Valid Pin Boundary Condition? 1

[duckhawk]

For a square moment frame, do you think this is a valid pin boundary condition at the base?

Where does the moment go?

 

[phamENG]

Maybe? Depends on the moment. And what deformation is expected at ultimate load and whether or not the connection will survive that deformation. Only true pin is, well, a pin.

Not sure what you mean by where the moment goes?

 

[Luceid]

Will it provide moment resistance? To some extent...yes as almost always. The moment would be resolved as base plate bearing on the concrete and tension in the bolts. But depending on the elongation and movement, this becomes more and more like a pin. Engineering judgement as always

 

[canwesteng]

It is certainly conventionally treated as a pin condition. Provided there is a sufficiently stiff load path above the base plate, you are ok to call it as a pin.

 

[rb1957]

"where does the moment go ?" ... in a pinned base, there is no moment ... though this will support "some" moment.

I guess the failure scenario would see the connection lose moment capability before it fails completely ... so pinned might represent the final state.

You could analyze as pinned, fixed, and/or partially fixed ... depending on how much time you thought this detail was worth.

 

[JoshPlumSE]

Yes, this is generally considered a "pinned" connection.

However, if there were a small amount of moment, then you could justify it with a hand calc. I had a case where I had a very small equipment support column with a base plate like this. No seismic load. Minimal wind and gravity loads.

I ran a calculation demonstrating that the wind load could be handled by the moment resistance at the base. IMO, the column was perfectly safe. But, the lead engineer objected and we moved the anchor bolts out to a more traditional moment resisting location. I didn't have a problem with changing it for him because he was the project lead and he had ultimate responsibility for my work.

 

[JAE]

Don't forget that the column base here rests on a footing which rests on soil that deforms under stress (i.e. moment from above).

So as rb1957 states there will be "some moment" as the true response of your structure is always somewhere between pinned and fixed.

As others have noted here it's usually assumed as pinned because that usually results in the structure above being stronger and stiffer.

 

[271828]

Consider a simple shear connection between a beam and column. The beam will have an end moment that is ignored for the most part. That moment will be small if the connection is detailed correctly, such as a double angle with non-thick legs.

Your base plate is kinda like that. If you don't make the plate thick, the moment should be small.

 

[duckhawk]

Appreciate everyone's responses! I think I get what's going on a little better now.

Here is the moment force diagram considering boundary conditions pinned. Units are in kip-in

 

[271828]

With realistic stiffness at the left base plate, the left column would have a small moment at the base and slightly less than 4500 at the top. The column design would not be appreciably affected. Great example of why we can use a pin.

 

[driftLimiter]

where is human909? Im curious to see what they would say to OP.

@duckhawk, remember that stiffness 'attracts' load. But also that ultimate limit states can aid our design process.

Sure the connection exhibits some moment stiffness as others have indicated, but as you keep increasing that moment, its behavior may change.

For example the baseplate may deform significantly and lead to joint rotation, or the foundation may be allowed to rotate as well. At this point from the moment frame perspective, you have a pin support because there is free (or rather fairly easy) joint rotation.

I think designers can get into more trouble when they assign a fixity to their frames, because then you need to provide significant rigidity in the load path even at the ultimate limit state. It can be done but it takes quite a bit more planning and material.

 

[human909]

where is human909? Im curious to see what they would say to OP.

You called?

Im curious to see what they would say to OP.

I'm not sure there is much to add, the above responses pretty much have it covered. Yeah this is typically considered a pinned connection but like others have said it will obviously have some moment capacity and attract some moment. (Especially true if axial loads are high.)

Normally assuming something is a pin is a conservative assumption as it forces the engineer to design stiffness elsewhere which normally limits rotations and makes 'nominally' pinned connections suitable to be assumed as a pin.

Sometimes however assuming something is a pin when it isn't is unconservative. Hence why engineers go to the trouble of using ACTUAL pins.

 

[SE2607]

Consider a simple shear connection between a beam and column. The beam will have an end moment that is ignored for the most part. That moment will be small if the connection is detailed correctly, such as a double angle with non-thick legs.

Your base plate is kinda like that. If you don't make the plate thick, the moment should be small.

The difference between the two conditions is that AISC has declared a shear connection between a beam and column as pinned. This position is reinforced by the fact that the bolt holes are larger than the bolts and some rotation is entirely possible. No such declaration that I am aware of has been made for a base connection.

Some have argued that the base connection will yield before failure which justifies a pin connection, but the analytical model is usually below yield conditions unless one is looking at collapse mechanisms or ASCE 41 criteria in which every connection can become a pinned connection.

In my opinion, those who cite "engineering judgement" without being reinforced by analysis is actually a business decision.

How many legs does a dog have if you call a tail a leg? Four. Just because you call a tail a leg doesn't make it a leg.

 

[canwesteng]

The rotation provided by 1/16 oversize bolt holes is not all that significant and is present in any splice plate moment connection anyway. They are called pins because they have been shown to have the flexibility or ductility to behave as a pin.

 

[271828]

The difference between the two conditions is that AISC has declared a shear connection between a beam and column as pinned. This position is reinforced by the fact that the bolt holes are larger than the bolts and some rotation is entirely possible. No such declaration that I am aware of has been made for a base connection.

Some have argued that the base connection will yield before failure which justifies a pin connection, but the analytical model is usually below yield conditions unless one is looking at collapse mechanisms or ASCE 41 criteria in which every connection can become a pinned connection.

In my opinion, those who cite "engineering judgement" without being reinforced by analysis is actually a business decision.

How many legs does a dog have if you call a tail a leg? Four. Just because you call a tail a leg doesn't make it a leg.

How would you model and design the base plate in the OP's case?

 

[joder]

For a square moment frame, do you think this is a valid pin boundary condition at the base?

Where does the moment go?

View attachment 14525

If you assumed it was pinned, it will satisfy statics regardless of how much moment the base develops, and the design will be safe. It will develop some moment.

But to answer your question:

The base plate will rotate by bending of the toe and stretching of the bolt, relieving the moment developed. Any moment developed by the base plate will go into the footing. If the footing was not designed as fixed, it will rotate and relieve the moment.

Once the rotation of the column end equals what it would be as a pinned base (per your analysis assumption), then it is effectively hinged. That anount of rotaion is very small. It should be readily available. A fixed base has ZERO rotation, which is diificult to achieve, requiring a thick base plate and a big footing. Because the rotation of a pinned base is small, the base plate bending and bolt tension should be well within the ductility of those materials, so it is safe.

 

[human909]

Since @driftLimiter called me out. I'm going to go on a nit-picking, pedantic crusade.

The rotation provided by 1/16 oversize bolt holes is not all that significant and is present in any splice plate moment connection anyway. They are called pins because they have been shown to have the flexibility or ductility to behave as a pin.

But most 'nominally' pinned connections also have the stiffness to behave as a fixed connection. Example this column would likely stand up just fine and remain plump during erection but itself which is an excellent demonstration that both the connection and the footing is fixed and rigid. Likewise I've seen many poorly designed non engineered steel cantilever platforms which cantilever off simple shear plates which, at least around here, every engineer would consider a pin connection. (clip angles aren't really used here)

Your statement lacks the nuance of referring to expected loads, expected or allowable rotations and the stiffness of the rest of the structure. The stiffest areas of your structure will attract the loads. If your stiffest connects are 'nominal pins' then they will be the ones attracting the load and most of our 'nominal pins' are still stiff enough and strong enough to act as fixed connection if loads are not too high.

The base plate will rotate by bending of the toe and stretching of the bolt, relieving the moment developed. Any moment developed by the base plate will go into the footing. If the footing was not designed as fixed, it will rotate and relieve the moment.

But it won't relieve the moment, the moment will remain. It will just be less than the moment of a 100% rigid connection or are more rigidly designed connection. So moment will develop but it will likely be limited by the stiffness of the rest of the structure.

Also the reality is that unless axial loads are quite low compared to the moment then this connection will reasonably stiff. If axial loads are high the bolts won't even be loaded by the moment attracted.

A fixed base has ZERO rotation, which is diificult to achieve, requiring a thick base plate and a big footing.

Ground beams to in plane columns quickly fix that. It is no more difficult to achieve that a fixed connection between steel beams. Likewise a deep and broad pile can provide some pretty good stiffness.

It is about economy, less about difficulty. This is especially the case for portal frame steel structures with low axial loads and long spans and relatively high lateral loads. The whole equation flips on its head for shorter spans and higher axial loads.