Pin Based Moment Frame... Column Base Stiffness 1

[cal91]

For a theoretically perfect pinned based moment frame the column base stiffness is 0. Many columns are modeled this way but I remember watching an AISC lecture about the Direct Analysis Method that talked about giving a stiffness to the column base. It was something like 6EI/L^2 or something like that. Is anyone familiar with this? I'm trying to find it but to no avail. Also, does anyone actually design their pinned based moment frames as having a column base stiffness? I remember it helps more than I expected for lateral displacements, and it seems to better capture what is actually going on than the perfect pin assumption.

 

[WARose]

Also, does anyone actually design their pinned based moment frames as having a column base stiffness?

When the anchor bolts are all within the perimeter box (formed by the flanges), I've never had an issue treating it like a pinned base. The one time I sat down and looked at it otherwise......it just wasn't stiff enough to attract much moment. I think where you start having to question that assumption is when your bolts start getting close to that perimeter (or are on it).

 

[SAIL3]

about the only time I consider a moment is when using post-installed ABs....

 

[JoshPlumSE]

I see a lot of base plates where the Anchor Rods are located outside the flanges. So, there is definitely some engineering judgment about whether this is fixed, pinned or something in-between.

If the base plate is grouted, the anchor bolts are outside the flange, and the anchors and footing can take the moment, then I don't have a problem treating it as fixed. Though I have, on occasion, used the modulus of sub-grade reaction and the footing area to come up with a rotational stiffness of the foundation. Just introduce a unit rotation and see what type of moment that produces in the soil (assuming equal resistance in tension and compression). Not a perfect number, but it should be in the ball park.

 

[BAretired]

Another idea is to consider a virtual member under each column with a moment of inertia equal to that of the anchor bolt group. The member could be considered pinned at the far end with a length equal to that of the column. The length could be varied to see how the resulting moment is affected.

BA

 

[Settingsun]

I have a document that gives the following recommendations for the column base stiffness and references British Standard BS5950-1 as the source:

[ul]
[li]Fixed/rigid base: 4EI/L[/li]
[li]Nominally pinned base - serviceability (deflections): 0.2*4EI/L[/li]
[li]Nominally pinned base - ultimate limit state: 0 to 0.1*4EI/L[/li]
[/ul]

I = I,column

I usually check deflections using zero for simplicity, then increase to 0.2*4EI/L if deflections are governing the design. I use zero base stiffness for ultimate strength design so that the footings don't need to be designed for a moment reaction.

 

[Contraflexure74]

I think the attached is what you're looking for. Hope you can read it. I always use this when I'm struggling with lateral deflection. Just make sure you've got a decent 4 bolt connection at the base.

 

[P205]

If you're using software, why not use a spring in the Mz direction, and iterate the spring value until you get a moment that the baseplate and anchor rods can comfortable take?

I'll do this until I have 10-15 kN-m of moment. Depending on how far apart your anchor rods are, it might result in a very reasonable couple force in your anchors.

 

[KootK]

I think the attached is what you're looking for. Hope you can read it.

I'd like to check that out in more detail. Can you share the name of the publication?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

I have a document that gives the following recommendations

I'd like to check this out too if you're able to point me too it.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Settingsun]

KootK,

Steel Construction Institute Publication P164 "Design of Steel Portal Frames for Europe" by C.M. King, 2001.

 

[BAretired]

If you're using software, why not use a spring in the Mz direction, and iterate the spring value until you get a moment that the baseplate and anchor rods can comfortable take?

Why bother iterating? Why not simply apply the desired moment to each joint and let the computer determine the remaining moments?

BA

 

[Guest262653]

I would had to the reference list NCCI SN045a - "Column base stiffness for global analysis".
It can be found at the following link: Link

 

[P205]

Why bother iterating? Why not simply apply the desired moment to each joint and let the computer determine the remaining moments?

Sure, depends on the size of the model. I usually iterate for small projects. Where 3 iterations (change value, run model) takes all of 5 minutes tops.