Cap plate for Pipe - How can release moments? 18

[X4vier]

Cap plate for Pipe - How can release moments?
Typical cap plates for pipes will have bolts around the pipe, is there a way to release moments in that type of cap plate?

Thanks.

 

[JAE]

Never mind - I see you corrected...

 

[JAE]

 

[Celt83]

Right, but I would interpret those things to mean that such connections can be used to transfer moment when desired.

My interpretation is slightly different rather than saying "connection can be used to transfer moment" I read "connection develops/resists moment"

The magnitude of the developed/resisted moment enters into these conversations and usually, as you've already indicated and as can be seen in Dave's Gerber model, we are proportioning the design to minimize the moment dumped into the column to the realm of engineering judgement of being ignored.

I believe you've also indicated though that folks are clicking the optimize button in the software, and a lot of these same folks aren't taught either through formal education or mentoring to think about any of this stuff in any sort of depth. That optimized design might be taking everything down to the gnats ass where all these hand waived away limit states now need to be considered.

 

[SE2607]

I stand corrected. There is a small moment transferred to the column (1.8 kip-ft). You can see that in the moment diagram above, but it is easy to miss because it does not show up graphically. If I load the middle bay with only dead load, the moment on the column becomes 3.5 kip-ft.

Sooooo...if you release the fixity at the top of the columns, how does that affect the CSR?

Curious minds want to know!

 

[phamENG]

I'm more curious about the curvature. To me, the 'make everything thin' argument depends on the ability of those thin parts to deform elastically to allow that rotation to take place without the top of the column rotating appreciably. So if we know the curvature of the beam at that point, we can determine if it's realistic for the plate to deform that much.

 

[human909]

I'm wondering what DaveAtkins has done to human909. LOL

Gave advice and made a statement that is plainly false. (sure you can ignore moments if you wish, but you can't just assume them away!) And everybody knows the best way to arrive at vigorous discussion on the internet is to lead with a plainly false answer to a question.

Hypothesis: A structure designed with pinned connections versus fixed or partially fixed will result in a conservative design.

I see above that human909 has an example initially claiming to meet this condition, but the comment was later edited to the contrary.

As I've said repeatedly in this thread and others in most cases you hypothesis is mostly correct regarding strength. However is it readily proven untrue in a variety of other circumstances, including serviceability and in strength. Examples of this little this thread and threads previous.

You are entitled to your opinion, and your method of analyzing this type of structure. As can be seen throughout this thread (in the comments by many structural engineers), as well as in the recommendations from Steel Tube Institute, my way of designing this type of connection is also accepted engineering practice.

You still don't get it. I don't have a problem if you design a structure with this connection as pinned. I do have a problem with your initial reply the the question.

X: "How can release moments?"
D: Simply assume there is no moment transfer between beam and column and design the members accordingly.

You can't assume away moments and make them disappear. This is very much poor engineering. You can decide (if you wish) that they are inconsequential and ignore them. But assuming them away is a completely different thing and FALSE. This false thinking leads you to such bizarre conclusions as:

Even though the columns deflect, they do not receive any moment. The moment in the cantilever equals the moment in the backspan. So maybe we are arguing over nothing?

That should have been a massive red flag for you. If the columns are deflecting with curvature they must have moment.

 

[human909]

I'm more curious about the curvature. To me, the 'make everything thin' argument depends on the ability of those thin parts to deform elastically to allow that rotation to take place without the top of the column rotating appreciably. So if we know the curvature of the beam at that point, we can determine if it's realistic for the plate to deform that much.

A good question.

In my analysis with IdeaStatic. A thin plate is quite effective at releasing moments for beam to beam connections. This however would be vastly less effective in the presences of a bearing load.

I haven't tested column to beam connections in this context but I have previously explored the rigidity of axial loaded connections and discussed it on Eng-Tips. This was done with base plates.

The axial load provides significant rigidity to the connection EVEN in the absence of bolts or any securing connection. Furthermore if separation does occur as deflection of the beam increases. While moment may be released you are then introducing eccentricity of axial load into the column and the effect becomes similar (but reduced as the curvature transfer is obviously less due to the separation)

To properly release the moment you need to centralise the load and provide a rocker type connection or a true pin. I and Smoulder have previously mentioned this solution.

 

[DaveAtkins]

I do have a problem with your initial reply the the question.

So it seems to me you would have preferred I qualify my response by saying something like, "There will be a small amount of moment transferred between the beam and the column, but many engineers will ignore that transfer of moment and design the connection for the axial load alone."

DaveAtkins

 

[DaveAtkins]

Unity check with column fixed to beam:

Unity check with column pinned to beam (restraint added to make frame stable):

So if moment to the column is considered it definitely increases the unity check.

DaveAtkins

 

[WesternJeb]

14% difference in unity check seems reasonable to me. In reality, it is somewhere between fixed and pinned, so call it a 7%± difference between designing as pinned or fixed (situation specific).

Designing everything to a .9 code check covers this differential and just changed my mind on how to design it. A lot less effort to design it as pinned, and designing to a .9 code check would likely cover the difference in forces for a scenario very similar to the one you have shown. Great work, Dave.

 

[SE2607]

So if moment to the column is considered it definitely increases the unity check.

Soooo...assuming pinned connection is unconservative.

 

[phamENG]

Jeb - I disagree. I think the 14% is accurate. The flexibility in this joint, if properly designed, comes from the fact that it's nearly impossible to proportion the column such that it dominates and draws significant moment. You'd have to run a W12x72 over top of an HSS12x12x5/8 (or a similarly proportional beam/column arrangement) to have the column really take the moment. Otherwise, the global response will be a bit 'soft'. But it's not due to the connection itself, if the connection is detailed properly

 

[driftLimiter]

How about the hinge at the Gerber splice ? Significantly more rotation occurs at that point in the model... A beam to beam shear splice has rotational rigidity as well. Why don't we hash that out as well??

If we buy into this too much we can't model pins anywhere simply because they are not true pins. We have to think about partial moment connections everywhere. Years of practice essentially amount to "ignoring reality".

In the past they didn't have ideastatica to to do a cbfem on every connection, they had their steel manuals and calculators. Seems they faired pretty well.

 

[STpipe]

Soooo...assuming pinned connection is unconservative.

Slightly, but that's the case with every simplifying assumption. It's how much of a difference does it make in the grand scheme of things.

 

[phamENG]

drift - funny thing, I was readying an AISC journal article from the late 80s today that talked about that...and encouraged considering that in serviceability design. They found that something like a 16% reduction in weight of steel was possible if you accounted for fixity from shear tabs at service loads in whatever building they were looking at.

They had sections in the paper on how it could be incorporated in practice for firms with 'minicomputers' running stiffness matrices, and for firms who's computer was "nothing more than a pencil and calculator."

 

[driftLimiter]

Jeb - I disagree. I think the 14% is accurate

Is it though? We still are modeling a pure pin at the base. If we jump on board the reality train, our 4 bolt baseplate exhibits rotational stiffness.
The moment at the top of column is now showing up at the base as well because the column wants to be in double curvature. This reduces moment demand on the column further than what Dave has shown. Even if its not fully fixed at the base it has rigidity against rotation.

 

[milkshakelake]

@phamENG The pic showed a bunch of 24' high W4 columns connected to W36 girders, which were done throughout the building to attach electrical boxes and such. Many of the W4's twisted out of place due to being welded and taking unintentional load from the W36 beams (which happened to be overloaded as well; separate issue). As you noted, there was also a fire there. There were tons of other problems, like wind bracing buckling from lateral overload. It was the shitshow-iest of buildings I have ever seen. I made a lot of money from that project. Anyway, the point of the picture was just to show a fixed weak-column-strong-beam connection improperly done, which causes twisting of the column below. Kind of heaping on the other serviceability problems others have noted.

 

[bones206]

Some old steel bridges used pin-and-hanger joints at the gerber splice to mimic true hinges and avoid the complexity of analyzing a partially fixed degree of freedom condition. Not really a practical or reliable joint for must structures though. Everything is a trade off.

 

[phamENG]

drift - where I am, yeah. Our soils are awful, so unless it's on piles there's a decent chance the column base can rotate a bit. Sure, it's more of a spring. But if I start asking for subgrade moduli on every project the geotechs will shoot me. Better to put the simplifying assumption there than on something I can easily calculate.

 

[phamENG]

MSL - yikes. Haven't seen one that bad.

bones - aren't those the fracture critical connections the bridge community started freaking out over a few years ago?