Analysis Discussion: HSS Baseplate, usually assumed as pinned. Yet in my FE analysis it seems Fixed. 2

[IngDod]

Greetings, attached is a summary of my FEA analysis.

I am currently designing a two story OMF building. My initial assumption was to assumed the columns pinned at the base, I did this because it seems to be the common practice. But now that the time has come to design the base plates I'm having problems (cant sleep) simply assuming its pinned and designing it solely for axial force and shear (using AISC's DG1), it troubles me that if this connection is actually fixed (or at least with high rigidity) the baseplate will fail.

Now, I've always had problems grasping the concept of steel connection design; some people say that since you assume is pinned and you designed for axial loads only then it will behave as such. I proceeded to do a FE model, if the connection is indeed pinned then I should see little (no connection is truly pinned of course) moment being transferred to the baseplate. What I get is that the base-plate bends considerably due to the moment, now i would expect this since it is precisely this bending which allows rotation; however the stresses in the plate are much higher than what it can resist... So while it rotates, it seems to me that it would fail too. Now i proceed to increase the baseplate size so that it can resist the stresses, now the rotation is much much smaller... giving me the impression that by increasing the thickness I have made the connection rigid.

Please see the attached summary as it is much easier to understand. The initial baseplate designed for axial load only is 1cm thick, while the one designed based on the FEA is 2.3cm thick. I added results for more thicknesses to study the behavior, with 5cm thickness the rotation seems very low.

I don't really know how to judge if the connection rotation is high or low, or whether it approaches rigid or pinned behavior. I'm getting rotations of 2 degrees for the 1cm thick plate and around 0.5 degrees for others... is this a lot? is this rigid?. How much should a connection rotate in order to be considered as pinned?

Thanks and I hope you can help me with this dilemma.

 

[AELLC]

The computer did all the math - no one needed to know how to set up all the matrices

The definition of a structural engineer: overdesign by a factor of 1.999, instead of the usual 2.

 

[JLNJ]

A have a few punch cards at my desk (they predate me, but not by much) just to show the youngsters!

May advice on the base plate fixity:

Start with a pinned case - regardless of how the bolts and column are arranged. If you can live with the additional moments driven up into the column and the sway is under control, then you are on the right track. Why throw moments into a foundation system? It makes the design needlessly and meaninglessly complicated.

Now, if you end up with an unmanageable column size, or unpalatable sway, then you start looking into other options. Keep in mind that full fixity through a base plate, anchor rod, pier, footing system will be virtually impossible to achieve (except for a cantilevered system). Now you are left with modeling a system which takes into account partial fixity. Keep you method rational and reasonable conservative - it may take some trial and error to get the reasonaby accurate results you are looking for. Just because it is FEA-modeled doesn't inherently make it reasonable or conservative.