Analysis Discussion: HSS Baseplate, usually assumed as pinned. Yet in my FE analysis it seems Fixed. 2

[IngDod]

Greetings, attached is a summary of my FEA analysis.

I am currently designing a two story OMF building. My initial assumption was to assumed the columns pinned at the base, I did this because it seems to be the common practice. But now that the time has come to design the base plates I'm having problems (cant sleep) simply assuming its pinned and designing it solely for axial force and shear (using AISC's DG1), it troubles me that if this connection is actually fixed (or at least with high rigidity) the baseplate will fail.

Now, I've always had problems grasping the concept of steel connection design; some people say that since you assume is pinned and you designed for axial loads only then it will behave as such. I proceeded to do a FE model, if the connection is indeed pinned then I should see little (no connection is truly pinned of course) moment being transferred to the baseplate. What I get is that the base-plate bends considerably due to the moment, now i would expect this since it is precisely this bending which allows rotation; however the stresses in the plate are much higher than what it can resist... So while it rotates, it seems to me that it would fail too. Now i proceed to increase the baseplate size so that it can resist the stresses, now the rotation is much much smaller... giving me the impression that by increasing the thickness I have made the connection rigid.

Please see the attached summary as it is much easier to understand. The initial baseplate designed for axial load only is 1cm thick, while the one designed based on the FEA is 2.3cm thick. I added results for more thicknesses to study the behavior, with 5cm thickness the rotation seems very low.

I don't really know how to judge if the connection rotation is high or low, or whether it approaches rigid or pinned behavior. I'm getting rotations of 2 degrees for the 1cm thick plate and around 0.5 degrees for others... is this a lot? is this rigid?. How much should a connection rotate in order to be considered as pinned?

Thanks and I hope you can help me with this dilemma.

 

[IngDod]

@AELLC: Actually this is the first time I am attempting to use FEA for this type of calculations... I usually do baseplate and foundation by hand. For the frames I usually do use FEA (stick frames), but I still double check some results using approximate methods and I feel confident interpreting the results from frame analysis. I don't argue that assuming pinned and designing the baseplate as such will work, I just wanted to arrive at the same conclusion using FEA analysis.

I got my M.S in Civil Engineering 3 years ago, I have taken several classes and courses in FEA modeling of frames.. Im not saying this validates using or not using FEA for relative newbies like me.. just saying that I know what Im doing when I do a FEA model of a frame.

 

[AELLC]

When I went to University, FEA was just an academic curiosity thing - I have done all my steel bldgs. either by hand or by using primitive 2D frame analyzers or STRUDL, ones like that - you input beam and column EI, and it gives you only moments, shears and axial forces at end of each member, and deflection and rotation at each node. Simple enough.

Actually, my first job was fixed offshore oil production platforms in the Gulf of Mexico, we had a sophisticated 3D frame analyzers for the underwater structure (jacket) up to the main deck about 100' above sea level, no FEA at all, and we did all the superstructure framing purely by hand. A lot of stuff with no Code guidance at all. BTW we only went by American Petroleum Institute code. That was in 1974-1975 and we were in only 350' depth of water then. The foundation piles went about 300' into the seafloor (mostly loose crud), and all the conductor pipes (piping for oil) went down thru the 2 large vertical legs of the jacket. The jacket had only 4 legs total, and the 2 were battered (sloped), leaning toward the direction of the worse hurricane. The 2 main vertical legs were always 64" dia and the 2 battered were always 48" diameter but we varied the thickness especially in the joint regions, they were much thicker.

 

[AELLC]

University "conditions" you to arrive at an exact, rational and mathematically-correct answer within 3% or you get no credit - real work is so different, it takes a while to change. So all I am saying, don't micro-manage the design of that baseplate.

 

[CELinOttawa]

Having established that FEA does not capture stress redistribution due to yielding, it would seem to me (feel free to call me an idiot if I am wrong) that it would still give a close to reality result for connections where no yielding is desired.. For example a fixed connection where i wanted to reduce rotation to the minimum.

You're not an idiot; You're learning... And you're smart enough to ask questions when you're not sure. That's a sign of intelligence, not stupidity.

SO: For a fixed/rigid connection, the model will only give you as accurate a result as your real world material is capable of conforming to with respec to the model. Remember, there is a whole lot going on in your section that we just "set aside" when doing design, ie: Metalurgical defects, fabrication errors, residual strains, etc. They are covered by our material reduction factors when designing in terms of strength, but they also have an effect on the actual (service) behaviour.

Your thinking is correct, just remember to think that you CANNOT model it at all accurately; You can only make enough assumptions and set up the conditions such that you can model and design SAFELY.

At three years out, this may still be of some help. You may not get the cultural joke, but a Canadian Intern would invariably find this something to chuckle at:

My personal rule to competent Structural Engineering is SSS DUCE.

Strength
Stability
Stiffness

Durability
Utility
Constructibility
Economy

 

[AELLC]

"Your thinking is correct, just remember to think that you CANNOT model it at all accurately; You can only make enough assumptions and set up the conditions such that you can model and design SAFELY."

At University, you had to model it ALL accurately for full credit. What I meant by "conditioning"

 

[AELLC]

@IngDod

Virtually everything I have ever done was according to precedence ("go-by") - what about other completed projects in your office, are they similar, how was the baseplate treated?

 

[IngDod]

@AELLC and CELinOttawa: Thank you both for your words and guidance in this, I don't have the benefit of a mentor or even a colleague who I can ask this kind of questions and expect a proper response.

@CELinOttawa: I understand, at least I can use this to better understand the behavior of some very non-standard connections that are sometimes required here.

@AELLC: The rule-of-thumb has been to design concrete columns as fixed on the base and steel columns without any stiffening as pinned on the base, for the base plates we usually design for axial load only, following the AISC Design guide. Using this design method the base-plate thickness for the current structure I'm designing is 1cm (about 0.4 inches)... Probably bumped to 1/2" since plate thickness is measured in inches here, having read yours and others suggestions in this post i feel inclined to take it up to 3/4". There is considerable shear on the baseplate, enough so that two bolts might not be enough. I might have to add a shear lug (not common here) or embed part the hss in the slab (they do that all the time here.. but by error.. the pedestal never extends above the slab on grade so they usually pour a 10cm slab on top of the baseplate).

 

[AELLC]

In my whole experience, no baseplate has ever been less than 3/4" for a one-story building.

 

[AELLC]

A residential steel post would typically get a 3/8" or 1/2" baseplate. It is pretty much standard because the standard details I use are the same as 90% everyone else uses.

 

[rowingengineer]

the component metbod for semi rigid is easy and repeatable. very similar to deflection analysis with springs. i would recommend taking a look.

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."

 

[IngDod]

@AELLC: I would like to see the design assumptions for this baseplates, when using AISC DG1 for axially loaded baseplates I consistently get a thickness of around 1/2"; I always do this calculations both by hand (a spreadsheet I developed) and using risabase and I get identical results. My guess would be that the plate is being designed to take some moment?

@Rowingengineer: Thanks, I had read about this method before but I have never used it before I will give it a try.

 

[AELLC]

IngDod,

I don't design in steel any more - it has been years, and I was working on memory - you are saying by calculation it comes to 1/2", that is fine if you are comfortable with it. Me, I would go to 5/8" or 3/4" because the cost increase is insignificant.

In "real world" at work, we are not calculating to get the "textbook answer" the professor is looking for, to get full credit on the test. We are doing things to save time and end up with a practical and cost-efficient answer.

Anyway, see attached procedure from the old AISC 7th edition here (ASD)



The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[IngDod]

@AELLC: Thanks for the information, I am still stuck in the "textbook" mindset I fear... I lose sleep over some details that seem rather silly the next morning, but I hope that as i gain more experience and "feel" for the structure this will improve.

 

[AELLC]

In my mind, it should be 3/4" to 7/8" because you mentioned 2-stories, however it may calculate out to be less.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[AELLC]

My first job was fixed off-shore platforms. The main part of the structure was pretty much cookbook recipe but we "winged it" a lot for stuff built into the decks of those things.



The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[IngDod]

@AELLC: that must have quite the first job!

 

[IngDod]

typo edit: *that must have BEEN quite the first job...

 

[AELLC]

However, it was frustrating compared to nowadays how computer work was done, and we did not have pc's at our desks.
We had to "code up" the structure on paper input sheets, and wait until the next day to receive a computer printout of results, many inches thick.

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[IngDod]

Wow it must have been incredibly hard... out of curiosity.. the input for the computers where the stiffness and load vector of the structure? meaning you had to assemble the whole matrix yourself by hand so the computer could solve them? I would imagine that any program that required paper input would have almost zero modeling capabilities... today its has become so easy to model structures that I constantly see people making stupid mistakes... making everything in the structure fixed or everything pinned without any regard of the actual behavior of the connection.. and even more stupid... since everything here is done with these dreadful steel tubes everyone think its just fine to weld them all together and call it a fixed connection... so no one bothers to actually calculate these connections.

 

[AELLC]

Not so hard - I meant code up by pencil and paper because there was no mouse, keyboard, and monitor - for example it was filing out pre-printed forms with all input data and some unseen person would create punchcards for feeding into some mainframe computer

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.