Instanteous Center of Rotation Help 1

[Jerehmy]

I was comparing Blodgett's method for analyzing stress in a weld to determine capacity compared to the AISC tables for a weldment group.

I get a large discrepancy. This isn't for a job or anything, just something I was doing on the side when bored. Anyone else that's more familiar with this can point out what's wrong?

I made a mathcad sheet for blodgett's method and did a quick check using the AISC table at the bottom and compared the two. attached

whatever is wrong I don't see it. Is it Jw? not sure


Thanks

 

[Jerehmy]

Oh, and blodgett's example is for a double angle but it still works for a single angle (multiplied by 2 for stresses) as far as I could tell. Also, the extra torsional moment caused by the double eccentricity (Lt) doesn't increase the stress very much.

I should have set Lt=0

attached with Lt = 0

 

[sbisteel]

The instantaneous center of rotation method is a plastic method, whereas Blodgett's method is elastic. The AISC verbage in the manual states that the elastic method is simple but extremely conservative since it neglects weld ductility.

 

[KootK]

I almost agree with sbissteel. Both methods are instantaneous center of rotation and I believe that both are elastic. The only difference is that Blodgett's method is linearly elastic and the latest AISC method is non-linearly elastic to reflect the latest research regarding force-displacement relationships for 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.

 

[Jerehmy]

So the Jw factor would be the elastic section modulus. If I calculated the plastic section modulus of the weldment and plug it into the blodgett's, I should get similar answers, no?

 

[sbisteel]

No, KootK is right; "plastic" was not the best word to use. The tables are made assuming a deformation at the highest stressed portion of the weld, and the remaining portions of the weld (they recommend 20 portions minimum) are included with their respective strengths assuming this deformation. It requires iteration to come up with the correct solution, and is pretty tough to do manually, hence the tables.

 

[Lion06]

I believe IC method is a plastic stress distribution. The AISC manual talks specifically about taking advantage of the ductility of the welds.

 

[Jerehmy]

I see. Do any of you know where there is more information on it, like an example showing iterations or explaining the process of analysis. The commentary references Lesik and Kennedy (1990). I'll prolly buy it unless there is something else that's preferable.

 

[sbisteel]

Someone was smart enough to figure it out and write a general program in excel to go through the motions, and he has a blog where he went into more detail about it.

http://www.steeltools.org/resources/viewdocument/?DocumentKey=40e83a4a-7d05-4e72-9417-fb0bcfea78bb

 

[KootK]

I believe IC method is a plastic stress distribution. The AISC manual talks specifically about taking advantage of the ductility of the welds.

I'm at a disadvantage as my AISC manual is at home. I'll check it out tonight. There is a a subtle difference between ductility and plasticity. If our welds truly went plastic, there would be permanent set.

If I calculated the plastic section modulus of the weldment and plug it into the blodgett's, I should get similar answers, no?

I don't think so. It's a numerical procedure that is not based on plastification as sbisteel has alluded above. If you make a spreadsheet using the modern non-linear method, and then switch your force-deformation relationship to be linear, you'll get Blodgett's results.

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.

 

[Jerehmy]

man I really liked blodgett's method, but it's so damn conservative . I feel as though I must have an error though in my mathcad sheet because I get 75% more capacity than blodgett's using the ductile method. Is that typical?

Also, found this article Link

I shall read.

 

[Lion06]

That second quote was not from me.

Regarding plastic vs. ductility - ductility is by defined by a materials ability to move past linear elastic behavior. What that means to me is plastic behavior. It doesn't necessarily mean permanent deformations until you get to the ultimate strength level. It's to a beam. It's elastic under service loading, but at ultimate strength is counting on the plastic strength of the section.

 

[Lion06]

Jerehmy - I think you'll see the conservatism increase as the eccentricity of the applied load increases.

 

[KootK]

@Jerehmy: yeah, pretty typical. Still, I use Blogett's method regularly.

@Lion: sorry about the misquote. iPhone copy/paste abuse. Regarding our debate:

1) plasticity = fibers yielding = some degree of permanent deformation. This is true even when only part of a section plastifies.

2) you can be non-linear -- and ductile -- without yielding any fibers. Just ask a rubber band.[pre][/pre]

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.

 

[Lion06]

I don't know if a rubber band is linear or non-linear, but the deformations are elastic. I don't consider a rubber band to be ductile. I consider it quite brittle. I've never found a rubber band to undergo any permanent deformations without snapping.

 

[Jerehmy]

my boss had my steel book but I snagged it back. It hase a pretty thorough explanation and example (salmon johnson and malhas).

Something I noticed though, none of the articles I have read talk about a double eccentricity such as a single angle connection to a beam. I modified blodgett's to include it. Any thoughts to why this is?

 

[KootK]

A rubber band may have been a uniquely terrible example. A little googling yielded some results that surprised me: Link

It seems that a rubber band is:

1) Non-linear.
2) Ductile.
3) Elastic? I'm not really sure.

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.

 

[KootK]

Something I noticed though, none of the articles I have read talk about a double eccentricity such as a single angle connection to a beam. I modified blodgett's to include it. Any thoughts to why this is?

a) because it's hard.
b) for that particular example, you'd normally assume that angle flexibility eliminates eccentricity in the one direction.

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.

 

[Lion06]

If you can stretch something to 3x its original length and it goes back to it's original length when unloaded, that's elastic.
That stress-strain curve does not look at all like a ductile material to me.
I don't really care about the elasticity/ductility of a rubber band.

I'm struggling to think of a material that doesn't undergo permanent deformation as ductile, regardless of the nonlinearity of the elastic deformations. S&J defines ductility as the amount of permanent strain (i.e. strain exceeding proportional limit) up to the point of fracture. The very nature of ductility in this context requires permanent strain (i.e. plastic deformations).

 

[Jerehmy]

For a single angle single bolt, angle welded to stair stringer(used almost exclusively for stair stringers), you can't ignore it. And how's it hard for elastic analysis? Blodgett gives the torsion, it's judt the in plane moment which is easy to distribute to the welds.