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Plastic Moment - Failing Flanges

Plastic Moment - Failing Flanges

Plastic Moment - Failing Flanges

(OP)
Anyone know a good way to design Shear + Moment connection with directly welded flanges with flanges failing in tension at Full Pen welds?

Not sure how to distribute the rest of the moment in the beam web - and not too excited about doing it at all.. Constraints: No flange plates, no extra field welding.

RE: Plastic Moment - Failing Flanges

The connection you're talking about, CJP welding the flanges to the supporting member, will develop the plastic moment of the beam.  Look at AISC Manual of Steel Construction, 3rd Edition LRFD, page 12-6.

RE: Plastic Moment - Failing Flanges

(OP)
Yes.. then you take Flanges in tension right? and what do you get..

For W24X117 beam
Mp=816k-ft

Flange Forces = 816*12 / (24.3-0.85) = 418 kips

Flange Tension strength = bf*tf*Fy/1.67 = 12.8*0.85*50/1.67 = 326 kips...

326<418kips - NG...

Then EOR says, well take the rest into the web.. and how would you do that good sir? :)

RE: Plastic Moment - Failing Flanges

Did you look at the reference I posted?

RE: Plastic Moment - Failing Flanges

(OP)
I don't have 3rd ed. I have 2nd..  

RE: Plastic Moment - Failing Flanges

Page 10-19, in Volume II of the 2nd edition.

The plastic moment of the supported beam phiMp can be developed with sufficient inelastic rotation and deformation capacity through such a connection. This apparent increase in beam strength above the prediction of elastic theory occurs because of strain hardening in the flanges. See Huang, et al. (1973), Krawinkler and Popov (1982), and Beedle, et al. (1973).

RE: Plastic Moment - Failing Flanges

(OP)
yes. so what does that mean? Mp= Fy*Z.. its the maximum there can be, but the flanges are not connected to the web at the location of the FP welds and thus require tension check. Such as I posted above.. am I wrong?

RE: Plastic Moment - Failing Flanges

(OP)
Exactly like detail on p.10-20 LRFD 2nd ed.. You see where the backing bar is.. that's where I check the flange for tension and get a failure..

RE: Plastic Moment - Failing Flanges

So stop checking the flange for tension.  

RE: Plastic Moment - Failing Flanges

(OP)
and even then.. the bolt holes for the shear pl or dbl angle reduce the beam Moment capacity even further..
 

RE: Plastic Moment - Failing Flanges

(OP)
Alright.. I'll just drop the check I guess...

RE: Plastic Moment - Failing Flanges

(OP)
Maybe the EOR will let it fly this time :)

RE: Plastic Moment - Failing Flanges

(OP)
Personally I am not satisfied with strain hardening as a way to justify the flange full pen welds due to beam plastic moment. I think that the strain hardening should be used as a region of safety not a design region. Anyone has any thoughts on this?

RE: Plastic Moment - Failing Flanges

where does this Mp=816k-ft come from?


the beam can be stronger than Mp= Fy*Z.


why do you need check that?

RE: Plastic Moment - Failing Flanges

You might get more response if you included a sketch.

RE: Plastic Moment - Failing Flanges

I am fairly certain these types of connections have been lab tested extensively.

I am with dcceecy here, where is this Mp=816 kip-ft coming from?
  

RE: Plastic Moment - Failing Flanges

Mp = Fy Zx / 1.67 = 50*327/1.67/12 = 816 k-ft.  This is ASD, and tabulated in the 13th edition AISC manual on page 3-51.


Quote (ToadJones):

I am fairly certain these types of connections have been lab tested extensively.

Yes.

RE: Plastic Moment - Failing Flanges

Oh, I was confused because there were two previous references to LRFD specs.
 

RE: Plastic Moment - Failing Flanges

(OP)
ToadJones,
nutte,

And what are the test results?
It doesn't help any if the EOR asks you to put the rest of the moment into the beam, you try really hard to figure something out, and then days later the EOR gives up and says "Well its ok, let it go then", but in the back of your mind you know that each flange is about 80+/-k kips short in plastic limit strength...


 

RE: Plastic Moment - Failing Flanges

(OP)
If I was EOR I would not rely on strain hardening. Any comments?

RE: Plastic Moment - Failing Flanges

What are the test results?

Quote:

See Huang, et al. (1973), Krawinkler and Popov (1982), and Beedle, et al. (1973).

If an EOR would not approve this standard connection, used successfully for a long period of time, approved and recommended by AISC, and supported by testing and documentation, I would say that EOR was uninformed.  If he wouldn't approve it after the above evidence was presented, I would say he was unreasonable.

RE: Plastic Moment - Failing Flanges

(OP)
nutte,

I am a recent graduate and I was wondering what do you think about strain hardening as a design limitation? In what situation would it/not be appropriate...

Another interesting journal on that topic:

"Truss Analogy for Steel Moment Connections"
Subhash C. Goel, Bozidar Stojadinovic, and Kyoung-Hyeog Lee
 

RE: Plastic Moment - Failing Flanges

iponom-
 
The beam would begin to yield at the yield moment, My=FySx.
Beyond that, the stress distribution on the cross-section is no longer linear. At some point the entire cross-section would in theory reach Fy, not just the outer fibers or just the flanges--> plastic moment Mp=FyZx.
Your calculation of the force in the flanges is more of an analogy to a force couple than the actual summation of force in the flanges. The flanges will not carry the entire moment. In reality, there will be some interrupted stress flow from the flanges into the web.  
In-other-words, you are doing a very rough calculation of the force in the flanges and expecting exact results.
Furthermore, it is okay to follow the code in this situation as you have already predicted loads and are dividing by a nice cushy factor of 1.67.



 

RE: Plastic Moment - Failing Flanges

iponom..

As others have stated, the code has approved this connection. They have done it for a reason.

I think you are looking for a physical description of what is actually occurring, so what follows is a summary:

In a fixed fixed connection, the ends of the beam begin to yield first. If both your flanges and your web were welded to the supporting member, then you would develop (theoretically) the plastic moment capacity at the joint.

In this case, however, only the flanges are welded to the supporting member so you get somewhat less than the plastic moment at the joint. However, the maximum moment is still at the support, so your beam has more capacity slightly away from the support. the joint will begin to rotate and the plastic moment will spread away from the joint.

This is a fairly long way of saying that there is ductility in the connection which allows the true plastic moment to form close to, but not exactly at, the joint. If you pick up a steel design textbook and look at plastic design, you'll see what I'm referring to (not to be confused with plastic moment capacity of a cross-section).

RE: Plastic Moment - Failing Flanges

I know people have referenced previous editions of the manual in earlier posts, but can anyone tell where this issue is discussed in the 13th edition?

Also, to add to this discussion:  I believe the reason you can rely on strain hardening in this connection is because it only over a small distance (i.e. the distance that the beam flange overhangs the web to make the weld).  

Consider a tension member with bolt holes; it is for this same reason that you check rupture (which relies on strain hardening)on the net section and yielding on the gross section.  Yielding occurs at the net section before rupture occurs, but since the deformation is so small because it is over such a small length (think PL/AE) that it is not significant enough to be considered a limit state. You can think of this overhanging flange as a tension member.  

RE: Plastic Moment - Failing Flanges

They didn't put it in the 13th edition.  That doesn't change all the testing that has been done showing that this connection develops the plastic moment.

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