Attached is a photo of the tower sections I have.

Regards
Hayden VK7HA