Pin Based Moment Frame... Column Base Stiffness
Pin Based Moment Frame... Column Base Stiffness
(OP)
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.






RE: Pin Based Moment Frame... 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).
RE: Pin Based Moment Frame... Column Base Stiffness
RE: Pin Based Moment Frame... Column Base Stiffness
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.
RE: Pin Based Moment Frame... Column Base Stiffness
BA
RE: Pin Based Moment Frame... Column Base Stiffness
- Fixed/rigid base: 4EI/L
- Nominally pinned base - serviceability (deflections): 0.2*4EI/L
- Nominally pinned base - ultimate limit state: 0 to 0.1*4EI/L
I = I,columnI 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.
RE: Pin Based Moment Frame... Column Base Stiffness
RE: Pin Based Moment Frame... Column Base Stiffness
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.
RE: Pin Based Moment Frame... Column Base Stiffness
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.
RE: Pin Based Moment Frame... Column Base Stiffness
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.
RE: Pin Based Moment Frame... Column Base Stiffness
Steel Construction Institute Publication P164 "Design of Steel Portal Frames for Europe" by C.M. King, 2001.
RE: Pin Based Moment Frame... Column Base Stiffness
Why bother iterating? Why not simply apply the desired moment to each joint and let the computer determine the remaining moments?
BA
RE: Pin Based Moment Frame... Column Base Stiffness
It can be found at the following link: Link
RE: Pin Based Moment Frame... Column Base Stiffness
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.