column-foundation connection 3

[Akhilkp]

Hi All,

I would like to hear your opinion about the column foundation connection. what is the actual condition of a column foundation connenction ? Pinned or fixed ? . does it depend on the foundation types ? what is common practice for the anlysis and design of high rise RC building ?

thanks

 

[KootK]

My thoughts:

- In my experience, people will design the connection as pinned unless the column is part of the primary lateral system.

- Obviously, most of these connections are fairly fixed in practice as a result of the details used.

- Fixing the column to the footing is one thing. It doesn't guarantee a truly "fixed" boundary condition, however, as stuff like soil flexibility etc also come into play allowing the joint to rotate.

- The actual degree of fixity is very dependent on foundation type. Big footings probably provide more fixity than small ones. Pile and cap systems, probably more fixity still. Sometimes designers will run large grade beams connecting footings to introduce even more fixity.

- If a column is to deliver moment to a footing that would create uplift, the footing probably ought to have top reinforcing.

- Even in the absence of reinforcing across the footing/column interface, you'll get a fair amount of fixity simply as a result of the compression created in the joint from applied column axial loads. It's essentially a prestressed connection.

- Due to carry over effects, moments applied to the top of a column will generate moments at the bottom of the same column when it is attached to a footing. This is true regardless of whether or not the designer assumed the joint to be pinned. As a result, while we typically check footing punching shear assuming only axial load, there will in fact be a concurrent moment needing to be transferred via punching shear. This would make conventional footing punching shear design potentially unconservative.

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.

 

[Guastavino]

Honestly, I do whatever is most convenient for the numbers. But of course, this requires checking to make sure it works, as moments are introduced when you fix it, and this typically leads to larger foundations, but benefits to the structure above (generally speaking of course).

I justify this because:

1. The loads are conservative.
2. The connections in the building itself are not modeled 100% accurately. So the loads to the foundations aren't really accurate regardless.
3. A load path is provided, and I would have a hard time proving it either way. And honestly, without checking the soil at each corner under the footing, who really knows?
4. Assumptions are stacked significantly by the time you get to the foundation, what's one more layer of assumptions?
5. It's been done for a long time without major issues.
6. The geotech factors of safety are high.

Caveats:
1. If you have a building with significant dead load (long term) moments on a footing when it's modeled fixed, I might be a bit more conservative.
2. It always takes your own judgment, but know that even if you were able to model every bolt, plate, connection and soil spring, I still believe you wouldn't touch reality. We are just enveloping and providing one possible solution to equilibrium. How it works itself out is another thing.

 

[MrHershey]

We typically consider them pinned unless we need them not to be for drift or whatever. Including for lateral systems. NEHRP's "Seismic Design of Reinforced Concrete Special Moment Frames: A Guide for Practicing Engineers" actually touches on this a little bit. It's a free download, would recommend reading it. The basic take there seems to be that it's typical to start with pinned and then adjust if you need. Just have to make sure your detailing matches your modeling. If you've included the fixity at the base to make the rest of your structure code compliant or work a little easier, then you need to design foundations that can handle the moment that fixity develops. If not, then you don't.

 

[TLHS]

I'm generally wary of designing isolated footings as fixed unless I'm really careful about it. If you haven't thought about the flexibility of the foundation system, you can end up designing something that has enough strength to resist a given moment but isn't necessarily stiff enough to take that moment until long after the system has moved to a pinned state through footing rotation. In that case, you may have underdesigned your structure by taking into account the added fixity.