Fillet weld sizes for HSS Tube column base plates 1

[oengineer]

I am designing a base plate for an HSS tube column. My column is a 10x10x1/4 and my base plate is 1 1/4" thickness. From the AISC design guide 1 for base plate it seems pretty standard to call out a 5/16" fillet weld size. But,I have also seen in the AISC design guide 21 for welds that "The minimum fillet weld size need never exceed the thickness of the thinner part being joined." I have read that 5/16" welds are economical. Is it possible to use a 5/16" weld size for my HSS 10x10x1/4 and my 1 1/4" thickness base plate, even though a 5/16" weld size is greater than my thinnest material? Suggestions/comments are appreciated.

 

[jayrod12]

1/4" weld for a 1/4" wall is all that you need. if a 5/16" weld is economical, a 1/4" is more so. I believe the notion of a 5/16" weld being economical comes from the fact they can lay that down in a single pass. Once you exceed 5/16" you need to do multiple passes which increases time.

At least that is how it was explained to me.

 

[KootK]

X2 for Jayrod's comments. If, by chance, there is something specific to the shop setup that would favour a 5/16" weld that would be just fine too I think. I'd let the fabricator propose that if that is their wish however.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JLNJ]

There used to be an AISC recommendation for minimum weld size based on the thicker part. Over 3/4" the recommendation was 5/16". I think this had to do with welding procedure and possible preheat requirements.

 

[BSVBD]

I agree with Jayrod regarding the 5/16" max for a single pass. I discovered that while estimating for a steel fabricator in the past.

However, referring to AISC 13th Table J2.4, it appears you could use even smaller welds than what is being discussed.

 

[nutte]

How can one say that a 1/4" weld is all that is needed for a 1/4" wall, without knowing the load in that 1/4" wall? The load could be such that a larger weld is required.

It is fine to have a 5/16" weld for the base plate condition you describe. That quote says "the minimum fillet weld size need never exceed ..." If the loads dictate it, you might need more for your scenario.

 

[KootK]

How can one say that a 1/4" weld is all that is needed for a 1/4" wall, without knowing the load in that 1/4" wall?

Likely based on being able to develop the cross section with the weld.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[nutte]

A 1/4" fillet weld on one side of a 1/4" HSS wall will not develop the wall strength. A double fillet weld sized for 5/8 times the plate thickness will develop the plate's strength. For a single fillet weld, it would be 5/4 times the plate thickness. 5/4 * 1/4" is 5/16". That 5/16" fillet weld is looking better and better.

 

[jayrod12]

I've never seen a column with so much lateral load that it needs more then the wall thickness worth of weld. Not to say there aren't situations that arise but I've never seen it. I've also always been told you can't provide a larger weld then the thinnest member. But that may just be an old wives tale.

 

[ARKeng]

I've never seen a column with so much lateral load that it needs more then the wall thickness worth of weld. Not to say there aren't situations that arise but I've never seen it. I've also always been told you can't provide a larger weld then the thinnest member. But that may just be an old wives tale.

I guess that'd be true if you've never designed a column to anything close to it's tensile or bending capacity.

A 1/4" fillet weld on one side of a 1/4" HSS wall will not develop the wall strength. A double fillet weld sized for 5/8 times the plate thickness will develop the plate's strength. For a single fillet weld, it would be 5/4 times the plate thickness. 5/4 * 1/4" is 5/16". That 5/16" fillet weld is looking better and better.

Pretty sure that would only apply if the load is pure shear. For example, if I have a 1/4" (nominal) HSS wall with Fy=46 ksi, the allowable tensile capacity is 0.233" (design t) x 46 ksi/1.67 = 6.42 k/in. The required weld to develop is D=(6.42 k/in) / (0.928 k/in/D) = 6.92 D ==> 7/16" weld.

So, by my understanding, if you provide a 1/4" weld, you're only providing 57.8% of the bending/tensile capacity. If you follow the 5/4 rule, you're only providing 72.2%.

 

[Tmoose]

http://www.hobartwelders.com/weldtalk/showthread.php?144937-I-get-a-call

 

[KootK]

A 1/4" fillet weld on one side of a 1/4" HSS wall will not develop the wall strength.

Pretty sure that would only apply if the load is pure shear.

I was thinking of shear load rather than axial load. I'd assumed that we were talking about a compression column finished to bear at the base plate. If it's a more exotic loading condition than that, OP will have to elaborate. If it's a predominately tension member, OP should rethink that enormous cross section and 1/4" wall thickness.

That said, it would be a pretty rare column that would be so heavily loaded axially that a base plate weld matching the wall thickness wouldn't provide an axial capacity exceeding the buckling strength of the column.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[nutte]

0.928 is the shear allowable on a weld when loaded parallel to its axis. If the HSS is in tension, the load is oriented perpendicular to the weld axis, and you get a 50% increase on its strength. 6.92 sixteenths becomes 4.61 sixteenths.

AISC mentions that a weld sized for 5/8 of the plate thickness will develop its strength, in bending, shear, whatever. They have also acknowledged that this doesn't calculate, but it has been verified in testing.

 

[KootK]

I dispute none of that nutte. My stance here is really much more qualitative in nature: I'm very confident that capacity greatly exceeds demand in most of these situations when a matching weld size is specified. That's all.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[nutte]

I was responding to the original question (Can you put a 5/16" fillet weld on a 1/4" HSS wall? Yes), and objecting to the implication that a 1/4" fillet weld develops the strength of a 1/4" HSS wall.

 

[KootK]

Duly. Noted.

Check out the attached table from CISC's steel manual which I've just "discovered". They spec a 10 mm fillet weld for shear development. I imagine that they're using calculation rather than the 5/8 / extended shear tab business from AISC. High falutin' technology like that can take a while to make its way across the border.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.