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Beam to column connection with eccentricity
3

Beam to column connection with eccentricity

Beam to column connection with eccentricity

(OP)
Hi all,

I have a question regarding a beam to column (hinged) connection (see the attached file).

The beam transmits a reaction F to the flange of the column with an eccentricity e from the centerline of the column. Should I check the column with a bending moment M = F*e?

Thank you in advance.

RE: Beam to column connection with eccentricity

I say yes.

“The most successful people in life are the ones who ask questions. They’re always learning. They’re always growing. They’re always pushing.” Robert Kiyosaki

RE: Beam to column connection with eccentricity

1) I do believe that the moment ought to be designed for, just as you've proposed. Whether or not that's common practice kind of depends on where you practice.

2) Through my participation on this forum, I've learned that some counties' standards explicitly direct designers to consider this moment as well as some moment from the eccentricity to the bolt line where the detailing includes that (yours doesn't). I believe that things work this way in AU and NZ. Not sure about Eurocode.

3) In my experience, it's pretty common to ignore these moments in North American practice. And there's some justification for that:

a) If you're not designing your columns to the nuts axially, there's a little margin there to cover this kind of stuff. Less so with automated design now.

b) The depth of the connection and the nominal fixity provided by it will tend to shorten the effective length of the column somewhat. There's truth in that but it's still a bit awkward in that you're weighing a benefit that you don't explicitly quantify in routine design (fixity) with a cost that is tangible (F*e).

The paper shown below supports this perspective to a degree although, in my opinion, it's kind of got the feel of "We like ignoring the axial load eccentricities and would like to continue with that".

RE: Beam to column connection with eccentricity

I'm of the 'other camp' that Koot refers to... I usually make sure the moment attachment of the bolts is OK to accommodate the shear load times the eccentricity to transfer the eccentricity of the load to the column centreline... specially if the beam stiffness is much greater than the column stiffness... forgot to add that for HSS sections, I use slip critical connections, usually...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

RE: Beam to column connection with eccentricity

Yes, you should design for this moment.

I always wonder however that since the beam “applying” the moment is also the same beam effectively restraining the column at the point which the moment is applied, how much moment really materialises. But that’s yet another make & break thesis I’d try if I were back in college now! Regardless, to answer your question - yes.

RE: Beam to column connection with eccentricity

If I were to justify omitting this, I could probably do some version of the attached sketch resolving forces around the bolts (so as you can see, your connection needs to be reasonably stiff, full depth end plate - not a pure hinge style pin!). I’ve taken an arbitrary diagonal line through the centroid of connection, you could probably refine this taking a number of diagonals to each bolt row.

But your code might still ask you to account for the moment anyway - mine does and I’ve always just accounted for it.

Edit - my T tension force might even be at the last bolt row... I dunno, I’d have to think about the plate stiffness..

RE: Beam to column connection with eccentricity

I get sick of agreeing with people especially Kootk. But I concur.

Should you check it? YES.
Do I always (and others) check it? NO. (despite code requirement) For pretty much the reasons Kootk has said. There is enough conservatism elsewhere to cover it in the vast majority of cases.

Likewise with beams. The reality is that shear plates (or angle clip plates) still can put a decent amount of eccentric load on beams. In most case this all washes out because any twist due to the eccentric load is counteracted by axial compression in the beam applying the load. Though this isn't always the case, in some cases if not properly accounted for can really cause severe torsion on a beam.

RE: Beam to column connection with eccentricity

Nice. After a week of being pegged as wrong about everything short of my shoe size, I'm down with a little harmony.

RE: Beam to column connection with eccentricity

Quote:

I believe that things work this way in AU and NZ. Not sure about Eurocode.

In Eurocode I believe it simply states something along the lines of designer must take account of all eccentricities. Which obviously means accounting for the moment induced in columns.

In NZ and AU codes for 'simple' construction (i.e. nominally pinned connections) connected to the face of columns, we are required to design the column for the shear acting at an eccentricity of half the column depth + 100mm or at the centre of bearing (whichever is greater). The moment is distributed based on the stiffness of column within the storey above and below, the design check is considered on a floor-by-floor basis in isolation treating the top and bottom of adjacent storeys being pinned to determine the moment diagram. The checks governs sometimes with higher axial load utilisation ratios, especially with heavy shear connections. I believe the additional eccentricity to consider is a nod towards pinned connections never truly being pinned, and the point of inflection is some distance past the line of bolts in reality. This additional eccentricity is only considered in the design of the column, not in the connections itself.

It seems like a logical way to check columns in simple construction if your code has no specific provisions.

https://engineervsheep.com

RE: Beam to column connection with eccentricity

The following excerpt from EC-3-8 ( Eurocode 3: Design of steel structures — Part 1-8: Design of joints )..


Quote (2.7 Eccentricity at intersections
(1) Where there is eccentricity at intersections, the joints and members should be designed for the
resulting moments and forces, except in the case of particular types of structures where it has been
demonstrated that it is not necessary........)



Apparently, designer's discretion is necessary and the eccentricity of the connection should be taken into account for global analysis..

The following doc. is useful to see how the axial load capacity is affected with eccentricity.



people.fsv.cvut.cz›…of…04…Structural_Modelling.pdf

RE: Beam to column connection with eccentricity

For tallying purposes, company where I work doesn't include a calc for this, just a rotational ductility check with the assumption that the column has been properly sized by the EOR delegating the conx. design.

RE: Beam to column connection with eccentricity

Quote (KootK)

3) In my experience, it's pretty common to ignore these moments in North American practice. And there's some justification for that:

Not saying you are wrong, but what is the justification?

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Beam to column connection with eccentricity

Quote (IDS)

Not saying you are wrong, but what is the justification?

Laziness based on experience?

My opinion on this is that this eccentric moment should be accounted for EITHER in the column or the connection, but that it's not necessary to design them both. That's my general view with pinned connections in general.

I think we often get away with not doing it for a few reasons:
a) The columns are frequently a little over designed anyway.
b) These gravity only columns are frequently laterally restrained by the slab or deck anyway, decreasing how much they can legitimately move laterally.
c) Usually the pure shear connections like this are a little over designed as well. And, it's really only the top bolt that would experience combined tension and shear (which is where we start to get nervous).
d) Even if you impart tension on the top bolt per your analysis, is it really enough to overcome the pre-tension force in a fully tightened bolt?

RE: Beam to column connection with eccentricity

Quote (IDS)

Not saying you are wrong, but what is the justification?
Expediency. If I designed every member to include the eccentricity of the connection it would take me 3 times longer.

As structural engineers most of us make reasonable and often conservative assumptions for the sake of expediency. We don't micro design everything. Just like we make design decisions based on 'pinned' connections or 'rigid' connections neither of which are perfectly rigid or perfectly pinned.

Ignoring eccentricity is unconservative. But for several reasons already listed other conservative assumptions more than adequately cover you in the vast majority of circumstances. The trick is recognising when further analysis is required. And that is where good engineering judgment comes in.

For the sake of the excercise I just threw in the to code eccentricity requirement into the model currently in front of me. I put it onto the beams coming into the most critically loaded column. The eccentricity applied is greater than what it would be in reality. I lost about 2% of capacity. I already have what I consider as comfortable margins on these columns. I also for the sake of modelling convenience haven't explicitly included area reductions on live loads, so that gives me a hefty chunk of extra conservatism for my columns. (I did look at it but I considered the savings of half a tonne of steel in a two columns not worth the extra modelling and documentation effort.)

RE: Beam to column connection with eccentricity

I learnt a long time ago to always include eccentricities due to poorly noded connections, whether it's a simple shear from a beam or a more complex arrangement in a heavier braced connection. Relatively small moments especially about the minor axis of columns (narrower UB's sections especially) have this habit of using up a disproportionate amount of the available capacity as you're usually sailing quite close to the wind on buckling under axial load about the minor axis.

It's just my preference, I don't care if the design segments have a capacity/demand ratio of 1 at the end of the process, provided I know what loads when into the capacity checks and that I've done my best to account for everything that should be accounted for including eccentricities I'm happy. Could I tell you how many times I had to bump up the column weight or size because of the requirement, not really, because that would involve checking it without the eccentricity and it would take longer....

I've peer reviewed plenty of external projects where people are very blasé about intentionally introducing sometimes quite significant eccentricities into connections and never once checking what the effect is. Many times they based their ignorance of the effects by deeming that there engineering judgement is up to scratch.

Many times the designer is asked to assess further, resulting in increasing member size or modifications to the detailing to achieve sufficient strength for all the permutations of the loading.

Moral of the story as far as I'm concerned is unless you actually check, you really don't get a feel for the effect of the eccentric loads for your particular situation. You don't know what you don't know.

Check several of the more critical locations on a job, and see how it pans out and go from there, eventually getting to a point where your judgement might be applied with some guidance from prior results, rather than ignoring completely! Sometimes it's just a matter of the designers not really understanding what they are doing, others are deliberately ignoring and sticking their head in the sand.

https://engineervsheep.com

RE: Beam to column connection with eccentricity

Alright, help me out here.

My philosophy has always been like Josh expressed. If the connection (and local effects on the column, which I include in connection design) are designed to transmit the forces and moments to the column centerline, haven't I replaced my physical beam with an 'effective beam' spanning from node to node?

In this case, what is left for design of the column?

(I'm a fan of minimum eccentricities to simplify analysis, but in my mind they wouldn't be strictly necessary for a robust analysis).

----
just call me Lo.

RE: Beam to column connection with eccentricity

Quote (Agent666)

I learnt a long time ago to always include eccentricities due to poorly noded connections, whether it's a simple shear from a beam or a more complex arrangement in a heavier braced connection. Relatively small moments especially about the minor axis of columns (narrower UB's sections especially) have this habit of using up a disproportionate amount of the available capacity as you're usually sailing quite close to the wind on buckling under axial load about the minor axis.
Despite that being a fairly different approach to myself I value that imput. Eccentricities about minor axis of UB column DO make me look carefully.

Quote (Agent666)

It's just my preference, I don't care if the design segments have a capacity/demand ratio of 1 at the end of the process, provided I know what loads when into the capacity checks and that I've done my best to account for everything that should be accounted for including eccentricities I'm happy. Could I tell you how many times I had to bump up the column weight or size because of the requirement, not really, because that would involve checking it without the eccentricity and it would take longer....
My approach is pretty much the opposite though I would not argue it is superior. I would much prefer a healthy capacity/demand ratio of >1.5 in my columns and not include every little eccentricity.** Out of curiosity, how are you ensuring that your combinations include the worst possible states of eccentric loads as some eccentricities can counteract others. When dealing with multiple floors the combinations and permutations add up to many dozens.

**The size and type of structures obviously play a big role here. In some circumstances having a raw capacity/demand ratio of >1.5 might $1k difference, in other cases it might be $1mil difference. So naturally the time and effort suitable to eek out everything depends on the job.

Quote (Agent666)

I've peer reviewed plenty of external projects where people are very blasé about intentionally introducing sometimes quite significant eccentricities into connections and never once checking what the effect is.
I agree with this. I continually see under designed connections in hollow section struts.

EDIT:

Quote (Agent666)

I learnt a long time ago to always include eccentricities due to poorly noded connections
Going back to this. What exactly do you mean by always include eccentricities? Almost all connections have eccentricities from minor floor joists to secondary, primary beams and bracing, where are you drawing the line? (Not that my line is clear, currently the line I have expressed is "where my engineering judgment says this deserves extra checking".)

RE: Beam to column connection with eccentricity

Quote (Agent666)

You don't know what you don't know.
You are right about this. I know you were talking generally but this certainly applies to me and that is why I read these forums. It often raises pertinent items that once I start pulling on the thread I learn more. For my own benefit I'll chase down the capacity change on some minor axially loaded UB columns in another project and post the results. The % change is probably decently high but the demand/capacity ration is very healthy.

RE: Beam to column connection with eccentricity

Quote:

Out of curiosity, how are you ensuring that your combinations include the worst possible states of eccentric loads as some eccentricities can counteract others. When dealing with multiple floors the combinations and permutations add up to many dozens.

Well if you're talking column design, don't rely on your analysis software to design in the first place. Some degree of post processing and hand checking is warranted. For example find worst unbalanced moment due to shear in combination with higher axial cases, check this column in isolation as I noted earlier (I'm guessing AS4100 requirements are identical to NZS3404 here). Keep going until you are happy that eccentricity is irrelevant. If you have a good method of importing design actions from an analysis, it becomes a click of a button to check every combination in post processing. Once you've setup a post processing process you are ok with you can obviously reuse it infinitely....

Quote:

Almost all connections have eccentricities from minor floor joists to secondary, primary beams and bracing, where are you drawing the line?

In terms of poorly noded connections in steel like I noted, I almost always try account for eccentricities in the design process (whether its member design, weld design, bolt group design, etc), it's just another load to add into the design checks. While we are on welds, I always see people working out total load = total length of weld, therefore let's create the most eccentric weld group we can and as long as it has the bare minimum length of weld we calculated we're ok aren't we..... eccentricities can be important

This raises a question around what defines a poorly noded connection I guess....

I think as an arbitrary/definitive point at which to consider or not consider doesn't exist, but if you consider the requirements for not requiring to assess combined actions in a NZ/AU sense when less than 5% of the axial load capacity is present. Similarly, if you were to say you had bending in excess of ~5% of the member capacity then maybe you should be assessing. But like I said earlier, small eccentricities can cause disproportionate reductions in capacity (members about minor axes, weld group and bolt groups also are especially venerable), but I don't disagree some judgement needs to (and can be) be exercised.

Things like eccentric cleats can fail because of 5mm eccentricities in the load path for example. For years people ignored this because they didn't know any better.

https://engineervsheep.com

RE: Beam to column connection with eccentricity

Thanks for your input. Greatly appreciated.

Post processing of column loads in steel design isn't something I have been doing extensively for steel column design. But I would normally delve into the calculations inputs and outputs and check the impacts on some common assumptions which may not be completely conservative. But I'll take your comments on board and explore my assumptions. I find better wisdom on these forums than from my engineering peers.

For what it is worth lots of my designs are probably on the conservative side of things. Though I also see plenty of insanity from some engineers who design things ridiculously overcapacity such as a 2.5m deep truss for a small walkway load spanning 16m. Or another engineer who calculated his foundation based on seismic overturning moment using the mass of a full vessel but then ignored the stabilising effect of the mass of the full vessel. (To his credit the consultant quickly corrected things when I asked the question and the foundation more thank halved.)

RE: Beam to column connection with eccentricity

Quote (IDS)

Not saying you are wrong, but what is the justification?

Right after the word "justification" in my previous post, I listed the three justifications that I'm most familiar with. And that's the lion's share of what I've got in my bag of tricks. Note that those things reflect less my own personal opinions than they do the opinions of others practicing in my market.

When I choose a low axial utilization ratio over a detailed look at eccentricities, my own "justification" is really:

1) I simply do not perceive that the world has a meaningful gravity post problem. One has to pick their battles as a structural engineer and this isn't the one for me. I feel that structural designs, and structural drawings, often have more serious deficiencies and that more attention in those areas represents a better return on investment.

2) As a structural engineer, I view myself as a moderately skilled, rough proportioned of things. As an industry, I feel that we tend to get too obsessed about accuracy in a space where any rational person must acknowledge that little meaningful accuracy is available to be had.

Now for a fun anecdote.

One of my first assignments as a newly minted graduate engineer was the design of a bunch of steel columns in a one story joist & beam building. My boss was an interesting character in that he was a very practical engineer but would never, ever utter the phrase "overthinking" to me. He'd adjust schedules and hemorrhage fees at will if that's what it took to buy me the space to do whatever I felt needed to be done. I think that he mostly just kept me around for company (two man dept) rather than production.

What I felt needed to be done was to check all of the "posts" for moments at the top and bottom arising from eccentricities in the two to four incoming framing members which may be pattern loaded. I had oodles of cases to check, oodles of things to consider, and built myself a giant spreadsheet to help. It took me the better part of two days and I almost choked on the assignment altogether. At the end of it, I asked my boss how he typically handled this stuff. I was ready to give up engineering and take up accountancy.

He said that, for a gravity post of a typical cross section and with typical shear connections, he ignored the eccentricity but kept the axial utilization under 80%. I looked at my numbers, realized that would have worked comfortably in all cases, and agreed that was a rational approach. 90 seconds per column design without the aid of software. More time to add missing details to the drawing set and wonder if the entire building might rack over sideways.

RE: Beam to column connection with eccentricity

Quote (Lomarandil)

If the connection (and local effects on the column, which I include in connection design) are designed to transmit the forces and moments to the column centerline, haven't I replaced my physical beam with an 'effective beam' spanning from node to node? In this case, what is left for design of the column?

How would that preclude moments developing within the column? I would argue that it doesn't and that you'd still have this to contend with in the macro-design of the column:

1) Combined axial and flexural stresses throughout the entire length of the column and;

2) Exacerbated P-baby-delta moments within the column associated with the flex induced in the column. This leads to a lower buckling capacity.

The trouble with structures is that their lousy listeners. Trying to tell them that their concentric doesn't necessarily make it so.

RE: Beam to column connection with eccentricity

I like your story Kootk. And it seems that your approach aligns more with mine than Agent666. I also think Agent666's point is pertinent, if you haven't asked the question or explored the results people might be quite in the dark about the degree that eccentricity affects their demand/capacity calculation.

To that end I checked some UBs columns in a model I have with weak minor axis. The difference was less than I expected, 2% difference at critical case and 12% at highest axial load (not critical case). All these UB columns have a capacity/demand ratio of over 2. Which seems excessive but the were sized to match stocky UC columns beneath. I could have spliced it a floor below but then the capacity/demand ratio would have been sailing much closer to the wind.

Anyway. I probably have not much further to add. This thread made me explore something a little deeper, which is why I read these threads. thumbsup

RE: Beam to column connection with eccentricity

I like the NZ method too. It seems to be the most rational out there in terms of clear guidance. This is one of those situations where I actually wish that North American codes would just prescribe what should be done. That way:

1) We'd all be doing the same thing.

2) Software would adapt.

3) The material costs would surely be inconsequential.

4) We'd free up our collective intellects for more worthy considerations.

I'm usually in the minority in that most engineers disdain heavy handed codes and would prefer to retain their autonomy for stuff like this. Not this cowboy though. I feel that codes do me a nifty service when they force my competitors to design consistently and safely on routine items. I'm happy to flex my creative muscles in concept design etc.

RE: Beam to column connection with eccentricity

Quote (Kootk)

I like the NZ method too. It seems to be the most rational out there in terms of clear guidance.
As Agent666 surmised, the Australian code reads the same. Though in my experience, Australian engineers ignore this prescription a fair bit. Either out of laziness, ignorance or confidence in their own judgment. Of course if you start ignoring code items you are going down a precarious path.

Australian codes and NZ codes are quite similar and are often shared or jointly produced. Though I do start to wonder if NZ is surpassing AU codes in quality. Their seismic requirements have driven the need for for comprehensive codes in some areas and an entirely separate code for seismic loads.


Most of us have likely seen dozens or hundreds or poorly or otherwise underdesigned structures. The biggest savior is likely that most structures rarely see ultimate loads. I've designed quite a few elevated bulk storage structures which have very high load in a relatively small space combined with a moment lateral restraint system. This means that the columns end up working extremely hard when axial loads combine with lateral loads. Yet I've seen similar structures built by others which have about half the capacity. I have little confidence they would stay upright with ultimate wind or seismic loads occur.

RE: Beam to column connection with eccentricity

Quote (Kootk)

KootK (Structural) 13 Apr 21 15:15



Quote (Lomarandil)

If the connection (and local effects on the column, which I include in connection design) are designed to transmit the forces and moments to the column centerline, haven't I replaced my physical beam with an 'effective beam' spanning from node to node? In this case, what is left for design of the column?

How would that preclude moments developing within the column? I would argue that it doesn't and that you'd still have this to contend with in the macro-design of the column:

1) Combined axial and flexural stresses throughout the entire length of the column and;

2) Exacerbated P-baby-delta moments within the column associated with the flex induced in the column. This leads to a lower buckling capacity.

The trouble with structures is that their lousy listeners. Trying to tell them that their concentric doesn't necessarily make it so.

On 1, I don't think this matters, as long as you have defined a load with a flexural compressive stress state that column passes. Basically the entirety of connection design, and steel design in general already works like this.

On 2, I think if the connection has some moment capacity, then it has enough restraint to reduce the P- small delta effects. And this would be true throughout the structure - all connections modelled as pinned have some capacity to reduce flexure in the column. So from a global stability perspective, you are already modelling a safe lower bound of structure stiffness.

My approach is, in order of preference
a- for columns with members framing in from all directions, or part of a large braced frame/moment frame where I'm confident there is a robust load path for the moment - keep utilization low enough in the column I'm happy. Still modelling the beams full length as well.
b- model the beams full length, make sure that the connections have some nominal moment capacity, and call it ok.
c- model the eccentricity, mostly in cases where there is a true post condition and I'm concerned about lateral stability, or if there some potential for weak axis moment in the column.

RE: Beam to column connection with eccentricity

Quote (canwest)

On 1, I don't think this matters, as long as you have defined a load with a flexural compressive stress state that column passes. Basically the entirety of connection design, and steel design in general already works like this.

I disagree strongly except for how connection design is envisioned to work. I feel that, if the column will see flexural stress in addition to axial, that will affect the column design as parts of the cross section will yield earlier, there will be an LTB demand this wouldn't exist otherwise, and Baby-P-Delta buckling tendencies will be exacerbated for axis coincident with moment. In my mind, having a place to redistribute moments to does not prevent moments from developing. One might cap the moment at the development of a stable flexural hinge somewhere in the system but, as far as I know, nobody is designing gravity posts that way. And I would expect that to yield prohibitively inefficient column designs.

Quote (canwest)

On 2, I think if the connection has some moment capacity, then it has enough restraint to reduce the P- small delta effects. And this would be true throughout the structure - all connections modelled as pinned have some capacity to reduce flexure in the column. So from a global stability perspective, you are already modelling a safe lower bound of structure stiffness.

I made basically the same point in my first post. But, then, I also made the point that this is a benefit that is:

a) difficult to quantify and;
b) is not being quantified by designers as far I know.

So I think that sketchiness remains in that you're trying to offset a tax (eccentricity) with an undefined credit (connection fixity).

Quote (canwesteng)

Still modelling the beams full length as well.

Quote (lomarandil)

...haven't I replaced my physical beam with an 'effective beam' spanning from node to node?

Obviously I can't read you guys' minds. I'm going to try though. These arguments sound to me like analogs to a common concrete design strategy: If I design my concrete thing to span to the center of the supports, I can ignore the torsion in the supporting girder.

That argument is substantially valid. However, when we do that, we also provide at least compatibility torsion reinforcement in the supporting member. And there's no analog to that in the case of steel post design. In steel post design, we are not taking any steps to ensure that the member as a whole has redistribution capacity.

RE: Beam to column connection with eccentricity

Concrete is an entirely different ball game, especially in that example, since you can be well underreinforced for torsion, and have sudden brittle failure of the member. Specific to the LTB case - as LTB develops, that load path will become softer and the moment will re distribute to the beam, assuming it is proportioned for it. Now for a true gravity post, sure you'd add some eccentricity to account for instability effects. For a column where the beams framing into it form part of diaphragm, I don't see an issue having the connection transfer small moments so that the reaction occurs at column CL.

RE: Beam to column connection with eccentricity

Quote (canwesteng)

Specific to the LTB case - as LTB develops, that load path will become softer and the moment will re distribute to the beam, assuming it is proportioned for it.

So you're actually willing to allow lateral torsional buckling to be a deliberate part of your redistribution mechanism?

Quote (canwesteng)

Now for a true gravity post, sure you'd add some eccentricity to account for instability effects.

So you don't design your posts for all of the eccentricity but, rather, some of the eccentricity? How do you decide how much?

Quote (canwesteng)

For a column where the beams framing into it form part of diaphragm, I don't see an issue having the connection transfer small moments so that the reaction occurs at column CL.

How do you see the diaphragm altering the situation? I see it as shown below which would have no impact on the stability design of a typical gravity column.

Moreover, how is it that you have any certainty that the reaction is coaxed into occurring at the column centerline? Your providing the capacity for a nominal moment connection at the joint is no guarantee that the structure will choose to exploit that 100%. You know, the stiffness thing.

RE: Beam to column connection with eccentricity

I concur with Kootk and am I am thoroughly confused by anybody who is trying to handwave away eccentric effects by saying (if I understand it correctly) that they have accounted for it in their connection design.

Eccentricity matters, how much detrimental effect it can hap depends on the circumstance so start ignoring it at your own risk. While I've discussed my rationale for not specifically allowing for it in some circumstances, I'm in no way arguing that it doesn't put extra demand on my members that isn't specifically allowed for.

RE: Beam to column connection with eccentricity

Quote (Kootk)


Quote (canwesteng)

Specific to the LTB case - as LTB develops, that load path will become softer and the moment will re distribute to the beam, assuming it is proportioned for it.

So you're actually willing to allow lateral torsional buckling to be a deliberate part of your redistribution mechanism?



Quote (canwesteng)

Now for a true gravity post, sure you'd add some eccentricity to account for instability effects.

So you don't design your posts for all of the eccentricity but, rather, some of the eccentricity? How do you decide how much?



Quote (canwesteng)

For a column where the beams framing into it form part of diaphragm, I don't see an issue having the connection transfer small moments so that the reaction occurs at column CL.

How do you see the diaphragm altering the situation? I see it as shown below which would have no impact on the stability design of a typical gravity column.

Moreover, how is it that you have any certainty that the reaction is coaxed into occurring at the column centerline? Your providing the capacity for a nominal moment connection at the joint is no guarantee that the structure will choose to exploit that 100%. You know, the stiffness thing.

Well, I don't think LTB occurs - before that there is certainly going to be a loss of stiffness. In any cases, where we have clean, ductile failure modes seems like an odd place to trying and button down the exact behavior - especially when column base plate are essentially end plate moment connections that we model as pins, and often concrete breakout would govern their failure. Taken further, do we need to look at the fixity in gusset plates adding moments to braces? There are probably a limitless number of examples where we design a ductile material by providing a lower bound solution, and rely on the material to behave as we've decided. Now in situations where this ductility has been overstated, certainly some failures have occurred.

Maybe an error in wording - I'd add all the eccentricity to it. Although we also have vertical loads on both sides of the column with counteracting moments, so in order to see a moment on the column you need to have an unbalanced load reducing column capacity anyway.

In the case of the diaphragm, the P-big delta effects from this eccentricity don't occur, only the P-small delta effects, which if you're happy to ignore the eccentricity then as a result you're happy to ignore these as well.

RE: Beam to column connection with eccentricity

Thanks for the explanation canwesteng. That said, I plan to flip flop and agree with Lomarandil's (and probably yours) approach, at least for a wide range of practical gravity post cross sections. More to follow.

RE: Beam to column connection with eccentricity

Quote (KootK)

That said, I plan to flip flop and agree with Lomarandil's (and probably yours) approach
Damn you does that mean that since I agreed with you earlier I need to flip flop too? :-p

For single story columns the eccentric pinned connection vs the stiffer moment connection almost balances itself out due to the effects on effective length. This was demonstrated in a thread here on a beam to column cap plate connection discussion.

For multistory type applications where there is significant axial load above the eccentric connection I believe you could see quite significant real effects of eccentric loads on columns particularly narrow flanged columns. I haven't demonstrated this to myself by Agent666 seems to have experience here. EDIT: I ran a quick check. It really isn't hard to come up with eccentricities where your capacity/demand ratio halves. Mind you the 100mm requirement of NZ and AU codes is pretty brutal on light columns, but even a more realistic 50mm is still rough on the column's minor axis.

RE: Beam to column connection with eccentricity

Quote (KootK)


How would that preclude moments developing within the column? I would argue that it doesn't and that you'd still have this to contend with in the macro-design of the column:

You're right, I didn't describe what is going on inside my head very well. Still working on a better explanation.

----
just call me Lo.

RE: Beam to column connection with eccentricity

Quote:

Although we also have vertical loads on both sides of the column with counteracting moments, so in order to see a moment on the column you need to have an unbalanced load reducing column capacity anyway.

Yes, very easy to get unbalanced moments when you consider for example 0.9G on a beam one side and 1.2G + 1.5Q on the other beam opposite.

This is based on net stabilising and destabilising effects and how they impact on the worst-case scenario for the column, most codes have some similar requirement. This is the requirement required to be considered in AU & NZ codes, least my interpretation of it. Assume 1.2G+1.5Q load from above, load below is load above +0.9G from one beam, +1.2G+1.5Q from other beam. Work out worst case out of balance load based on highest beam reaction minus the lowest, apply the 100mm eccentricity from face of column. Check column for axial load and the bending from unbalanced eccentricity moment. This is usually more critical than taking the net unbalanced moment generated from full load on both beams (even if the axial load is slightly higher if you were to do this). Check all columns top down in this manner.

As well as above whilst we are on column design, if in a seismic region check the actions resulting from deformation compatibility in addition to eccentric moments, obviously if these columns are attracting axial loads form the seismic system this case can govern over and above the gravity ultimate limit state cases.

https://engineervsheep.com

RE: Beam to column connection with eccentricity

Agent666 is busy putting the fear of god under me and making me want to go check everything I've ever designed and comprehensively include eccentric effects of the connections. rednose

(I'm actually fairly comfortable. I had a very early learning experience with columns bending like a bananas on a jobsite. It has made me somewhat wary of excessive eccentricities OR inadvertent moment transfer to columns. At the time I was working a fabricator not the engineering firm responsible.

Still I value your input Agent666 because you seem to be the loudest voice saying eccentricity MATTERS!
)

RE: Beam to column connection with eccentricity

Just to clarify one thing I said in my last post, the 0.9G I consider is only the permanent dead load, i.e. no superimposed dead load or any dead load than cannot be removed is included. For the NZ or Australian folks following along, this is covered in the commentary CL C4.2.2 of AS/NZS1170.0.

So, for a simply supported beam system, just self-weight of the beam and structural slab would be included on side, and the full ULS loading from the other side. If you consider real world ULS loading, it's more likely to occur on a single beam, rather than over the entire floor system at once. So you're more likely to see the out of balance moment if you consider one side being overloaded in practice.

https://engineervsheep.com

RE: Beam to column connection with eccentricity

Totally agree on that last point Agent -- I teach the same thing to my engineers. Personally, I've borrowed the term "Dead Collateral" load from the bridge world.

----
just call me Lo.

RE: Beam to column connection with eccentricity

Interesting, I've never heard that term before, most stuff that isn't part of the permanent structure is termed superimposed dead loads (SDL) round these parts.

https://engineervsheep.com

RE: Beam to column connection with eccentricity

As far as I am concerned it has always been a bit fuzzy the line between long term live loads and dead loads.

I deal with bulk material handling and storage. So sometimes the main structural load is a fairly well known volume of a well defined material of known maximum density. Calling it a live load with a 1.5 factor ontop of an already conservative maximum density start excessively conservative. AS3774 has a load factor of 1.25 which is a bit more sensible, but few other structural engineers know what I'm talking about if I refer to that code. That said calling it a dead load also isn't suitable because for any wind calc you really can't afford to treat it as a dead load or empty storage bins would fly away in a stiff breeze.

I mostly end up using a 1.5 live load factor which might be excessive but its nice to keep plenty of margin up your sleeve, especially since you do know that the structure will be regularly filled with full live load weight.

RE: Beam to column connection with eccentricity

Quote (human909)

Damn you does that mean that since I agreed with you earlier I need to flip flop too?

We'll see how it goes. I'd be grateful if you'd help me vet it.

THE THEORY

1) Model per the sketch below.

2) Make a bunch of simplifying assumptions that I feel are pretty reasonable for a gravity post.

3) I contend that the process of column buckling may be envisioned as occurring in two phases:

a) Magenta; single curvature; [K = 1.0]; column top moment encourages buckling; design case; load eccentricity to RIGHT of post centerline.

b) Green; double curvature; [0.7 < K < 1.0]; column top moment restrains buckling; ultimate buckling capacity; load eccentricity to LEFT of post centerline.

At the limit of the single curvature mode shape, the apparent flexural stiffness of the post drops to zero for the single curvature buckling mode. This causes it to kick to the left until it engages the stiffness of the beam as rotational restraint.

4) In transitioning from magenta to green, the post must pass through a moment in time where [e = 0; K=1; design case]. Moreover, after the post passes through this moment in time, it's capacity continues to increase. So this moment in time represents a lower bound on in-plane buckling capacity.

5) I see one hiccup with #4. At the moment in time described, the column is not yet shifted to the left enough to reach equilibrium. Another way to look at it is that it's an [e = 0; K = 1.0] design with a baked in displacement, delta, that adds to the normally assumed column imperfections. So the "moment in time" column capacity is some degree lower than that calculated at [e = 0; K = 1.0]. I feel that the impact of this is going to be small will relatively stiff beams and larger with relatively flexible beams since, the larger the beam stiffness, the less movement is required to re-establish equilibrium.

SOME TAKEAWAYS

I feel that this theory supports some of the limitations that many engineers are already applying as a matter of intuition and judgment. Namely, if one is going to ignore the eccentricities in a gravity post in favor of a nominal moment connection:

1) It should be a stocky, post appropriate section like an HSS, W14, etc. It should not be a W12x14, W18x35, or anything else with relatively narrow flanges and an affinity for LTB.

2) The [EI/L] of the beam(s) should be relatively stiff compared to that of the post. No limp noodles, no joist seats unless the BC ties in too.



RE: Beam to column connection with eccentricity

Quote (Agent)


Yes, very easy to get unbalanced moments when you consider for example 0.9G on a beam one side and 1.2G + 1.5Q on the other beam opposite.

This is based on net stabilising and destabilising effects and how they impact on the worst-case scenario for the column, most codes have some similar requirement. This is the requirement required to be considered in AU & NZ codes, least my interpretation of it. Assume 1.2G+1.5Q load from above, load below is load above +0.9G from one beam, +1.2G+1.5Q from other beam. Work out worst case out of balance load based on highest beam reaction minus the lowest, apply the 100mm eccentricity from face of column. Check column for axial load and the bending from unbalanced eccentricity moment. This is usually more critical than taking the net unbalanced moment generated from full load on both beams (even if the axial load is slightly higher if you were to do this). Check all columns top down in this manner.

As well as above whilst we are on column design, if in a seismic region check the actions resulting from deformation compatibility in addition to eccentric moments, obviously if these columns are attracting axial loads form the seismic system this case can govern over and above the gravity ultimate limit state cases.

Of course we have to consider this load case, at least implicitly. I would wager that even those modelling the eccentricity aren't going to this level of detail though, at least for all columns. Since you're basically removing all the live from one side of the column and a chunk of dead, for gravity columns you might as well be removing half the demand from the column. Are there cases where removing half the demand from the column and adding this eccentric load are critical? Yes, and those cases are the cases I'd be checking the eccentric moments, but 90% of the time, by inspection this will not control. Generally, following good practice for columns (W8, W10, W12, W14 that are roughly square) means that this won't control in most cases.

RE: Beam to column connection with eccentricity

Just removing live load from one beam supported by the column virtually leaves the axial Load unchanged in multistorey scenarios. Its not something that I would model. Just applied by hand when I'm doing the design checks. So you're ending up checking for say 95% of the axial load and the worst eccentric moment you can derive. Most of the time it probably doesn't govern, sometimes it probably does. But the point is that it is checked irrespective. The thread is about whether you should be checking it in the first place after all.

Of course for a multistorey building the axial load barely changes when you drop a small overall proportion of the live load from a single member, for single or double storey it's another matter of course. Totally depends on the scenario, I'm in the make some simple conservative assumptions & check it and see camp, takes just as long to check either way if your design tools/methods are up to scratch. It's not as onerous as people make out, and to me it's better than almost ignoring completely as some folks advocate.

https://engineervsheep.com

RE: Beam to column connection with eccentricity

@Agent666: I'd like to ensure that understand your method accurately. Is the model shown below correct?

Beyond a certain height, it's been my experience that columns pretty much just stop caring about the moments induced at individual floor levels other than, perhaps, code minimum eccentricities on the axial load as a whole. As you said, though, it depends on the scenario.

RE: Beam to column connection with eccentricity

Yeah that's more or less what the standard says to do. Simply looking at each level in isolation to determine the eccentricity moment. All other actions from other calcs, analyses, etc superimposed.

https://engineervsheep.com

RE: Beam to column connection with eccentricity

That does seem pretty simple to apply.

RE: Beam to column connection with eccentricity

I think I have to retract my earlier statement ("What is left to design for the column?").

I implied that creating a load path for eccentrically applied loads to be transmitted to the column centroid removed the need to design the column for the net moment from that eccentricity. However, that implies that the column would not see any moment along its length. And I can't rectify that in my mind. Certainly, with typical detailing, column moments must exist (due to unbalanced spans, pattern loading, etc.)

----
just call me Lo.

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