Column Base Plate - Fixed or Pinned 3

[PEFLWI]

I am involved in a dispute with a contractor about a base plate connection to a concrete pier. The base plate is anchored to a concrete pier similar to the attached detail taken from Design Guide 1. The contractor's engineer is adamant that this can be a pinned connection. He is suggesting that the anchor bolts will stretch to relieve the moments. What do you think?
Can the moment be relieved by stretching the anchor bolts? If so, how much stretch of the anchor bolt would be required?
The anchor bolts only extend 5 inches from the top of the pier to the top of the base plate. I don’t think I can count on stretching the anchor rod inside the pier.
A complicating factor is that the concrete piers are already poured. Adding anchor rods would be expensive.

 

[WinelandV]

What is everyone's role here? Are you the EOR and did you provide a baseplate detail?

As to your question, I'd guess I'd like to see the actual baseplate and anchor detail with diameters and thicknesses, as well as the design loads before offering an opinion.

Please note that is a "v" (as in Violin) not a "y".

 

[PEFLWI]

I am an engineer that works for the owner of the project. The base plate is 23"x23" and 2-3/4" thick.

 

[KootK]

1) I don't buy the bolt stretching argument.

2) For some purposes and situations, I do buy the argument that this joint could be conceived of as pinned or pin-ish.

Help us help you:

a) Why do you want this joint to be fixed?

b) Why does the contractor's engineer want this joint to be pinned?

c) Is this column part of the building's lateral system?

d) What is the building's lateral system? Moment frames? Concentric braced frames?

At 2.75" thick, I'm guessing that the EOR means for this to be a restrained base in the final condition.

 

[JLNJ]

Did the contractor put in fewer anchors than specified? Or no anchors?

Or were the anchors mis-set and the contractor is trying to get out of a proper remediation?

 

[DoubleStud]

Did the contractor messed it up and tried to wet set the anchors?

 

[GeorgeTheCivilEngineer]

Would need to consider the flexibility / restraint in the concrete pier as well.

 

[JedClampett]

If you can get the system to work as pinned (deflections, stresses, etc.), I'd accept the assumption as pinned. With that short of anchors, you're going to have a hard time getting them to work, especially as they look pretty close together. Plus your work as not done once the anchorage is designed. The concrete needs to be able to carry the moment.
It seems that with that heavy of a base plate, someone wanted rotation restraint, but unless you absolutely need a moment supported base, I'd try to make a pinned configuration.

 

[PEFLWI]

KootK:

I don't want the joint to be fixed, but I believe it will function that way. The concrete piers were not designed to resist moment, but I think they should be. The contractor's engineer wants this to be pinned so he won't have to redesign the base plates. If moments are transferred, it will affect the concrete pier design. The foundations were not designed by the same engineer as the steel. Yes, the column is a part of the lateral load resisting system. The lateral system is moment frames.

JLNJ & DoubleStud: All the anchors were installed correctly.

JedClampett: The anchor rods are not short. The anchor rods are embedded more than 4'-0" into the concrete. The projection above the top of the pier is not long. In my judgement for this to be a pinned connection, there has to be some flexibility to allow rotation. I don't see any flexibility in this connection. If I wanted it to be a moment connection, what would I need to change? I'm not saying I want it to be fixed; I just think it is.

 

[KootK]

Yes, the column is a part of the lateral load resisting system. The lateral system is moment frames.

Yeah, there really is no perfect way to make a pin of that. Common approaches that I've witnessed in my time practicing:

1) Call the joint a pin and, if you're still good for drift, leave it at that. Obviously, structures are bad listeners so the joint will see some moment.

2) Design the joint and anything about the foundation that would fail in a brittle fashion for 50% of the moment that would arise from assuming a rigid joint. Why 50%? No idea.

In my judgement for this to be a pinned connection, there has to be some flexibility to allow rotation.

In my opinion, very few real world base connections possess enough flexibility to be assumed to be pinned on that basis. Even a 3/4" base plate with anchors inside the flanges will be pretty fixed prior to something yielding / breaking.

The contractor's engineer wants this to be pinned so he won't have to redesign the base plates.

Those base plates are 2.75" thick! If they weren't designed for moment transfer, what were they designed for. It would take a crap ton of axial to justify a plate thickness like that.

 

[BAretired]

The anchor rods are not short. The anchor rods are embedded more than 4'-0" into the concrete. The projection above the top of the pier is not long. In my judgement for this to be a pinned connection, there has to be some flexibility to allow rotation. I don't see any flexibility in this connection. If I wanted it to be a moment connection, what would I need to change? I'm not saying I want it to be fixed; I just think it is.

The connection is neither pinned nor fixed; it is somewhere in between. There is flexibility to allow rotation; pier reinforcement will yield and the pier will rotate in the soil, just enough to resist the residual moment. If the structure works when the connection is assumed to be either pinned or fixed, then it works when it is in between.

 

[jhnblgr]

So how was it foundation designed without reactions from the metal building?

Shear load and axial load?
Wouldn’t this still create bending in the pier? Then this would be transferred to the footing. Then require overturning and sliding checks.

 

[sandman21]

Yeah, there really is no perfect way to make a pin of that. Common approaches that I've witnessed in my time practicing:

1) Call the joint a pin and, if you're still good for drift, leave it at that. Obviously, structures are bad listeners so the joint will see some moment.

2) Design the joint and anything about the foundation that would fail in a brittle fashion for 50% of the moment that would arise from assuming a rigid joint. Why 50%? No idea.

AISC has been doing lots of looking into bae plate connections. “Column Base Connections: Research, Design, and a Look to the Future” Author Amit Kanvinde did a AISC Webinar in April going over the new design methodology in the 3rd edition of the AISC Base plate Design guide 1. The proposed pinned connection uses an "upset" anchor rod, threads upset, debonding tape and a shear key. According to the research even the original detail would be closer to a pin.

 

[KootK]

Thanks for the heads up on that stuff sandman21. For anyone interested in the webinar that sandman21 mentioned, I believe that it can be viewed for fee here: Link

 

[PEFLWI]

Several people have commented about considering the flexibility of the concrete pier. The piers are 3'-0"x3'-0" and 2'-0" tall and sit on a 4'-6" thick mat foundation. Not much flexibility.

The base plate, welds and anchors were not designed for any bending moment. When a fixed condition is used in the structural steel model, the EOR says the anchor rods are overstressed by 27%. No bending moments were supplied to the foundation designer.

sandman21 "According to the research even the original detail would be closer to a pin."

Is this research shared in the webinar?

 

[human909]

The base plate, welds and anchors were not designed for any bending moment. When a fixed condition is used in the structural steel model, the EOR says the anchor rods are overstressed by 27%. No bending moments were supplied to the foundation designer.

Well who designed the baseplate and anchors. Because a baseplate that is 2-3/4" thick sounds like it was designed for a moment connection. Not to mention the position of the anchors.

the EOR says the anchor rods are overstressed by 27%

Well if we have claimed to establish that the foundation, the pier, the column and the base plate are rigid and strong enough for the moment then it seems the the EOR has found where the hinge will form. Even if the connection does behave rigidly then at ULS a hinge will form at the base and you'll get your pinned connection.

I suspect that given the structure seems to have been designed for this being a pinned connection then it should be sufficiently stiff enough that the EOR "overstressed by 27%" will never occur. If the EOR replaced his fixed assumption with a very rigid spring to represent the real connection you might get a suitable result.

 

[dauwerda]

So, when it is considered a pin, how much does the column actually rotate at the base? How much would the anchor need to stretch to allow that rotation? How much force does it take to make the anchor stretch that much? Is it less than the capacity of the anchor? If so, you don't have an issue.

Just like at the ends of simple beam connections, the rotation of the bottom of the column is self limiting. Much like bolt plow in a shear tab, this allows us to take advantage of local yielding effects (whether it be stretch of anchor bolt, bending of base plate, localized concrete crushing, or a combination of all of these and others) without it being detrimental to the structure.

 

[robyengIT]

I found the attached doc

 

[JLNJ]

I attended the aforementioned seminar on base plate flexibility. I applaud AISC for looking into this. In my book, it is one of the least understood among our most commonly used details.

Intuitively, there is almost no way to achieve 100% fixity, right? But I have seen simple base plates with four anchors, no stiffeners, no "stretch" length, and no pretensioning assumed as fixed. Just a big 'ol column with an all-around weld to a wide flange and four bolts.

I have assumed the exact same base plate assumed pinned, maybe with slightly smaller anchor rods.

I have seen people model springs at the base level. I have personally attempted to model the stiffness of the soil and foundation and seen the weird effects when combined with gravity column loads.

For a tower, I have used pretensioned anchors with long stretch lengths. Not every contractor can perform and verify this type of work, that's for sure.

I would not want to stand before the judge and try to explain why the same detail can be 100% fixed and draw a large percentage of the frame moment into the base OR 0% fixed and push all that moment up into the frame. Or anything in between. "That's just the way we do it, Your Honor. We don't understand it either! Pretty funny, eh?"

 

[structSU10]

I have tried the PCI method to determine fixity at a base plate and used rotational springs in a model to see what is happening. Usually the foundation is the largest component to the flexibility at the base.