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Strength of Fillet Welds 2

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MarkAJohn

MarkAJohn

Structural
Nov 28, 2001
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Here's one for the welding experts:

In the old AISC 1993 Spec. (J2.2a) you used to be able to
get extra effective throat on a fillet weld if it was done with the SAW process. In the 2005 Spec. this has disappeared and been replaced with a testing
requirement. This could apply to all processes, not just SAW.

So, does anyone do anyone do anything like this kind of
testing? If so, what test is used?

In other words, what’s a ¼” fillet worth, 4 times 1.392 kli, or is it more based on test?

thx,
MJ



 
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My interpretation of that clause is that if you can demonstrate through a test using production processes and procedure variables, you can increase the effective throat to that average value demonstrated.

This typically requires welding a test specimen then doing a fillet macroetch test to determine the throat depth when penetrating beyond the diagrammatic weld root.
 
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AISC does allow for an increase in fillet weld strength relative to the direction of loading. For example a fillet weld loaded orthogonally (in tension) has increased capacity "k" = 1.5.

Why would you used submerged arc welding (SAW) for fillet welds? Typically this is used for long groove welds, on plate girders or built-up sections.

SAW consistently has a greater depth of deposit. Basically the effective throat is deeper for a given weld size. The increased strength can be considered if effective throat can be verified.

Generally and engineer does not design fillet welds for a specific weld process, since control of the welding process is not within the engineers scope. The cost of using SAW for single pass fillet welds would be extreme relative to the increased strength. Using an increased single pass size would be more economical. For example increasing for 1/4 fillet (5.56 k/in) to 5/16 fillet (6.96 k/in), both single pass fillets.

For a fillet weld in shear (loaded parallel) the k = 1.0. Your statement is correct 1.392 k/in (LRFD)

 
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Thanks to you guys for the responses.

I believe Connectegr is saying you don't get the extra penetration with SMAW. That would be consistant with the old spec. which only allowed extra strength for SAW. Am I reading you right?

Are you also saying that practically all fillet welds are done by the SMAW process?

Regards,
MJ
 
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I think in most modern structural steel shops, GMAW or FCAW are the preferred welding processes. These are wire feed machines with gas shielding. In the field FCAW-S or SMAW (stick) are frequently used. These are self-shielded processes and work better outside in the wind. These processes represent most of the fillet welds.



 
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More thoughts:

You are, of course, right about who picks the process, the fabricator.

If your in seismic country where the engineer picks the size, a fabricator might be more apt to use SMAW. If the fabricator is given the required strength and gets to pick the size, (not seismic country) I wonder what he would do? Maybe test for extra penetration in a SMAW weld?

Regards,
MJ
 
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I am proudly east of the Rocky Mtns, but the seismic requirement for welds have more to do with filler metals and quality control within demand critical areas. Filler metals are available for all welding processes which qualify for AWS D1.8 Seismic.

Just in case...
SMAW (Shielded Metal Arc Welding) and SAW (Submerged Arc Welding) are very different. SMAW or stick erectrode is an entirely manual process. I have never met a welder that can manually match the quality of an automatic SAW welding machine. The SAW machine controls the wire feed, travel speed, weld angle etc. This weld quality and increased deposition rate are why the SAW welds can have additional strength. But, SAW is inefficient. The setup and limited welding positions make it impractical in most cases.

Even if the fabricator chooses the weld size, seismic or non-seismic, east coast or west coast, the weld must be sized for the required strength. If the fabricator is qualified to complete this calculation and the EOR is willing to share this liability, the weld size is not arbitrary and is rarely based on the welding process.

One example of a process specific weld which I use frequently is a BTC-P4-GF. This allows the prep and effective weld size to be the same, and minimizes material thickness. But, this requires a FCAW-G or GMAW welding process in limited welding positions. This is standard equipment in most shops and the positions can be controlled in fabrication. But, this is rarely practical for field welds.



 
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One more thought...
Testing for additional weld penetration in manual SMAW welds is not allowed and would be impractical. The lack of control makes it nearly impossible to determine when the thinnest cross-section may occur. Fillet welds are measured based leg size and convexity (shape) of the weld.

 
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Connectegr,

Many Thanks for your time and knowledge!

Some of that I knew. I didn't know it wasn't allowed to test SMAW to justify higher strength.

Best Regards,
Mark Johnson PE
 
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You can UT test to verify effective weld depth of a fillet weld. But, the minimum depth establishes the weld strength. So testing may also prove the weld size to be smaller than expected. Measuring the surface dimensions is also determined on the minimum measurement.

NDT testing of fillet welds may be helpful if a strength issue occurs. But testing is more expensive than laying a little more weld. Even when weld sizes are determined to be undersized it is generally less expensive to overlay the weld, which may exceed the required weld size.

One of advantages if fillet welds is that only visual inspection is expected.

 
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Connect-
Can GMAW and FCAW welding be performed on base metals of any thickness? In shops, are these processes automated?

I will say this, you seem like a very knowledgeable guy. Your input on this forum is second to none in my opinion.
I will admit, even if others will not, that as an engineer who has been "calling out" welds for a long time, I still have a lot to learn, but I love learning.

Thanks for your dedication to your profession. I know it often seems like that it goes unappreciated, but know that it does not.
Same can be said for Ron, Paddington, BA, and many others who are willing to help.
 
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Thank you. I definitely learn more from the posts on this sight than I can contribute.

Depending on the wire diameter and machine settings GMAW and FCAW can be used for nearly all shop welds. These are semi-automatic processes. The machine controls the wire feed but most of the other factors are controlled by the welder.



 
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The wire diameter is what had me wondering...i have done some MIG welding....a GMAW process (i think).
Maybe it was the size of the welder i was using but I didnt think the little wire was really sufficient for structural welding.
I guess there are some larger wires?
 
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I'll throw another question or two in as well:

(1) Connectegr, you mentioned that it isn't allowed to test SMAW for the purpose of increased nominal strength. Where is that, AWS1.1?

(2) I see fabricators weld 14 gage stair pans to stringers (e.g. MC10 or plate) without a little supporting angle. They use little stitch fillets. How good an idea is that? Can you go as thin as 16 gage?

TIA
MJ
 
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As a welding process SMAW is the oldest and least consistent process. The strength of the weld is ultimately dependent on the effective throat thickness. Regardless of the weld process the cross sectional area, material fusion, and the properties of the filler metal will determine the strength. The effective size of a fillet is determined by measuring the leg size and shape. My intent was to say that additional testing of fillet welds for greater penetration is impractical. But based on the principals above it is not prohibited. SMAW will have the least consistent results. The quality of SAW may show enough consistency to justify the additional testing and the savings in a large quantity of welds.

I am not aware if a specific prohibition in D1.1.

Regarding the welding a Gage metal

 
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