Thread: How To Design Twist-Free Brace/Bracket?

I wish you could do a picture too. But if I grasp it anyway, diagonal braces (in an X-pattern between the upper and lower outriggers) would increase the stiffness. One diagonal brace between the inner flanges on the near face, and the other connecting the flanges on the far face for the other diagonal of the X.

The text from Lincoln Electric Welders, "Design of Weldments," which I can't find at the moment, includes a comparison of welded machine bases with stiffeners in either rectangular grid or diagonal grid against torsional distortion. The diagonal grid is much, much stiffer than the rectangular, all other things being equal.

There's no such thing as "rigid" BTW.
It's what we call a convenient fiction.

Cheers,
Joe

Thankyou, Joe, I think you do understand what I'm trying to say and I think I understand from what you say, what I should do.

I've put pictures up on photobucket.

arial pictures by abrogard - Photobucket

If you think the whole setup is wrong please let me know.

You can see from the pics how I slapped the 2" angle right on top of the original, one facing this way and the other that way. Pic 3.

Pic 1 shows the whole thing - looks to me like I might be pushing it trying to support such a height and weight of pipe in this way. It is leaning back even when at rest. I think that just is the 'prongs' out of line and plan to put washers in to straighten it up or maybe bash it straight before welding the bracing in.

Pic 2 doesn't help much.

Pic 4 perhaps is the clearest shot of what's going on.

The whole thing survived winds gusting to maybe 50kmh yesterday so that's a relief. It was too windy to even try taking it down single handed.

The way it looks to me from the photos your brackets are relatively rigid in the direction across the house but are going to be flexible along the direction of the gutters,.

Have you considered guy wires from near the top of the mast back to the roof, roughly in line with the guttering but angled back a little bit, lead them in under the lip of the tiles and secured to the battens?

That should reduce the leverage on the brackets considerably.

Yes, I was thinking about a guy wire yesterday. Down to the fascia at the corner on the left as you look at the house. Because, yes, it is twisting in a line along that front gutter.

But I'd also like to build a decent bracket.

Probably should have started from scratch instead of mucking about with the old light duty one.

Basically I guess I'm looking for the technical 'how to brace' answer. The theory. What's the way to build the strongest such thing.

Like in two dimensions when they're teaching us geometry at school they talk about making triangles and that figure is then rigid, immutable.

In my working, making things, that's still the only guide I've got and I try to brace by making trianges. But here I found myself puzzled - where to make the triangle?

It is like trying to brace a piece of paper or something, basically, isn't it?

Held rigid along one side. The parallel opposite side having a twisting force applied to it.....

Anyway, I'm thinking of going along with Joe's bracing (and maybe a guy wire - belt and braces ) with what? 1" angle or 2" angle?

And I've done a rough sketch showing what I think Joe means. Hope it is intelligible.

I'll try incorporate it in the body of this post:

The problem seems to be in the horizontal plane. You could fix a diagonal strut to the facia. That will take a lot of load off the bracket and be reasonably low profile. It will however just move the point of stress concentration a little bit higher. Guy wires coming from higher up solve that but won't help with forces coming toward the house. The correct answer I think is to use a tapered upright, like a flag pole or goal post. This spreads the load more evenly. Just using a two stage column would probably do the trick. By that I mean a larger diameter pipe/tube for the first maybe third, with the smaller one shoved inside and welded/screwed. But the bracket needs to be supported first.

I'm not sure I'm with you, Bryan. 'Forces coming towards the house' ? I don't see any forces (of concern) coming towards the house.

Towards and away from the house the forces ares taken by the verandah pipe to which the bracket is bolted. I think.

But parallel to the house - now that's where the problem is. Isn't it? The arial pipe is like a lever trying to force open the two 'jaws' or 'arms' of the bracket.

I'm no engineer, not even secondary school level unfortunately, so I don't know how to calculate the forces but common sense tells me that if I were up there at the top of the arial pole and had four metres of leverage on those arms I'd open them up alright. With that kind of leverage I could lift a car if the poles didn't break.

I've decided I don't like guy wires. I was thinking to have one running off to the left as you look at the pic of the arial and house - that's where the weather comes from - that's the direction the arial pipe is leaning back away from.

So I wanted to brace against that with a guy wire. But now I realise if I don't make it taut then weather from the other side will slacken it and let go with a bang and slacken and let go..... and break something. Guy wires need to be all around and taut. No good.

I think I've decided to weld in a rectangular piece at the front, closing the front off, making it look like a solid rectangle.

Cut bits out of it so's it'll fit in. Get the idea? Look at that pic number 4 - the one that shows the front of the bracket where it bolts the arial pipe on. You can see that for all my stiffening we've still got two independent arms, one above the other.

Something for the arial pipe to exert its leverage on, forcing one arm one way and one the other until it either bends right back over or snaps one of my bodgie welds and the whole thing comes crashing down on the roof.....

But if I close in that rectangle than it'll have to move the whole rectange, the whole bracket, as one thing, a much harder job. So hard I think we'll see no movement at all after that. So no 'working' of the bracket, looking for metal fatigue and wear and weakness.

I think that's where the 'triangle' reinforcing is required here. Think I'll have a go at it tomorrow, all being well...

BUT: I do like the idea of having a lighter, smaller diameter pipe up the top there. This is 35mm outside diameter stuff I'm using. Pretty thick, too, maybe 3mm, maybe 2.5mm I don't know. I don't know what weight of pipe would be needed to do the job but I guess 1" water pipe might be enough for the top 2 metres?

Originally Posted by abrogard

I'm not sure I'm with you, Bryan. 'Forces coming towards the house' ? I don't see any forces (of concern) coming towards the house.

The force in question is the wind.

So I wanted to brace against that with a guy wire. But now I realise if I don't make it taut then weather from the other side will slacken it and let go with a bang and slacken and let go..... and break something. Guy wires need to be all around and taut. No good.

Exactly. Sorry, I glazed over after this. Just ignore everything I said except this bit: You could fix a diagonal strut to the facia.

Hi Abrogard,

Probably not the answer you wanted to hear, but can that mast be run down to ground level, and get rid of the other bit of pipe completely, then all you would need is a bracket from the wall (or fascia). Most of the masts of that type that I've seen have the anchor point at ground level.

Regards
Ray

Oh, I see, Bryan. Well I was thinking those forces are no problem because they just feed straight back into the verandah post which is set in concrete at the bottom in the verandah slab. Which is above ground. An above ground verandah or porchway slab.

I see what you mean, Ray, I think. Anchor it on the ground and then these 'twisting leverage' forces I'm talking about would be much diminished. Much.

Great. But then I've got a pipe coming down to the ground getting in the way all the time.

'Get rid of the other bit of pipe entirely..' ? But that bit is integral to the verandah thing. Don't exactly know why. Maybe keeps the roof overhand up at that point. Probably does. So I wouldn't be getting rid of it.

Or you mean just get it out of the picture as a support? But it is the best, easiest, available support.

I guess I could angle the arial pipe down to meet that verandah slab at the bottom where that verandah pipe enters it. And then restrain it there. I'd have two pipes coming down there then, not that strange. Not in the way for walking past, so much, being up off the ground. That slab is about a metre off the ground.

I should have posted a picture of the whole setup. Sorry. Or drawn one.

I've just been looking through our snapshots hoping to find one and not only did I not find one, I found two years of pics missing, totally! Beware of keeping your stuff on computer.....



Well, I've drawn something. A bit rough. You can't see where the arial pole finishes and the verandah post begins but that's not important. You can now see why it has been chosen for the job. Perhaps.

Hi Abrogard,

I didn't mean to remove the verandah support I just meant with respect to the antenna mounting.

Just an idea, if it was going to be in the way, maybe you could do a "cranked" set up, like these... TV Aerial and Satellite Poles, Masts & Brackets

Regards
Ray

Hi Ray,

yep, I think I'll let it sit on the ground. I'll put an extension on the bottom of it - the 'mast' (that's what I should have been calling it all this time, isn't it) - with a couple of cranks in it so's it comes in close to the verandah post and then follows it down and sits on the concrete slab at the bottom right next to the verandah post.

And fill in that rectangle in the front I was talking about.

That should take all the drama out of it.

Very interesting site that, Ray. See them at work in England. Doesn't like putting them on fascia boards, does he? He'd freak out in Australia.

I don't like the problems they face fastening to brickwork.

He says a 16' pole on a wall requires a pair of brackets at least 18" apart.

That's the point - spread the grip. If I lengthen the pipe (the 'mast') downwards and clamp it then I've done that. I've got the leverage then, not the wind.

Did you notice he says this, right at the

When deciding how strong to make your install remember that the weight of the aerial/satellite/weather station is almost irrelevant,
it`s the wind loading that counts !

Yeah, but it is that weight that the wind is moving and using to exert a force on your brackets down lower on the mast.

I'm pretty sure I could hold up a ten foot pole flexing in the wind with nothing on the top and have real problems if it had a blob of weight up there moving around.

That's what I meant about the maths, the physics. I never knew any really but I do have that vague memory we've all got about multiplying weight by distance to get a force - right? Something like that. And once the wind has moved it off the vertical then that weight is acting like that, I'd suppose. Makes sense to me.

So here's the end of it. I'll trouble people no more. To solve my problem I need to spread the grip and ideally drop the weight down onto the ground.

BUT - if for some reason I had to do it this way, held up in the air, I think I figured out for myself how to best brace the bracket - fill in the vertical cross section at right angles to the bracket arms. That would minimise any possible flex in that bracket.

That's how I see it.

And thanks everyone for helping.

Hi,
Three things
1 I think you put your reinforcement in the wrong place.
2 You really shouldn't drill through pipe like that, clamps are much better
3 photobucket upsets my pc for some reason. sorry about the picture lol


Stuart

p.s. Forget 3, with windows photo viewer the aspect ratio is all funked up, but it displays ok in internet explorer. ?

You're so right. Why didn't I think of that? So dumb. All I needed was buy two pipe clamps and set them a metre or more apart on the pipe and grasp that mast and it'd be held secure.

I'll do just that as soon as I can. Throw all my stuff away.

Still interested in the academic question, though - how to best brace such a structure. i.e. a bracket with a top arm and a bottom arm.

I'm inclined to think putting diagonals inside it - like a st andrew's cross, is the way to go.

Originally Posted by abrogard

Still interested in the academic question, though - how to best brace such a structure. i.e. a bracket with a top arm and a bottom arm.

You mean if you starting over or with what you already have?

Stuart