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.
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
RE: Plastic Moment - Failing Flanges
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
RE: Plastic Moment - Failing Flanges
RE: Plastic Moment - Failing Flanges
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
RE: Plastic Moment - Failing Flanges
RE: Plastic Moment - Failing Flanges
RE: Plastic Moment - Failing Flanges
RE: Plastic Moment - Failing Flanges
RE: Plastic Moment - Failing Flanges
RE: Plastic Moment - Failing Flanges
RE: Plastic Moment - Failing Flanges
the beam can be stronger than Mp= Fy*Z.
why do you need check that?
RE: Plastic Moment - Failing Flanges
RE: Plastic Moment - Failing Flanges
I am with dcceecy here, where is this Mp=816 kip-ft coming from?
RE: Plastic Moment - Failing Flanges
Yes.
RE: Plastic Moment - Failing Flanges
RE: Plastic Moment - Failing Flanges
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
RE: Plastic Moment - Failing Flanges
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
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
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
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
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