Is it possible to do a partial fixed base plate? How?

[X4vier]

Is it possible to do a partial fixed base plate? How?
Considering the foundation is capable, is there any way to do a partial fixed base plate for a steel column?
I guess if partial something in the connection will yield. Is that safe?

 

[Tomfh]

It won’t necessarily yield.

What’s the aim here?

 

[1503-44]

It's pretty difficult to design a totally rigid anything, but something can appear rigid, if the loads are small.

Design the plate to flex within its elastic stress limit to give you the rotation you want.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[human909]

Yes you can design a semi-rigid base plate. Many baseplates even nominally pinned ones are semi-rigid.

Quantifying the rigidity of connections is HARD. More advance tools are now available that do make it easier (eg IDEA Statica) but even then there are still enough variables that the actual calculated connection stiffness still has significant variance.

There really is alot of work to deal with semi-rigid connections and not a huge amount of benefit. That said I have recently designed a full structure of semi-rigid connections. The particular requirements of the job required a moment frame but it also had limitations on the member choices (hollow sections) and the connection design (clean environment). The connection chosen were calculated to not achieve the threshold of full rigidity. But the structure was still stiff enough and was highly redundant.

 

[Klitor]

When dealing with retrofitting of existing structures, some benefits can be gained.
Though so far I used it (with ideastatica) only when I was "in my mind" sure the structure is ok, and needed the numbers to prove it.
Wouldn expect much from the method though.

 

[bones206]

You can assign a rotational spring to the column base in the structural model, then design the baseplate, anchors and foundation for the resulting moment reactions. Some software lets you assign a percentage of the member flexural rigidity as a boundary condition, which makes it easier than hand calculating. I’m trying to get RISA to incorporate that, but I’m not sure they see it as a useful feature. It would be useful for checking wood trusses too, since the truss industry uses partial fixity in their software for web members etc.

 

[JoshPlumSE]

There are no perfectly rigid or perfectly flexible supports. The reality is that supports are always somewhere in between.

For structural ANALYSIS, the practice is generally to look at the type of connection you have and assign it to be rotationally rigid (i.e. restrained against rotation) or flexible (i.e. free to rotate). Then you design that the connection and foundation so that they can handle the forces that are generated by this fixity (or lack thereof). If this is new construction, then I would strongly advocate for this practice. This is a good design procedure and deviating from it is (IMO) poor practice.

In some rare conditions (often academia), you may be required to be more precise with your analysis. In those cases, the most challenging part is QUANTIFYING the rigidity of the support. This means you have to come up with a force-deformation curve. Hopefully, it's linear or near-linear. If not, your analysis will get more complicated. How do you come up with a force deformation curve. You may have to do some physical testing of the system. Maybe you review someone else's published testing. It's not an easy procedure. Even when you do it, you want to be cautious about trusting the accuracy of your assumptions.

 

[HTURKAK]

EURCODE allows the use of semi-rigid base connections . Although it takes more time , sometimes provide some economy.
The following doc. prepared as per EC-3 rules gives an idea for modelling of semi rigid connections and base connections.
...

He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[human909]

I'll preface my comments below by saying that the are many people on this forum, including JoshPlumSE, that have significantly more knowledge, experience and understanding than myself. That being said I disagree with some of the comments above.

This is a good design procedure and deviating from it is (IMO) poor practice.

If you are going to make a statement like that then you need a better argument than that. lots of methods that were standard design practices are now obsolete or even considered dangerous.

There is absolutely nothing wrong with choosing to model a structure in a more accurate way. In many cases it is going to be MORE conservative than making an assumption than is inherently inaccurate.

If you are modelling a moment frame with completely rigid connections then you may be slight or even significantly underestimating deflections. There several common rigid connections which are significant away from being truly rigid.

(All that said I'm not advocating deviating away from the pinned/rigid dichotomy. It significantly simplifies analysis and suitable conservatism can be adopted elsewhere and is generally codified anyway.) Itt is the approach I use 99% of the time, but a more detailed analysis isn't 'poor' practice.

How do you come up with a force deformation curve.

You calculate it. Some tools have been described above. But as said the calculative effort is likely not worth it most of the time, even with advanced tools

 

[bones206]

I often have trouble getting moment frames to get under drift limits for wind using a purely pinned base model. I will apply a rotational spring of 2EI/L at the column base for the wind drift load cases, and a pin for all strength cases. It seems reasonable and still quite conservative for typical base plate configurations, but can make a big difference in the drift calculation.

I feel like the pure pinned assumption is overly penalizing designers trying to meet wind serviceability targets like H/400 or H/500.

 

[human909]

That is a sensible approach bones206. It is also an approach that has some AISC basis if that makes people more comfortable.

When things are heavily deflection controlled making some sensible, easy and conservative assumptions seems like a good choice.

 

[JoshPlumSE]

There is absolutely nothing wrong with choosing to model a structure in a more accurate way. In many cases it is going to be MORE conservative than making an assumption than is inherently inaccurate.

Well, I have done a lot of work in the software industry and seen a lot of users waste a lot of time trying to "model a structure in a more accurate way" that ended up being (IMO) less accurate than if they'd just made the standard assumptions in the first place.

Sure, it can be done and it can be done well. But, in my experience, there are a lot more people out there doing it poorly than doing it well. Ergo my opinion that deviating from standard modeling assumptions when it is NOT needed (or not well understood) is poor practice.

If you've got experience with ASCE 41 where you're constantly evaluating force-deformation relationships for the various elements of your structure, then you could certainly do a good job with your base rigidity assumptions.

I just think the majority of structural engineers are NOT experienced with these types of procedures and would be better served to stay away from them. I certainly haven't seen any reason to think that this thread's OP currently has that level of sophistication.

 

[bones206]

I agree with Josh here. It's definitely advanced territory that requires judicious engineering judgement on how and when to apply.

I've only recently started doing this regularly, since architects' highly-constrained designs are forcing me into using more and more moment frames, with smaller and smaller member depths.

 

[Celt83]

I agree with Josh. For an introduction AISC has this paper from the 80’s on the topic:
Link

 

[lexpatrie]

If you are going to make a statement like that then you need a better argument than that. lots of methods that were standard design practices are now obsolete or even considered dangerous.

I'm gonna go with "Josh was right".

(As a side note, if anybody is interested, you can "copy image link" directly from firefox and paste that path into the "Image" tool here.) Just a little tip...

Here's a tip!

While Human mentions there are "old ways" of doing things that are wrong (cough inflection point is a brace cough), that's outside the context of what Josh said referring to base plates. Factually correct, but not on point, and also, demanding specificity of JoshPlum while making a generalization, poor form.

We have a fairly long history of structural failures associated with "ambitious modelling techniques" (Kemper arena, Hartford arena, Hyatt Regency) that weren't all that legit and or not justified or done incorrectly. Our field tends to evolve via both research and mistakes. And research on the mistakes. And yes, there's usually more in that picture as well, bad connection design delegation, "novel" floating roof, lack of roof slope, cruciform space frame when 2D analysis was pretty ... nascent?

Base plates are generally not designed as rigid, it's a design simplification, but it's not wildly inaccurate. If you design it as both fixed and pinned, you're pretty likely capture the design behaviour in there somewhere (similar to Justifiable Precision and Accuracy in Structural Engineering Calculations, Sputo, Pract. Per. on Des. and Const. 2005) though I don't think that's quite what Sputo is driving at, but maybe you can see it from there). That's how I view JoshPlum's comment. You could design it for a small amount of moment a lot more readily than a fixed end, unless you're embedding the shape into concrete outright, then a fixed end is probably more realistically justifiable.

K factors back in the day allowed a G of 10 for a pinned connection because a perfect G of infinity is rarely achievable and unrealistic. Unless that's a Mandela effect "memory" of mine.

I feel like there was a good period of time where the PEMB folks considered all based rigid until the various foundation engineers pushed back on how unrealistic it was and how much it messed up the foundation design. I will defer to others on when that was, but I'd say prior to 2000-2010 as a general timeframe.

 

[bones206]

Speaking of pre-engineered... this company PACO designs and fabricates moment frames:

https://www.pacosteel.com/wp-content/uploads/2016/06/PACOsmartMomentFrame2016.pdf

It's interesting that they have pinned, rigid and partial fixity base options available. I would be curious to see the cost differences for a given loading and drift criteria.

 

[human909]

Hmmmm... That is a fair bit more and broader push back than I expected...

I'd like to think that a fair bit of this is simply agreeing furiously.

Case in point:

If you design it as both fixed and pinned, you're pretty likely capture the design behaviour in there somewhere .... That's how I view JoshPlum's comment.

That is what I do whenever I have concerns that I wish to investigate more. It takes a minute of design time. And doesn't involve the complexity of quantifying a semi rigid connection.

Maybe I took JoshPlum's comment too literally. "deviating from it is (IMO) poor practice" I would rarely deviate from it. But I don't believe that if you do choose to deviate from it then it necessarily is poor practice.

 

[bones206]

As someone who does employ semi-rigid base plates (although in very limited situations), I did not take offense to Josh’s comment about poor practice. I interpreted the subtext of that statement as “structural engineering hubris can get you in trouble”, which I wholeheartedly agree with. Semi-rigid modeling implies you think you can characterize boundary condition stiffness with a degree of accuracy, rather than bounding with the known extremes of fixed or pinned. Unless you feel strongly that you can back up a calculated stiffness value, you should stick with traditional bounding analysis.

 

[JoshPlumSE]

Semi-rigid modeling implies you think you can characterize boundary condition stiffness with a degree of accuracy, rather than bounding with the known extremes of fixed or pinned. Unless you feel strongly that you can back up a calculated stiffness value, you should stick with traditional bounding analysis.

You said it better than I did, but this basically captures the gist of what I was trying to say.

FWIW, if I had said it this well, I don't think I'd have gotten any pushback from Human909.

 

[bones206]

It’s all good! Pushback is healthy