HSS square tube to HSS square tube

[bisandcan]

I am having a "discussion" with another engineer. He wants to weld a square tube directly to a square tube, without following the criteria for a moment connection outline in K.3, one tube is a 4x4 and the other is a 8x6. This connection does not fall in line with the spec because the tubes will not have the same center line, we have to maintain top of steel.

My argument is that the smaller beam will not be able to rotate independently of the larger beam, therefore we are making a moment connection, and therefore need to follow the guidelines for moment connections laid out in Chapter K. I was under the impression that the beams need to rotate independently in order for the connection to not be a moment connection.

Is there something I am missing, he is several years my senior.

 

[KootK]

Your colleague is right about standard practice being to consider that connection a shear transfer only connection when moment restraint is not intentional.

You are right about the rest save, perhaps, one point: if the supporting HSS has torsionally flexible end connections, then it will just twist "along for the ride" and little moment will be developed.

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.

 

[bisandcan]

The problem I have is, whether or not the moment restraint is intentional or not, won't the connection still be a moment connection? How does the behavior of the connection change because I say I don't want it to be a moment connection?

 

[KootK]

How does the behavior of the connection change because I say I don't want it to be a moment connection?

It doesn't. As I said above, you're right about the theory and your colleague is right about what is done in practice. You can't survive in structural engineering unless you pick your battles. There's just. Too. Much. Stuff. This isn't the hill to die on.

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.

 

[MrHershey]

Agree with KootK. Yes it will behave like this, at least at first. But have to think of the consequences too. You haven't detailed it like one, so I wouldn't count on it being a moment connection. But say it behaves like one, then what happens? As long as earthquakes aren't involved (i.e. this isn't part of the lateral system), then you've provided a stouter connection than you need. Yes, there might be extra strength. Yes, it might try to rotate and put some out of plane bending on the webs of the larger beam. But does that mean you've failed? Given decades of standard practice, the answer is more than likely 'no'.

But would encourage you to keep thinking this way. Most engineers don't, especially younger engineers. It's fantastic to recognize that the real world doesn't really care what your analysis or modeling assumptions were.

 

[jayrod12]

What are your issues with it being a moment connection when it isn't intended? Perhaps if you can convince yourself that there are few situations where it is detrimental to the structural performance of a building you'll see why Koot is so non-chalance about it.

 

[Lomarandil]

I've wondered about this too. If I may piggyback on the OP's post, my issue is this:

How do I know that a connection (which is intended to be pinned but physically has some small moment capacity) will be ductile enough to develop the rotation required, and not just develop a progressive/"unzipping" failure (due to the attracted moment) which will sever the connection completely?

 

[racookpe1978]

If the two HSS were the same size 4x4 to 4x4, or 2x2 to 2x2, or even 6x6 to 6x6 then they'd reinforce each other against moment much better. Theory (and practice) then would need to account for the added stiffness.

But here, they are not symetric about the intersection, and much of the intersection then actually in "mid air" between 4x smaller square tube steel walls into a skewed "flat plate" that is relatively thin - thin at least compared to the intersecting small TS walls with their curved corners and weld fillets.

So that isolated "flat plate" in mid-air receiving all of the 4x4 loads will bend and twist rather than resist moment loads. IF this is a significant compression force - where buckling could begin to be a problem in the 8x6 TS, then the longer vertical (compression) member should be substantially reinforced. Not against moment exactly, but against wall deflection at the intersection.

 

[KootK]

How do I know that a connection (which is intended to be pinned but physically has some small moment capacity) will be ductile enough to develop the rotation required, and not just develop a progressive/"unzipping" failure (due to the attracted moment) which will sever the connection completely?

You don't. Two factors that tend to prevent real world problems:

1) As I mentioned above, the supporting member itself often does not have rigid torsional support at its ends. As such, it twists and reduces the moments delivered by the supporting member.

2) Folks, including me, tend to weld the snot out of these things, often for aesthetic or durability reasons. Or just laziness. By the time that you throw an all around 1/4" fillet weld on an HSS 4x4, you're probably pretty close to being able to develop the full shear and moment capacity of the supported member through the weld.

you'll see why Koot is so nonchalant about it.

Exactly. And as any of the locals around here will attest, I'm not known for being especially nonchalant. If I don't care about something, it's a pretty safe bet that it's a pretty safe bet (rotten wood to steel connections excepted). But yeah, I totally agree with Mike too. My favorite junior engineer is one who's been bothered by this, has thought about it in the shower for a few days, and has subsequently set it aside as way less scary than the other scary stuff that we assume on a day to day basis.

There are some situations where the distribution of weld stress is poorly known and AISC will have you multiply your demand by 1.25 to compensate. I think that provides a rough indication of their level of concern.

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.