Thread: Welding 120mm steel box section to 5 inch box section?

I have the main post for an amateur radio mast. It's second hand and like most has been angle grinder cut off at ground level as they are usually in a reinforced concrete block of about 2 square meters. The last post was 120mm square galvanised mild steel with about a 4mm wall thickness. I got a 4 foot lenth of similar box section, cut some 8mm plates to reinforce all four sides of the join, and got a friend with a big MIG to weld it up.

This "new" mast has a similar post, again having been cut off at ground level, but is older and Imperial. I hadn't noticed it wasn't 120mm outside dimensions, but 5 inch. I have bought an extension tube that's 120mm square outside dimension.

Should I get four 10mm plates and mill a 3.5mm step half way down each plate so as to adapt the 7mm outside diameter difference, or just get my pal to fill the void with weld? I may mill two 1 inch holes in each reinforcement plate to allow puddle welding, too. The post supports a steel fabricated latticework, telescopic and fold over forty foot antenna mast, and elevation and tilt winches, so needs to be a decent joining job! Thanks for any advice.

A rather poor photo of an existing mast attached to give a bit of perspective. About four foot depth of the main post is in a concrete block underground. When people sell the things they hack off the main post at ground level of course...

Buy a whole new piece of steel in the right length will be way less work. You also wont have joins to rust.
120 SHS Galv in 5mm wall, whilst its not bargain basement cheap, isn't particularly expensive by comparison to the epic you are about to embark upon to join two pieces of steel together.

Welcome to the forum.

Weld a square plate to it with 4 bolt holes and have a matching square plate welded to the length that you are going to use for the footing.
Then it only needs to be unbolted when moved again....

Hi Mr Basil, Guys,

I used to have a 75 foot mast ! It was rated for a 256 (16' X 16') square foot wind loading at 55mph. So the issue you have is ensuring that the base is sufficient to support the load that you are going to put up there. My mast was on a four foot square steel plate with four 1" inch anchors on 36" inch centres, with welded gussets against an 8" square tube support. The support tube being 7' foot six inches tall. The whole thing sitting on a big concrete block.

At one time I had 16 144Mhz antenna up there. The amount of sway/twist in the mast was quite unnerving ! It made moonbounce difficult at times.

Originally Posted by racingtadpole

Buy a whole new piece of steel in the right length will be way less work. You also wont have joins to rust.
120 SHS Galv in 5mm wall, whilst its not bargain basement cheap, isn't particularly expensive by comparison to the epic you are about to embark upon to join two pieces of steel together.

The problem with that outwardly very sensible suggestion is there's some fairly intricate pulley block mounts, cable feed through tubes and the mast hinge U plate all welded to it, and it's galvanized. The one in the photo has the new, underground piece triple coated in Galvafroid paint, that previous experience has shown to be the next best thing to hot dip galvanizing, so it should last a good many years. Thanks for the idea though, welding an extension on is MUCH simpler than recreating a new post!

Originally Posted by Mr_Basil

The problem with that outwardly very sensible suggestion is there's some fairly intricate pulley block mounts, cable feed through tubes and the mast hinge U plate all welded to it, and it's galvanized. The one in the photo has the new, underground piece triple coated in Galvafroid paint, that previous experience has shown to be the next best thing to hot dip galvanizing, so it should last a good many years. Thanks for the idea though, welding an extension on is MUCH simpler than recreating a new post!

Derision.. meh... because internet..where people use vague terminology like mast support...

Originally Posted by BaronJ

Hi Mr Basil, Guys,

I used to have a 75 foot mast ! It was rated for a 256 (16' X 16') square foot wind loading at 55mph. So the issue you have is ensuring that the base is sufficient to support the load that you are going to put up there. My mast was on a four foot square steel plate with four 1" inch anchors on 36" inch centres, with welded gussets against an 8" square tube support. The support tube being 7' foot six inches tall. The whole thing sitting on a big concrete block.

At one time I had 16 144Mhz antenna up there. The amount of sway/twist in the mast was quite unnerving ! It made moonbounce difficult at times.

I work on the thesis one cannot have too many aerials, masts or dishes, if the gaff doesn't look like GCHQ you ain't really trying

Originally Posted by shedhappens

Weld a square plate to it with 4 bolt holes and have a matching square plate welded to the length that you are going to use for the footing.
Then it only needs to be unbolted when moved again....

The post came with a half inch steel plate and gussets welded on, it was such an appallingly bad job I took the angle grinder to it and cut them off, there was no way I could live with such a monstrosity in case someone thought *I* had adapted it It was easier to just describe it as cut off at ground level though, rather than get all involved with unnecessary details. Once in it won't be going anywhere during my time here, so it's neater with a fully buried post IMHO, which helps keep `er indoors happy.

Originally Posted by Mr_Basil

The post came with a half inch steel plate and gussets welded on, it was such an appallingly bad job I took the angle grinder to it and cut them off, there was no way I could live with such a monstrosity in case someone thought *I* had adapted it It was easier to just describe it as cut off at ground level though, rather than get all involved with unnecessary details. Once in it won't be going anywhere during my time here, so it's neater with a fully buried post IMHO, which helps keep `er indoors happy.

Hahaha.......so it had an ugly plate with gussets and you cut it off because your wife didn't like the look of it.....

If I was doing this job, I think that I would firstly dress the end of the post that you wish to extend, making sure that the cut was square if it is not already, and then drill suitably sized holes for later plug welding to the extension. I would place these holes so that they landed on the 1/8th plate (or 3 mm) packers that I would weld to your extension post, 2 on each side placed such that when inserted into your original post, 300 - 350 mm, the lower packer is fully inserted. Tack together and once you are sure everything is straight, do the plug welds. If you feel that some extra reinforcement is advisable, some flat bar wrapped around the base of your post and stitch welded would be easy enough. You could even make a socket from some 5" SHS if you can locate some, otherwise suitable channel or angle section, or even flat bar, and bury that in the concrete and then you would be able to just remove the mast when it comes time as ShedHappens suggested.

Originally Posted by Mr_Basil

I work on the thesis one cannot have too many aerials, masts or dishes, if the gaff doesn't look like GCHQ you ain't really trying

Hi Guys,

You wouldn't say that if you had seen a tower collapse !

Anyway my tower went a bit back, the base is used to support the wife’s washing line nowadays. I've only in the past couple of weeks lifted that base and rotated it 90 degrees so that I can get my new shed in the space at the back.

25-05-2021-003.jpg 25-05-2021-004.jpg 25-05-2021-002.jpg

Anyway this is all that is remaining. Those hold down bolts are 1" diameter. There is also an intriguing story behind this tower installation.

Good looking gussets there John, spoze if you didn't like the look of them you could put a pot plant there....

Originally Posted by shedhappens

Good looking gussets there John, spoze if you didn't like the look of them you could put a pot plant there....

Shush, don't give the missus ideas

Originally Posted by shedhappens

Good looking gussets there John, spoze if you didn't like the look of them you could put a pot plant there....

He could just about hide the car behind them Shed