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

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

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

(OP)
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.

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

I have done steel OMF in the past, and I always provided a conservatively thick baseplate without worrying at all about the partial fixity issue.

In fact, one was 11 stories tall. and was built about 1986. It is fine to this day. We did not have FEA on computers back then where I worked. We had a simple 2D frame analyzer that ran on a strange "micro-computer".

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

About the same time I did a one-story OMF, but the columns were about 20-24 feet tall. The drift was not good, and I provided a full fixed base plate. Like I said, we had no FEA and little Code guidance so it made liberal use of stiffeners-

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

If you run the frame with pinned base condition, you are probably going to see only a very small rotation there at the base of the columns, and that isn't trying to stress that baseplate as much as you may think.

If you run it as 100% fixed base, than your moment maxes out as the rotation is zero.

Now when you run it FEA, that may create a false impression of localized high stresses because I am assuming this a linear FEA analysis. Also, the more you thicken the base plate, then you are increasing the bending moment in the base plate as it approaches that theoretical 100% fixed condition.

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

(OP)
Thanks for your reply, I feel that sometimes all the codes requirements and load factors are an overkill; so many building went up before all this fancy design methods and most of them are still standing. But still, it nags at me not to know how it actually works... Cause if the base-plate does take moment it will go straight to the footing... and moment at a footing not sized for moment could be pretty bad.. specially during an earthquake.

I found this classification system for connections and would like to hear your thougts:

Link: Starts at bottom of page 46
http://books.google.co.ve/books?id=J3SJsyhpLBwC&am...

It says that if the connection moment capacity is lower than 25% of the fixed end moment capacity then the connection is pinned. How could I apply this to my baseplate situation? basically i would like to come up with a criteria that allows me to say if the connection is pinned or not.

I supposed i could model the frame with fixed restraint at the column base.. this would give me the fixed end moment capacity when i perform the analysis. Here is the part i dont understand... say I design the baseplate to have 0 moment capacity.. very thin plate... according to this it would be pinned... yet when the structure is loaded would the plate not yield and tear due to the moment.. and should a FE analysis not show you no moment at the baseplate?

Thanks.

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

(OP)
Im using non-linear analysis in SAP2000. I have very high spikes in stress near the anchor bolts but otherwise the values seem agreeable.

My problem is that.. true i can run the analysis with pinned ends.. i get no moment at the base.. then i detail the baseplate with four bolts and the thickness for the axial load only... is this supposed to make it so that the baseplate takes no moment in real life?

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

The moment gets bigger as you make the baseplate thicker, but if the footing compresses the soil a little bit as it starts to rotate, than the moment decreases.

You are overthinking this. If you had only the AISC code book, your frame designer software, and no FEA, you would be OK with this.

Besides, the rotations we are talking about are very small. The columns bases are not rotating very much according to your output, right?

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

'

Quote:

I feel that sometimes all the codes requirements and load factors are an overkill; so many building went up before all this fancy design methods and most of them are still standing.

Be careful... Modern codes and methodology have allowed our modern world's construction methods. Very few buildings have survived from before modern codes (gravity codes in 1910s & 20, seismic with a reasonable middle step from the 30s to the 60s). I do a great deal of work on historical structures, and not a one could be built today without losing your shirt. There is a good reason for that: premium materials, expertly installed and finished BUT with tons of redundancy because so few knew what was really going on.

Shake your head, apply the codes correctly and fight to correct them when they become unnecessarily complex.

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

(OP)
@AELLC: So in your opinion i should treat this as pinned and design the baseplate and foundations for axial and shear load only? Have you designed buildings with HSS and baseplates similar to the one in my file and assumed pinned? (four bolts, one at each corner) if you have it would definitely make me more comfortable in proceeding with my design. I ask because there is little literature regarding hss column baseplates, if it were a W section I would not be conflicted as there is ample researh in to the behavior of this connections.. basically you put the bolts between the flanges and this results in plenty of rotation.

@CELinOttawa: Thanks; I might complain a lot about the codes and curse them for the many complexities they bring but I always follow them to the letter.

While right now im inclined to designed the base plates as pinned, I still would like to hear more from the community in the matter... specially regarding classification using the results i obtained.

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

The primary reason most buildings still stand after many years is that they have never seen the loads for which they were designed. When buildings are subjected to near their design loadings, we see more failures.

As for your model, if you have the capability in your FEA program, animate the model. This will show you how the model is acting under load and you can tell if you have a modeling issue.

Just because you model something in a particular way does not mean that it will act that way in the field. Model it the way you expect it to react....don't model it then expect it to react to your model.

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

I have designed plenty of buildings with baseplates having the anchor bolts in the 4 corners instead of being "tucked-in" between the flanges of a W-column. No problems.

I am just wondering why you refer to the baseplate as HSS, and the printout says it is A36.

Also, why are you using a square tube for the column? Is it large enough? - I did notice a very large rotation in two cases.

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

the first cut from an FEA program gives one a fairly accurate initial distribution of stress based on the initial stiffeness of the various components....what it may not capture is the redistribution of that stress as portions begin to yield...that is why one should have a solid grasp of the underlying engineering principles involved so you can make an independent engineering judgement on the program output. In reality, to design a truly fixed base pl condition would take a dedicated effort as there is usually enough "give" in the fdn as awhole to render the condition pinned. I usually design HSS base plates as pinned, however, if I am using post-instaled AB's , I design the AB's themselves for a fixed condition. My advise would be to turn the FEA progam off and analyze it as pinned and use a pracical base pl thickeness. For a typical stucture, I would not use anything less than 1/2" for base pl thickeness and would tend to go to 3/4" as a min.

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

As AELLC and SAIL are saying, the stiffness of the model is a moment in time... It does not reflect the changing conditions of reality, and maybe never will.

Frankly every engineer should read "Wooten's Third Law" five times before being allowed to touch a computer.

I never design exclusively by PC, and I would not have looked twice at the output you're stressing over. That's *not* an insult; I am setting you a challenge: Look beyond the output, after verifying there are no errors. Every design I put out is checked to within 20% "back of the envelope" style before I ever believe the computer or sign and seal.

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

As far as the Code being "overkill" - I have no problem with that.

I recently received news that the floor in a wood residential structure that I had designed long ago was finished with 3/4" mortar setting bed and natural stone flooring - had never heard of that during the design. It weighed a lot more than the ceramic/porcelain/travertine flooring that is commonly used here.

Also we get a lot of instances where the roofing is changed from plain concrete tile to clay tile with buttered end joints (without notification to me) - I actually heard from a builder that a roof over a porte-cochere sagged noticeably, and he went in and installed larger beams without any legal action taken against me, so I was lucky that time, also.

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

I second the requirement to read Wooten's law.

If you design it as pinned, but some moment is actually transferred to the base plate, what happens? If you base plate yields is that a bad thing? I would say no. Yielding a flat plate in the weak axis is very predictable and ductile. You can do it many times for small strains without adverse effect. Sail3's recommendations are good. Base plate yielding is fine, anchor bolt failure is bad.

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

Timber is a pretty large exception to most of the common structural rules, particularly under the US and Canadian codes. The majority of structures are so far above the code minimum due to the 5% rule applied in timber design. This is necessitated by the fact that timber is such a variable material, but stress grading is brining the inherent redudancy of timber down a great deal.

I think we will start seeing timber collapses of the nature of the occassional steel and reinforced concrete collapse we see now, but not for another thirty or so years (20 at an absolute minimum).

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

I think SAIL and Gumpmaster concisely nailed it.

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

CEL-

I never head of Wooten...that was interesting.

I have read Russell Fling's (Designer of 30,000 concrete beams by ASD elastic method) essay of why USD design is nonsense. That was good read. (But I lost my copy)

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

There have been a few post on this forum about semi rigid analysis of base plates for portal or similar frames, I prefer this method.

This allows you to estimate the defections a little better and also predict the moment distribution due to your detailing a touch better.

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

USD? Do you mean Limit States Design (LSD), the trip which most every Structural Engineer "takes" daily? I'd be very keen to read that, and either my google fu is down or it is hard to find on the net...

Oh, and frankly Wooten is the man. I mention the third law weekly, and daily if not hourly when training an intern.

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

I think we referred to that nonsense as Ultimate Strength Design, back in the day. I went to many a seminar where they championed it. We should start a separate thread of what nonsense it has created, and how many engineers are confused.

I was hi-tech when I went to graduate school, however. My thesis was Progressive Collapse of Steel Frames.

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

Lol... Well AELLC, you're certainly an American.

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

>>>I have read Russell Fling's (Designer of 30,000 concrete beams by ASD elastic method) essay of why USD design is nonsense. That was good read. (But I lost my copy)<<<

I would love to read that as well; it sounds interesting. Do you remember if it was a stand-alone article or part of a book?

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

I always have had this perception of engineers in the UK, Europe, China, NZ, Australia, Canada, etc as being more advanced than here in the USA, but we make better softwares, computers, and airplanes.

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

I don't think it is at all fair to say that the Engineers are more advanced outside the US, but I do think that the rigour with which most countries train their engineers is no longer "standard" in the US.

From school to school the quality of graduates varies much more greatly out of the US than out of most other countries with which I am failiar... As a result your codes tend to me even more "recipe book" than anywhere else. That holds back true understanding and lends itself to the comoditization of Engineering.

VERY good engineers (and quite sh*te engineers) exist in all jurisdictions.

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

I think the biggest problem here in the USA is:

We test and categorize the heck out of our High School students, but we don't know how to teach them math and science properly.

BTW - my HS counselor recommended to me that I don't go to college. Still trying to figure that one out.

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

I'd say you've proven your BS Guidance ClownSeller wrong with aplumb(bob). *evil smiles*

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

I wonder if IngDod got his project finished without any more fretting. We all spent a lot of time helping him.

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

(OP)
@AELLC: Sorry I had not posted... I had plenty of non-engineering things to do today; but i have been following the thread all day from my phone.
The baseplate is A36, the HSS tube is a different steel (im not in the US, I dont know if HSS in the US is also A36). I use HSS because thats produced in my country, W shapes are imported so the price is very high. Here we use HSS for both beams and columns.

@Ron: I agree with you on structures not actually seeing the loads for which they have been designed. For example I would say that the 1.6 amplification on the live load pretty much guarantees that the live load will not be exceeded, unless of course the owner decides to change the use of the structure... say turn a second floor of a commercial building into a storage area.. that would be a big problem and indeed I have seen it happen; however I would say this is not a design error but rather a user error.

I checked the model and qualitatively it seems to behave as expected, I recently took a SAP2000 class that covered modeling baseplates and I based my model on this.

@Sail3: Thanks for such a thorough answer, I understand what you say about the redistribution of stress; my intention with the FEA model is to determine the connection stiffness at the onset of yielding in the plate; My understanding is that as the plate further yields and redistribution occurs the rotation of the connection would increase; thus a connection that behaves as pinned in the FEA model should approach such condition in real life. However I do feel uncomfortable not being able to model or predict this redistribution.. While I know that redistribution occurs I do not know how to calculate or estimate it. If anyone knows of a way to do this I would be most grateful.

@CELinOttawa: I agree with you, I also do not design solely by black box programs. In this case I am trying to better understand the behavior of the connection, in the past I have relied on AISC DG1 for calculations. I use FEA as a tool, I do not blindly accept the results; if I did we would not be having this conversation. If you have a "back of the envelope" method to quickly estimate connection stiffness and rotation I would be most grateful if you could share it.

@Gumpmaster: Thanks for your reply, I would pose the question how can I determine if there is too much yielding? There must be a method to ascertain this or at least to help in making an engineering call.

@rowingengineer: To be honest I have never designed a semirigid connection, my understanding is that you would need the moment-rotation curves for such a connection. In the past I have chosen connections that are widely assumed to be pinned or fixed. But I can see how such analysis and design would be better, specially since no connection is truly pinned or rigid.

@johnbridge231: I added springs to the area elements that form the baseplate, this springs are compression only and are very stiff to simulate concrete pedestal. The bolts are modeled as a pinned restraint; the loads are non-linear and the analysis is as follow: apply axial load, then use the matrix resulting from this load to apply the moment load.

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

IngDod,

We don't see A36 but rarely in the USA now. Most steel is Fy = 50ksi.

HSS means Hollow Steel Section, not High Strength Steel. Fy can be from 36 to 50ksi, for rectangular tube section, the 2 most prevalent here are 46 and 50 depending on the exact steel spec.

I am surprised you use so much HSS instead of W-section. Here, the W is cheaper and much easier to make connections. Plus, with HSS and you have a non-thru shear tab, the question is, does the tab buckle the wall of the HSS, or does it punch thru? Sounds like a can of worms.

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

Here's a quick and reliable trick for checking and working with stiffness of a connection in steel: %(EI)

I'm not trying to be smart... Most software will allow you to enter this value as a stiffness at a connection, and in hand calcs you can apply a moment based on the partial stiffness.

Whatever moment you develop at that point and are happy with, you design your system to sustain without failure (both SLS & ULS).

Thus is exactly the type of situation Wooten was envisioning: You will never know the exact state of stress, even with FEA, so pick a possible state of stress and design against that. Plastic materials WILL conform.

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

(OP)
Here we use A36 mainly for baseplates and such. Yes for some reason the hot-rolled W-Sections produced locally only go up to 14cm depth.. everything else is imported. There are companies that manufacture W-shapes by welding three plates, but even that is not very common. The HSS beam and column situation is truly terrible.. For gravitational loads i would say is not that bad.. but for lateral loads there is of course the risk of punching trough the wall of the columns when using moment frames. Dont start me on the connections... its awful.. directly welding steel tubes is a recipe for disaster if you intent to develop any moment, I usually detail all the beams with a haunch in both ends to decrease rotation in the connection and avoid plastification and/or punching of the column wall.

I base most of my calculations in the design guides provided by CIDECT, which is an european agency that promotes and research the use of rectangular and circular hollow sections. The top of the cherry is that the hss sections produced here are not compact and 90% of the country is under the risk of a seismic event. Dont try to convince anyone here to use a single bolt on their structure... absolutely everything is welded... the baseplate.. they weld rebars to the plate instead of using proper anchor bolts... The rebar manufacturer clearly states that they rebars are not weldable. Thank god that most structures in my city don't go over two floors, I studied my M.S in the US.. imagine my shock after coming back here and facing all this steel tubes beams and columns... even in universities here they teach steel design with only W-Sections... Its absurd.

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

Very interesting...what country?

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

Sounds like Indonesia, or maybe Sri Lanka.

Vietnam still uses Bassemer steel for EVERYTHING.

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

(OP)
After reading all your helpful posts I am inclined to design the baseplate as pinned. Following SAIL3's advice to design the bolts for a fixed condition seems like a very good practice and on the safe side.

As I see it right now a column-baseplate connection fixity depends primarily on the thickness of the plate (supposing no stiffeners are used), a thin plate behaves as pinned because it bends considerably, causing the column to rotate and thus not gaining moment. Is this correct?

My dilemma starts here: Since I assume the baseplate to be pinned I size it solely for axial load, which results in a thin plate; But in reality the plate has to bend so it must have stress... If I assume there is no axial load how can I verify that the bending of the plate is not "too much"?

@CELinOttawa: I have done this before.. But how would this allow you to calculate the connection stiffness?. Or are you saying that the connection stiffness will be conditioned by the moment I design it with?.. say i model my frame for only 10% of the columns moment capacity to be transmitted to the base... I design the baseplate for this moment.. And since I designed it for this moment this results in a stiffness that will attract approximately only this moment?

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

(OP)
The country is Venezuela.. I will try to take some pictures of connections tomorrow... I'm pretty sure this things don't exist anywhere else.

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

IngDod

Make the baseplate 3/4 to 7/8" thick. That is common for 2-story steel building.

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

IndDod: You've got it. After the capacity is reached, rotation and yielding transfers the load to other parts of the structure, making the structure conform to your model.

So long as you have a stable state that provides sufficient Strength, Stability, and Stiffness, plastic structures will comply. It is effectively a close-cousin of the Hardy Cross Method (though I have no idea if you've learnt it, it is DEFINITELY worth learning about at a minimum).

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

(OP)
@CELinOttawa:
Thanks, you've truly opened my eyes on this subject... And I can totally see how a FEA model cannot capture this behavior. I was taught the moment distribution.. which I believe is the hardy cross method... So I can see what you mean, however I never thought of applying a similar concept to the connection - member interface. I think my error is thinking of connections as a separate part of the structure.. a habit probably enforced by the use of "stick frames" software where connections are idealized as nodes.

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.

@AELLC:
Thanks, I will aim for this during my calcs.

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

IngDod,

You are still too FEA-oriented...others have designed countless low to high-rise steel buildings without any FEA at all.

How many years since you got your degree at University?

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

(OP)
@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.

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

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.

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

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.

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

Quote:

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

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

"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"

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

@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?

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

(OP)
@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).

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

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

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

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.

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

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."

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

(OP)
@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.

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

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.

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

(OP)
@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.

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

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.

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

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.

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

(OP)
@AELLC: that must have quite the first job!

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

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

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

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.

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

(OP)
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.

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

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.

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

The computer did all the math - no one needed to know how to set up all the matrices

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

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

A have a few punch cards at my desk (they predate me, but not by much) just to show the youngsters!

May advice on the base plate fixity:

Start with a pinned case - regardless of how the bolts and column are arranged. If you can live with the additional moments driven up into the column and the sway is under control, then you are on the right track. Why throw moments into a foundation system? It makes the design needlessly and meaninglessly complicated.

Now, if you end up with an unmanageable column size, or unpalatable sway, then you start looking into other options. Keep in mind that full fixity through a base plate, anchor rod, pier, footing system will be virtually impossible to achieve (except for a cantilevered system). Now you are left with modeling a system which takes into account partial fixity. Keep you method rational and reasonable conservative - it may take some trial and error to get the reasonaby accurate results you are looking for. Just because it is FEA-modeled doesn't inherently make it reasonable or conservative.

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