Weld design using HSS rules?

[SuG89]

Hi,
I have come across situations where the weld group (hollow rectangle) is under combined eccentric loading. In such situations I calculate the section modulus of the weld group about both the axes and the torsional sectional modulus (using Chapter H (H3) in SCM 13th Edition). Can I use the rules given in Chapter F for rectangular HSS to decide whether to use elastic or plastic section modulus for the weld group check in bending? Or one should always use the elastic section modulus for the check of welds?
Regards,
Su

 

[dik]

I normally only use plastic capacity if I'm looking into the strength of something that failed. In general connections are the area you want to avoid saving a 'buck'. CIDECT has research for HSS sections that increasing the weld size to approx 10% of the section thickness yields a strength similar to a CJP weld.

Dik

 

[SuG89]

Hi Dik,
Thanks for your response!
Do you remember which CIDECT book states that increasing the weld size to approx 10% of the section thickness yields a strength similar to a CJP weld?
Regards,
Su

 

[dik]

I'll see if I can dig up the reference.

 

[JAE]

Maybe I'm confused (very well could be) but we've never used plastic section modulus for a weld group design check.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[dik]

JAE, agreed... only if investigating a failure...

Dik

 

[SuG89]

I'll stick to the elastic section modulus for checking weld groups in bending.

Thanks
Su

 

[KootK]

1) Agree with others, I don't even think that most welds are capable of going plastic.

2) One needs to be sensitive to the stiffness of what you're welding to. Unless the supporting thing is very rigid, even an elastic distribution will be wishful thinking. As an example, if you weld and HSS to another HSS, as is ubiquitous, the weld stresses are very much not linearly varying. I think that there's a 1.25 factor in AISC for this but I'm at a loss to produce the relevant code clause.

3) It is an interesting, and often overlooked feature of connection design that the connection should be able to do whatever is being asked of the member at that location. You can't rightly design the incoming HSS for it's plastic moment capacity if that same capacity can't also be passed through the welds.

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.

 

[dik]

Kootk... not all welds are brittle... some have ductility.

Dik

 

[KootK]

Hence my “most” qualification.

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.

 

[SuG89]

Hello,
Table J2.5 of the SCM 13 states that for CJP welds the strength is governed by the base metal provided a matching weld filler material is used. I have a base metal which is a round bar. Section F11 of SCM 13 states that the nominal flexural strength shall be the lower value obtained according to limit states of yielding (plastic moment) and LTB, as required. If I follow table J2.5 then I just have to specify a CJP weld. But this CJP weld actually fails when I do a hand check using the elastic section modulus. What should be done in such a case?
I think this relates to KootK's comment (point no. 3).
Regards,
Su