Steel structure base fixity (theoretical)

[StrEng007]

How do I determine if the base connection for a frame/bent steel structure erroneously has enough flexibility to be considered a pin instead of a fixed connection? The same question would go for a free-standing cantilever structure?

I realize the flexibility lies in the weakest member (ie, the base plate, foundation, anchorage, or possibly the member itself), so these connections are not just about having a full penetration weld. I expect some of you will mentioned the absence of gusset plates, which I have utilized in the past thanks to Blodgett (although I'm not sure if my approach was spot on).

This is a question that occurred after I saw a DOT truss/frame structure for holding highway signs. The base of the frame had a considerable stand-off baseplate attached to the top of a drilled pile. What caught my attention was the diameter (small) of the anchors that holds/levels the baseplate. Additionally, nobody floated grout under the base plate, so all these anchors are in bending. It's possible the structure is designed to have a pinned base and this is all intentional. Note that I do not work with these kinds of structures.

Seeing this got me thinking about building structures and all the possible ways an assumed fixity it not occurring in real loading scenarios.

 

[andrews2]

It all comes down to what design assumptions you make and making sure you design your structure to accommodate those assumptions. If you design your structure to have a fixed base, then you should design the base to be rigid and provide the fixity you are counting on. If you design the base as pinned, you don't need to design it as rigid, so some deformation may occur. That deformation is ok since it is in line with the pinned assumption you made.

For the highway sign structure you mention, I would say one of the following is the case (assuming it was properly designed):
1. The structure was designed as having a pinned base.
2. Loads are small and the small anchors can provide adequate fixity.

You also mentioned cantilever structures. Those must always be fixed base. If the base isn't fixed, the structure isn't stable.

 

[phamENG]

The lack of grout doesn't make it a pin. Just because the anchor bolts are in bending doesn't mean it lacks the ability to transfer the moment from the column to the foundation with near zero relative rotation. You may get a minuscule amount of lateral movement as the anchors bend, but think about it...how much can a 5/8" steel rod bend when cantilevered 2" without yielding? Not much. Probably not enough to matter. And for most non shrink grouts, you'd have to assume those bolts are bending anyway. The only real change is how the compression side of the couple is being dealt with. Instead of a large bearing area, you have discrete points where the plate is bearing on a nut. Actually makes the analysis easier.

 

[BridgeSmith]

Our sign structures are typically designed so that there is less than one bolt diameter between the leveling nuts and top of the concrete. For that configuration, we're allowed to ignore bending in the bolt itself due to shear. The moment is resisted by the force couple developed through tension and compression in the bolts. The exception would be break-away bases designed to shear off at a specific location when impacted. Those are beyond my expertise, though, so I just take the manufacturer's specs at face value and move on.

 

[StrEng007]

You also mentioned cantilever structures. Those must always be fixed base. If the base isn't fixed, the structure isn't stable

I think the way I asked the question gave the wrong impression about my understanding or capability. Let me backup to say, I understand the differences in fixity and the types of forces that need to be accounted for in a design. So yes, it's understood that a cantilever structure will need to handle moment, shear, axial compression, and possibly torsion, all through the same base connection.

Moving past the fact that I can design connections, ones that consider steel members connected to baseplates/foundations.

The question I am asking is, how do I determine if there was a fault in those assumptions during design? If I've run a rigid basplate design through Hilti's Profis software, how can I be sure that the blackbox calculations done on the plate are adequate?

Additionally, if I were to gusset a connection to 'assume' the proper rigidity for a fully fixed connection, how do I actually verify this? I know it has something to do with the degree of rotation at the connection, but with most structures being neither truly fixed nor truly pinned, what rabbit hole can I go down to get a better understanding of how this is being verified. Is this being done through a program? I can't image we have the budget to be able to verify these things by hand.

 

[human909]

what rabbit hole can I go down to get a better understanding of how this is being verified.

That is a VERY deep rabbit hole. I've delved down a few of the pathways... So have many research papers.

Most pins aren't pins. And many 'rigid' connection fall into the 'semi-rigid' category.

For example most 'pinned' columns have enough rigidity in the connection to act in a rigid manner as far as compression goes. AKA they their effective length is significantly lower than you would assume if you consider the base plate as pinned.

IdeaSTATIC does an excellent job of calculating the rigidity of connections. It is another black box, but it is damn useful and its results have been verified.

 

[andrews2]

The question I am asking is, how do I determine if there was a fault in those assumptions during design?

I see. This is on me for misunderstanding your question. I thought you were asking about assumptions that may have been made for existing structures.

To answer your actual question, I think human909 said it best. Nothing is truly "pinned" and nothing is truly "fixed" and there are probably many papers you can read that provide their own methods and recommendations for determining the actual degree of fixity in a connection.

To determine degree of fixity based on your profis results, my thought would be to take the anchor stress it gives you and convert that to anchor elongation and column base rotation. Then use your engineering judgement to determine if the rotation is minimal enough to justify calling the connection fixed.

 

[SE2607]

If you call a tail a leg...

Fixity is established based on rotation of the joint. If it doesn't rotate, it's fixed. If it's free to rotate, it's pinned. Most real world structures are somewhere in between. One exception I can think of is railroad bridge intermediate supports.

The prudent thing to do is to look at both possibilities and determine the effects the fixity condition has on the structure. In the case of a cantilevered column, it is unstable if it is pinned at the base. In this case, one should design the connection as fixed and consider the effects of the deflection for a partially fixed connection in which the joint rotates causing more deflection at the top of the column. In the case of a moment frame, one should consider the effects of a pinned-based frame (more moment in the beam-column connection) and fixed base (moment at the foundation).

The real world is more complicated than the theoretical world and requires a substantial amount of judgement, hopefully judgement reinforced with rationality (enveloping, testing, published positions on various connection types, etc.) rather than "I've been doing this for 30 years and nothing has collapsed".

Another quote I enjoy (other than the partial quote from Abraham Lincoln I opened with) is this:
"In theory, there is no difference between practice and theory, but, in practice, there is."