Footing: pinned or fixed?

[gotlboys]

Hi everyone,

In my building design class, my professor said it is a structurally acceptable practice to assume low to mid rise buildings' column-footing connection to be pinned, although fixed condition may be assumed in some situations.

I would like to hear from your experiences and the reasons behind your assumptions.

Thank you
Gotlboy

 

[hokie66]

If you try to fix a column at the footing, it requires a much more substantial footing than if the column is pinned. So most of us used pinned base columns as a matter of course, so in this case I agree with your professor.

 

[KootK]

I also agree with your professor. Sometimes, for concrete columns, I'll design footing punching shear assuming a moment 1/2 that at the top of column in acknowledgment of potential fixity as punching shear is a brittle failure mode. Even that isn't all the way conservative but you've gotta draw the line somewhere.

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.

 

[MrHershey]

We do pinned generally. Only do fixed when we have to like at cantilever columns, or when we otherwise need/want some fixity at the base. But pinned is pretty standard in the US, including in high seismic regions.

Important to remember that these are approximations. Very few connections are truly pinned or truly fixed. Also important to remember that even though your column to footing connection may be 'fixed', that doesn't mean you have a fixed base from a modeling standpoint. If you're on a spread footing that can rotate or if you're on a deep foundation that doesn't have a moment connection from cap to pile or tie beams that have been designed to develop the moment, then you're still not fixed at the base regardless of how rigid your column to foundation connection may be.

If you do model and design with a fixed base, it's important to remember to carry that moment through your entire foundation design.

 

[gotlboys]

Thank you everyone for those insightful information.

1] For pinned connection, it is applicable to buildings in moderate to high seismic regions where the SMRF (columns and beams) are to carry lateral seismic forces plus gravity loads, consequently these columns' moments have to be resisted by footings (spread or mat) to maintain stability.

2]It seems that 'pinned support' makes the 1st floor column having max. moment at top, which requires bigger column cross-section than if it were fixed.

It is a good point that footings have to be designed for axial and moment forces if 'fixed' base has been assumed.

 

[MrHershey]

1. Generally you do not need fixed base connections to maintain lateral stability. Go ahead and run a simple moment frame in the analysis program of your choice or even do it by hand. It'll run just fine with pinned bases.

2. Perhaps, but usually this effect is fairly small compared to the impact on the foundations. And if you're doing a special moment frame, your column design is often governed by strong-column/weak-beam behavior anyways. So even if the moment demand is higher with pinned bases than with fixed it often won't drive the column design.

 

[gotlboys]

If pinned is assumed, do we terminate longitudinal bars in the interface of column-footing? Any sample detailing is helpful.

It seems that minimum transfer dowels that code requires [0.005Ag] plus concrete hardness could still provide some fixity. Thus partially fixed looks more representative.

 

[hokie66]

Yes, there is always some fixity, which is a bonus. But you asked about design assumptions. Most of our design assumptions are a bit on the conservative side, as they should be.

 

[MrHershey]

I would venture to guess that 99.9%+ of connections are 'partially fixed'. You will never be fully pinned, which would mean that the two connected elements are able to rotate independently of each other at even infinitesimally low moments. You will also never be fully fixed, which would mean that no amount of moment will ever cause the two connected elements to have different rotations.

But let's go ahead and follow the line of thinking that a connection with minimum dowels is partially fixed and plug it into the structural model that way. What's your rotational spring constant? Keep in mind that your foundation can rotate too, so factor that in to your calculations as well. Also your material behavior in real life is not linear, especially in the plastic stage, so your rotational spring really shouldn't even be a constant, but resistance should vary by moment level.

The problem can get incredibly complex really quickly. You could write a whole dissertation on this topic alone. Both 'pinned' and 'fixed' are approximations that help us come to simple solutions for this extremely complex problem. Don't get too hung up about not being perfectly in line with these approximations, you'll never get there.

 

[dik]

Agreed that for low buildings I generally treat the footing to column junction as pinned, in reality, with the axial load on the column, and the relative stiffness of the footing, the condition is more likely nearly fixed if you have reasonable soils.

Dik

 

[gotlboys]

Thank you all. I have to say I learned something from this conversations.

 

[271828]

For multi-story steel moment frame buildings, we typically use fixed base plates to reduce the frame sizes required for wind drift serviceability. Of course, this results in large footings, base plates, and anchor rods.

 

[JasonMcKee71]

Fixity factor also depends on the soil modulus, the possible account for footing uplift etc.

Jason McKee
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