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Fixity of Column Base plates for axial load only?

Fixity of Column Base plates for axial load only?

Fixity of Column Base plates for axial load only?

(OP)
Recently, our office has been challenged concerning the fixity of the column baseplates.  Typically, we use 4 anchor bolts that are outside the column flange.  A reviewing Engineer for an industrial project states we have to give a value for moment for this baseplate since they are designing the footings.

We modeled it pinned and have always assumed it to be pinned.  How are you other Engineers handling this?

RE: Fixity of Column Base plates for axial load only?

There is nothing "wrong" with what your doing, it is conservative. I prefer to use semirigid design, there are a few past discussions that you may wish to review: thread507-261922: Fixed base on tube steel colum

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

RE: Fixity of Column Base plates for axial load only?

Why not just give the guy what he wants?  Designing the footings for a given moment can only help your steel structure.

RE: Fixity of Column Base plates for axial load only?

There is a P*e moment.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

RE: Fixity of Column Base plates for axial load only?

i ALWAYS use 4 bolts outside as FIXED support.. 4 bolts inside are pinned..  

RE: Fixity of Column Base plates for axial load only?

With the flexibility of the base plate and the stretch in the anchor bolts I'm not sure you're getting full fixity ever.

And assuming fixity reduces your required column size, under-estimates your building drift, etc....all un-conservative assumptions.

Partial fixity - maybe some.  Hard to determine.

RE: Fixity of Column Base plates for axial load only?

this is standard practice in oil and gas EPC.. 4 bolts outside is fixed.. not sure in other industry

RE: Fixity of Column Base plates for axial load only?

The way you model the baseplate tends to be self fulfilling prophesy. If you tell the foundation designer there is a moment, she will design a foundation to resist the moment; if you tell her there is no moment, she will design for axial load.

Do you ever get a fully pinned foundation or a fully fixed foundation? No, of course not, but I don't know how you can say there is no moment. If you tell the foundation designer there is no moment when there is obvious moment capacity, they have no option but to question it.

Now, consider if you are the foundation designer, would you not worry that you would design a zero moment foundation and then get blamed if it rotated due to the actual, non-zero moment?

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Fixity of Column Base plates for axial load only?

Michael,
I fail to see your point, the method Berniedog has chosen to design his column connection is conservative for his structure. If the foundation designer is concerned about his design then he can also be conservative.  

I assume the foundation designer is an engineer, he can analysis the whole structure and get the design moment you consider to be "real" or worst case for his foundation.  
 

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

RE: Fixity of Column Base plates for axial load only?

Just because it has moment capacity doesn't mean that it's actually resisting moment.  Sure there is some moment there from uneven load distribution, connection eccentricity, etc, but is a 5k-ft moment really going to amount to a hill of beans compared to a 50k axial load?  

Additionally, I would consider it a waste of time to go around an check all baseplates for what moment capacity they have based on axial load and baseplate/anchor rod configuration.  To then go around and design all of the footings for that theoretical baseplate moment capacity seems, to me, to be an inefficient use of our time and the owner's money.

RE: Fixity of Column Base plates for axial load only?

(OP)
Thanks for the discussion.  I suppose I should give some moment.  We have never done that before.  On page 14-4 of the 13th Edition Steel Manual, fig. 14-2, it shows an axially loaded column base plate with 4 anchor bolts outside of the flanges.  AISC Design Guide 7 sheds no light on it and AISC Design Guide No. 1 pg. 36 thereis discussion but it is not conclusive (they do not tell you what to do).

We are going to call AISC this morning.  I will keep you posted.

RE: Fixity of Column Base plates for axial load only?

I agree with SEIT here.

In the case of a braced frame with veritcal braces, there really would be little moment, BUT, there will be base shear---> which will cause moment on the foundation. This might be a bigger concern for the foundation guy.

Is there base shear at the column?
In some industrial structures this shear can cause a rather sizeable moment on the foundation.  

RE: Fixity of Column Base plates for axial load only?

(OP)
There is about 3.5k shear.  I do need to look into this.

RE: Fixity of Column Base plates for axial load only?

rowingengineer, If I am asked to design something with input from another company, I won't have money in the contract to check or redo their work if I have, and I always have, had to bid competitively for the contract. If I see that they may have made a mistake, I would be stupid not to ask, and to request an upper limit.

If I blindly went ahead, and if there was a problem later because there actually was a moment on the foundation, even though the problem is not of my causing, I would still have pay a lawyer buckets of money to defend me, eating up my profit and more.

Michael.
Timing has a lot to do with the outcome of a rain dance.

RE: Fixity of Column Base plates for axial load only?

Bernie-

As for the shear, I'm sure all you'd have to do is report that as one of your column reactions.  

RE: Fixity of Column Base plates for axial load only?

(OP)
This is the response we recieved from AISC:

Subject: RE: Ask AISC | Anchor Rods, Base Plates and Embedded Plates

 

Ryan,

 

The AISC Specification does not define what assumptions the responsible design professional should make in regards to base fixity considerations. There is probably no such thing as a completely fixed or completely pinned base. It is usually a matter of it being considered as closer to fixed, than pinned or vice-versa. The specific details of the base play a large part as to what goes into making these assumptions. But the type of foundation system also may be a major consideration. Bases with the anchor rods located outside of the column shaft are often assumed an 'pinned' due to flexibility of the base detail, as well and rotational characteristics of the foundation itself. These are really engineering judgment calls that must be made by the design professional.

Kurt Gustafson, S.E., P.E., F.ASCE
Director of Technical Assistance
American Institute of Steel Construction
866.ASK.AISC
 

RE: Fixity of Column Base plates for axial load only?

Nice to see an engineer from a code agency tell you to use engineering judgment rather than referring you to a bunch of obscure research or code provisions.

I would just list a nominal design moment along with the gravity load, base shear, and uplift if applicable.

RE: Fixity of Column Base plates for axial load only?

Agree with Hokie here...I would have thought for sure AISC would tell you to do some extremely detailed analysis to determine the degree of base fixity or refer you to some committee.

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