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Thread: fillet weld leg size.

1. Member
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fillet weld leg size.

Can anyone give me a link or explain to me how you actually determine the proper leg size when doing a fillet weld. From what I understand is that with 1/4" materials you should place a 1/4" weld, or with a 1/4" plate welded to a 3/8" plate the weld should be 3/8". Is that right? Would there be any reason to put a 3/8" weld on 1/4" materials. What i was taught is that would be a waste of time and materials. I have read that putting more weld than necessary will cause more stress to the toes of the weld, but nothing actually says what size weld is needed for certain material thicknesses. So please someone clear my mind on this.

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The rule of thumb, on a T joint with fillet welds on both sides, is 3/4 of the thinnest material thickness equals a full strength weld.
This assumes no beveling of the joint, which will reduce the required leg size.
So, if you are making a fillet weld on both sides of an unprepped T joint, 1" plate welded to 1" plate, a 3/4" fillet will meet or exceed the material strength without overwelding.

JTMcC.

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This is what I was taught in the D1.1 certification class-

Fillet welds should have a throat depth equal to or greater than the thinnest of the two plate sizes in the t-joint or lap joint.

Throat depth is the distance from the root to the face, measured on an angle that bisects the joint angle, which is 45* for a lap joint or 90* t-joint.

For a flat-face fillet weld, this means the leg lengths will be greater than the throat depth and thus greater than the thickness of the thinnest of the 2 plates in the joint.

For example-

A 1" plate to 1" plate t-joint requires a fillet weld with a throat depth of at least 1".

A 1" plate to 1/2" plate t-joint requires a fillet weld with a throat depth of at least 1/2".

A 1/4" to 1/4" plate t-joint requires a fillet weld with a throat depth of at least 1/4".

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Using throat depth just adds more work than is required.
Drawings will call out fillet leg size. No money to be made in converting a conventional call out to a throat dimension. And throat dimension measurement requires more tools. That just puts one more step in the process, and one more mathematical conversion (which means one more shot at making an error) in the fabrication of a "thing".
Just like having dimensions called out in inches, you can convert all dimensions to metric......you can, but why?? Unless you are dying to lose money. And right now welding/fabrication outfits are dropping like flies. efficiency rules the day. When work is tight efficiency keeps you in business, when work is abundant it just makes you more money.
That's my take only so take it or leave it.

JTMcC.

5. Originally Posted by Desertrider33
This is what I was taught in the D1.1 certification class-

Fillet welds should have a throat depth equal to or greater than the thinnest of the two plate sizes in the t-joint or lap joint.

Throat depth is the distance from the root to the face, measured on an angle that bisects the joint angle, which is 45* for a lap joint or 90* t-joint.

For a flat-face fillet weld, this means the leg lengths will be greater than the throat depth and thus greater than the thickness of the thinnest of the 2 plates in the joint.

For example-

A 1" plate to 1" plate t-joint requires a fillet weld with a throat depth of at least 1".

A 1" plate to 1/2" plate t-joint requires a fillet weld with a throat depth of at least 1/2".

A 1/4" to 1/4" plate t-joint requires a fillet weld with a throat depth of at least 1/4".
Originally Posted by JTMcC
Using throat depth just adds more work than is required.
Drawings will call out fillet leg size. No money to be made in converting a conventional call out to a throat dimension. And throat dimension measurement requires more tools. That just puts one more step in the process, and one more mathematical conversion (which means one more shot at making an error) in the fabrication of a "thing".
Just like having dimensions called out in inches, you can convert all dimensions to metric......you can, but why?? Unless you are dying to lose money. And right now welding/fabrication outfits are dropping like flies. efficiency rules the day. When work is tight efficiency keeps you in business, when work is abundant it just makes you more money.
That's my take only so take it or leave it.

JTMcC.

So how do you know when it is correct?? Is the 3/4 thing an actual spec? or are you saying an engineer will specify anyhow and that is all that matters?

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Originally Posted by FusionKing
So how do you know when it is correct?? Is the 3/4 thing an actual spec? or are you saying an engineer will specify anyhow and that is all that matters?

No, I'm saying that engeneering will commonly use 75%, full length fillet on both sides of a T joint meeting at 90 degrees as developing the full strength of the parent metal, it actually developes quit a bit more in actual as welded condition (engineering calculations underestimate "as welded" weld strength by a large margin, they sometimes underestimate material strenght by quite a bit as well. But we don't have a lot of structural failures in this country so that's a fine thing ).
The hard number is 71% but 75% is used commonly as a rule of thumb to compensate for any dimensional variation such as slightly concave fillets.
Don't take my word for it though, do a little research on your own.
But this is a simple rule of thumb that can be reliably used by the back yard types. If your project is so tightly designed that this rule won't work for you then you either need thicker material or professional engineering help.
My take only.

JTMcC.

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so drawing it on paper with 1/4" legs and a flat to slight convex face from toe to toe your effective throat length should measure a 1/4" or probably more. So i guess it's alright to go by the leg size a drawing may call for.

I can understand getting things done quick and making more money, but i'm more of a perfectionist and would rather shoot for perfect every time. Whether it matters or not to anyone else.

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Originally Posted by Jeep80CJ7
so drawing it on paper with 1/4" legs and a flat to slight convex face from toe to toe your effective throat length should measure a 1/4" or probably more. So i guess it's alright to go by the leg size a drawing may call for.

I can understand getting things done quick and making more money, but i'm more of a perfectionist and would rather shoot for perfect every time. Whether it matters or not to anyone else.

It's not "alright" to go by the leg size called out on an engineered drawing, it's a mandatory requirement. Engineering will design in an industry standard safety margin. Overwelding causes as many (or more) problems than underwelding.
Your "perfectionist" comment is ridiculous. If you are making proper, code quality welds then adding more than is called out means you will likely get to scrap your completed items, buy all new material, and weld it properly according to the drawings. That will put you out of business unless you are a large company and in that case it will likely mean turning a profitable year into a total wreck.
There is a proper amount of weld for every joint configuration. More, or less, causes proplems to everyone involved.
To say that "I'm a perfectionist so I'll overweld everything" is just plain stupid.
"Perfect", in the welding world, is to apply the proper amount of the proper quality weld deposit in each joint. Period.

JTMcC.

9. Member
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Originally Posted by JTMcC
It's not "alright" to go by the leg size called out on an engineered drawing, it's a mandatory requirement. Engineering will design in an industry standard safety margin. Overwelding causes as many (or more) problems than underwelding.
Your "perfectionist" comment is ridiculous. If you are making proper, code quality welds then adding more than is called out means you will likely get to scrap your completed items, buy all new material, and weld it properly according to the drawings. That will put you out of business unless you are a large company and in that case it will likely mean turning a profitable year into a total wreck.
There is a proper amount of weld for every joint configuration. More, or less, causes proplems to everyone involved.
To say that "I'm a perfectionist so I'll overweld everything" is just plain stupid.
"Perfect", in the welding world, is to apply the proper amount of the proper quality weld deposit in each joint. Period.

JTMcC.

wow. ok. i never even said "I'm a perfectionist so I'll overweld everything". Do you realize what you just got mad about? just wondering cause it makes no sense on my end. I actually asked the question of what is proper so I would know for sure and not just guess it like most people do. What I was trying to say is I was leaning more towards desertriders input on the subject as the whole 3/4 thing didn't seam perfect enough.

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Why did you ask the original question if you intend to argue the response you get? What are you, 15 years old??
Don't take my word for it, run it past a structural engineer or a couple of dozen.
Or do the research on the interweb and locate (hopefully) the same info without spending several thousand dollars.
But to ask a question, get an answer from someone who actually knows the answer, and then to argue with them. That's a waste of everybodys time.
I regret answering your simple question in the first place, hopefully it will be helpfull to someone other than you.

JTMcC.

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