Plastic Moment - Failing Flanges

[iponom]

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.

 

[nutte]

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.

 

[iponom]

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?

 

[nutte]

Did you look at the reference I posted?

 

[iponom]

I don't have 3rd ed. I have 2nd..

 

[nutte]

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).

 

[iponom]

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?

 

[iponom]

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..

 

[nutte]

So stop checking the flange for tension.

 

[iponom]

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

 

[iponom]

Alright.. I'll just drop the check I guess...

 

[iponom]

Maybe the EOR will let it fly this time

 

[iponom]

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?

 

[dcceecy]

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


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


why do you need check that?

 

[apsix]

You might get more response if you included a sketch.

 

[ToadJones]

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?

 

[nutte]

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.

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

Yes.

 

[ToadJones]

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

 

[iponom]

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...

 

[iponom]

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