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Theoretical vs Actual Boundary Conditions
5

Theoretical vs Actual Boundary Conditions

Theoretical vs Actual Boundary Conditions

(OP)
What are your opinions on using boundary conditions that don't represent the real world reaction a support will see?

Example: designing a frame for a pinned-based connection with a base-plate (that will develop some unintended moment capacity). The intent is to hold the moment in the upper portion of the frame (post to beam) only. Consider adding a diagonal brace to take some sway out of the frame if necessary. However, the base plate being specified (post welded to plate) will develop some sort of moment capacity (not intended) due to the base plate and anchor configuration. In order to design the plate & host reaction, an assumption to whether it is pinned or fixed has to be made. However, the intent to call this pinned could be considered wrong as the plate will develop an unspecified moment capacity. Even though it's intended to be pinned, the pin will place a moment on the host structure that needs to be considered.

Opinions?

Thanks

RE: Theoretical vs Actual Boundary Conditions

Quote (StrEng007)

What are your opinions on using boundary conditions that don't represent the real world reaction a support will see?

In general, boundary conditions that match reality are obviously preferable. Where this is not possible, or desired, the connection should be able to morph into the assumed boundary condition in a ductile fashion.

In the situation that you've described, I'll usually try to bracket the design by assuming pinned base connection for the design of the superstructure but fixed base connection for the design of the connection itself. If I don't like the answers for the base connection, sometimes I'll take steps to make the base connection more flexible and try to assess how much moment I really think will be drawn to the connection.

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.

RE: Theoretical vs Actual Boundary Conditions

kk solution sounds ok, but i prefer semirigid modelling, easy to do, works well

http://www.nceng.com.au/
"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

RE: Theoretical vs Actual Boundary Conditions

Quote (rowingengineer)

i prefer semirigid modelling, easy to do

Surely you jest? If you know of an easy way to accurately assess the rotational flexibility of a base plate / anchor bolt / grout bed / pier / foundation / soil assembly... do tell. All I know of is the abbreviated PCI method and full on FEM, neither of which leave me brimming over with confidence.

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.

RE: Theoretical vs Actual Boundary Conditions

In general, it's good to detail such that you avoid non-ductile failure mechanisms. Then just make the most reasonable assumption you can.

But also don't overthink it in real design cases. Spend time learning and understanding the consequences of the assumptions, but keep things straightforward during actual design. Feel free to use partial fixity when you have good control over the situation, but don't force it in applications where it may be fuzzy. In my opinion, you've got more risk of making a dangerous mistake by complicating the design process in a typical case than the benefit you're getting to cost or safety by possibly becoming more accurate.

Be comforted by the fact that structures have been designed based on these kinds of simplified assumptions throughout the history of steel design without systemic failures because of those assumptions.

RE: Theoretical vs Actual Boundary Conditions

I've become a bit discomfited by the poor performance of "properly" designed base connections in the only full scale testing program that I know of: Link. It's great that nothing bad has been happening out in the wild. It would be much better, however, if there was reason to believe that that had anything at all to do with our ability to actually evaluate these connections accurately.

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.

RE: Theoretical vs Actual Boundary Conditions

KootK. I follow the same procedure that you do. Members designed for worse case, connections designed for worse case. And sometimes i adjust my assumptions if answers don't work well.

And nice link. I know and have worked with the Author of that paper (Professor Kanvinde is one smart and funny man) thumbsup2

RE: Theoretical vs Actual Boundary Conditions

Oh yeah, anchorage design is definitely a different story.

That's a really neat looking paper. I think I have my weekend reading. None of the issues in the paper really surprise me. Shear key performance especially has always seen incredibly optimistic to me.

RE: Theoretical vs Actual Boundary Conditions

If you ever want your confidence in typical looking "fixed base" connections shaken to the core, look at what they suggest in the AISC seismic manual for situations where it counts (plastic hinging at moment frame column bases). It's all columns embedded to China in giant grade beams with deformed bar anchors etc...

@EngEric: Master's work I assume? Color me envious.

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.

RE: Theoretical vs Actual Boundary Conditions

KK this is where i started my liking of the component method

http://people.fsv.cvut.cz/~wald/COST-C1-Column-Bas...

Eurocode 3 base d a bot of what they have doe off the above.

there are a few resources around, I recommend reviewing it, it is very helpful for all sorts of connections.

http://www.nceng.com.au/
"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

RE: Theoretical vs Actual Boundary Conditions

Next frontier is when software will be able to reliably implement methods such as the component method.

Very fast and accurate designs ahead. Perhaps too accurate if you know what I mean...

EDIT: RE I can't seem to download any of those PDF links in the webpage link you provided.

RE: Theoretical vs Actual Boundary Conditions

@RowingEngineer: thanks for the info. I'm dying to check it out but I also cannot download anything from the web page that you linked for us.

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.

RE: Theoretical vs Actual Boundary Conditions

Attached PDF file - again this only is a starting place, eurocode 3, part 8 will also help, but there is more to learn.

http://www.nceng.com.au/
"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

RE: Theoretical vs Actual Boundary Conditions

From what I am led to believe, Autodesk Robot will iterate the rotational stiffness of your connections = based upon material in the Eurocode.

Can anyone confirm the reliability?

RE: Theoretical vs Actual Boundary Conditions

Hey guys,

Got an email today from Istructe regarding the new steel design method. Shame I'm on the other side of the world...

Link

Assume you'll be interested RE & KootK.

RE: Theoretical vs Actual Boundary Conditions

Quote (Trenno)

Assume you'll be interested RE & KootK.

Ohhh yes. Thanks for sharing Trenno.

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.

RE: Theoretical vs Actual Boundary Conditions

Looks like a very simple yet sophisticated piece of kit.

I've downloaded the trial and am interested see what sort of rotational spring constants it comes up with for various arrangements.

Tutorial

Especially portal frame baseplates!



RE: Theoretical vs Actual Boundary Conditions

If you have a spare two minutes, give this video a watch.

Professor Wald from Czech Technical University in Prague talks about creating of new method for design of steel connections - CBFEM.

I've had a play on the software and it seems like it's a very intuitive and simple tool to use. Engineers can certainly pick it up and start using it right away.

But I still can't get my head around how using this software accounts for and adequately replaces (for example) the ~52 code based connection design checks required for a portal knee.

RE: Theoretical vs Actual Boundary Conditions

2
I have seen few different methods for handling the boundary condition described above including the PCI method KootK refers to. I also remember attending a few seminars given by the AISC.

One speaker said that as long as we have a method for resisting the loads the structure will perform as intended. He gave an example of how a an old structure analyzed by his company had a bunch of over stressed members, so many that they couldn't figure out how it was standing up. A few years later they stumbled upon a design method that verified the way the structure was designed, and all the over stresses went away. If I remember correctly, this was a seminar given by the AISC in regards to connection design.

In the other seminar, the lecturer presented materials that would justify using a 10% fixity at the base of the columns (or 0.4EI/L) based upon assumptions made in utilizing the old K charts found in the previous versions of the AISC. In these charts, one would assume G=10 for a pinned based column.

RE: Theoretical vs Actual Boundary Conditions

3
LOL.... That video is little more than a PR / Sales video. Nothing really technical about it. There's so many non-linear aspects of connections that it just seems too far fetched.

Are residual stresses from rolling or welding included in the FEM model? If so, how? Does FEM model include geometrically non-linear effects such as web local buckling or stiffener buckling? How do you model bolt pre-tension or bolt bearing and friction? What about fracture mechanics for block shear rupture or such? These are not simple questions!! FEM modeling of connections is just really, really tricky.

During the post-northridge years, there was all kinds of research done on moment connections. Including lots of FEM analysis. The end result is that if you want to do something that isn't already pre-approved, you have to go through physical testing per AISC 358H.... Because FEM models of connections just aren't reliable by themselves.

RE: Theoretical vs Actual Boundary Conditions

Couldn't agree with SteelPE and Joshplum more. There are millions of variables that FEA can't account for, I think trying to model connection stiffness/behavior is insane. Just detail a ductile failure mode (in this case, call the base pinned and make sure the anchors fail before the concrete breakout) that can realistically occur.

RE: Theoretical vs Actual Boundary Conditions

Quote (JoshPlum)

LOL.... That video is little more than a PR / Sales video. Nothing really technical about it.

I posted the video in the attempt to introduce the concept of CBFEM to people, who much like myself, had never really heard of or used these methods. If you want to know the technical side of it, I'm sure the information is out there.

You can do buckling analysis, yes.






RE: Theoretical vs Actual Boundary Conditions

As I understand it, you are not designing actively for the buckling mode, but determining when buckling stress takes place and then adding a stiffener to preclude the buckling mode?

RE: Theoretical vs Actual Boundary Conditions

Ended up asking for some questions (including yours, Josh):

1) Can you provide more information on how the program calculates the ‘equivalent stress.’

Equivalent stress is another name for von Mises stress. If you would like more information on this you can always to the wikipedia entry which is quite extensive. The reason for using this, is that according to EN 1993-1-5 and our approach to general connection design, this value is essential as it takes into account both shear and principal stresses (see also point 4)

2) I’m just wondering how things like residual stresses from rolling or welding are taken account of.

Right now, we are investigating how to take into account these effects into our models.

3) How would one model pre-tensioned, friction or bearing bolt types?

These kind of bolts are scheduled for development in 2016

4) Can the CBFEM method address shear rupture in plates?

Actually shear rupture is one mode of failure. In reality all possible modes of failure are combined and one of them usually prevails. And to make things worst, the loading coming from FEM models (which are more realistic) is complex and not simply a moment or a shear force. This is the reason that the component method is not applicable to strange connections (I'm trying to squeeze an one hour lecture in three sentences so please be forgiving). The CBFEM results always visualize these interactions.

Since it's impossible to elaborate this in the context of an email, I will try to demonstrate this in an IDEA Connection model.

I create the following model of two HEA 200 beams. I connect them with a very thin plate (4mm) and a lot of strong bolts to ensure that they will not fail, and also that the bolt hole will not fail. Then I apply a big enough shear force.



The equivalent stresses check in the plate (which take into account shear stress) have the following distribution:



Green part is over 80% and yellow parts are above the limit. The shear rupture failure is clearly located in the yellow area and passes through the bolt column.

Since bolt utilization is low (picture below) we know that they do not fail



But in the end we see that the plate is also not failing since the strain of plate is quite low.



Which is not the case if I double the acting shear force



To conclude, our global criterion is the strain check (this comes from EN 1993-1-5 material model and experiments)




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