Thread: Bending Flat Stock To Form Polygon Then Weld

This is an interesting decision I have to make regarding the bending and welding of this polygon (attached).

100-130 mm flat stock x 6-10mm thick mild.

Where would you weld it shut? where would you start? Does it really matter?,

and as an aside, are welded joints (properly executed one or two pass both sides) stronger or less than a bend around?

We could go into a lot of detail about fitness for purpose etc but just wanted an opinion as a starter if someone had the time.

It's a component from a shed design which I will show a struct engineer ( maybe someone on this site if they are interested enough ?) can you guess where it will go?

cheers for now!

edit: those measurements are mm (courtesy of my cad loving surveyor friend)

backbround: furniture maker, developing welder, budding shed maker.

Hi there,

Wouldn't really know about the strengths of welds V's bends but I'm guessing it would depend on the nature of the forces applied & directions oh and the intended end use (which I equally have no idea of). I'm just impressed with your intended dimensional tolerances!

Simon

Just off the cuff I would say weld at the 120 deg. It's easier to weld from both sides at that point.
Strength should be the same if done properly. Strength also shouldn't be an issue if this has been designed without any gussets.
Yup, that's about all I got
Oh, and welcome to the forum

Phil

How rude of me! It's your first post.
A BIG WELCOME FROM ME TOO!

Simon

cheers,

i live in the country where sheds are practically everywhere.

Some of the designs i see for truss to column connections for tubular pipe frames are attached below.

my slightly modified treatment of the juncture (pictured also) allows for eaves overhang and fascia attachment via cleats.

i would like to add stub locators to the faces and then weld on the ends once aligned.

after looking at buying a bender etc, i will probably have the prototype bent at an engineering shop, unless someone here wants a project? hint hint.

gussets may have to be welded yet, not sure....any retired engineers want to get involved?

the connections will be seen by anyone who comes to the shed /showroom so like to have it neat but still old school if you get my drift. there were so many great sheds built strong and not "blow away" that I'd like to carry on the tradition.

anyhow great site......been lurking for a few months z

I'm assuming the bends are on the flat, so in that case I'd just heat and bend to the angles, layout the shape on a flat surface and fix in position while establishing the angles.

If you had to make a number of them, it would be worth making a jig. Only one corner needs to be welded, doesn't matter too much which one, but as Phil suggests the 120 corner is a bit easier to get welds on both sides..

I'm curious as to where this fits into a shed construction? The shape won't have much rigidity, triangles are the key to rigidity...

Oh, and welcome to the forum!

Regards
Ray

Originally Posted by meadow street

This is an interesting decision I have to make regarding the bending and welding of this polygon (attached).

100-130 mm flat stock x 6-10mm thick mild.

Where would you weld it shut? where would you start? Does it really matter?,

Yes it matters a lot.

For starters you should not be joining or welding the polygon at a corner.

To avoid distortion you should butt weld (double sided) at a neutral position midway between two corners.


Cheers

Rob

Welcome to the mob Meadow Street.

As a metal fabricator I can't picture the item being bent as a single unit in a fabrication sense. It is not really practical to bend in one piece with any standard equipment found in the average engineering shop. Possibly with a special purpose made bender,though economy of scale might change that.

What about 2 bent pieces welded at 120 and 60 degrees. Much easier and quicker to fabricate. If gusseted, these need to be welded anyway.

With 2 pieces or more it lends it self to tacked and /or welded in a jig/fixture.

I'm not an engineer, but just looking at the design, a polygon is not very rigid, as there is no diagonal support.

That's why triangulation is used,

You could in essence just use a triangle with a vertical riser comming off of the lower left corner , using the roof bearer as a stressed member and have a stronger, easier to make unit.

The whole poygon idea is not great engineering in my view.

Rob

I'm with you on this one Rob. What size shed are we looking at here? There are easier and cheaper ways to do it. Do I hear an echo somewhere?

You could just as easily extend the top pole as per the last picture, with the design in the first picture. I cannot see any difference with doing this except for the better structural method of picture one.

Dean

thanks for all the responses

i have lots and lots of 48mm od black pipe at my disposal.

shed will be:

10-12 m span

3-4m bays over 28-30 m.

perhaps Dean the pic below might be something for me to focus on ?

i guess I liked the angles of the polygon because the pipe ends would weld at 90 degrees...

I am adaptable though

How well the shed is designed ultimately depends upon the planning and inspection process in your area.

It varies between country/city/various states.

If you require approval and inspection then you will need to use an approved design and dimension specification.

So best to run the end design past the local council engineer to see if it is acceptable.

Even some of the older designs (eg series angle braced between twin bearers is no longer approved in the original dimensions - ridiculous.

The fact that the sheds have stood for near on a century seems to be irrelevant. So even buying secondhand trusses can be bad news.

Cheers

Rob

Meadow street, that last pic you sent is a much better way to do the connection than your polygon idea. Its a far stiffer joint, and you do need maximum stiffness at that point.

Originally Posted by meadow street

thanks for all the responses

i have lots and lots of 48mm od black pipe at my disposal.

shed will be:

10-12 m span

3-4m bays over 28-30 m.

perhaps Dean the pic below might be something for me to focus on ?

i guess I liked the angles of the polygon because the pipe ends would weld at 90 degrees...

I am adaptable though

You must have lots of pipe to build that big. Wish I had that much.

That pic looks good to me but then I am not an engineer.

What are you cutting the pipe with? A dropsaw or bandsaw could be set for the angle and used to cut all pieces needed at that angle in one go then have the angle reset.

Dean

In your latest design the roof will push the walls out.
Structures are usually stronger, especially in corners, if the forces push the building together.

This is one way of doing it, also it's very simple.