Footing: pinned or fixed?
Footing: pinned or fixed?
(OP)
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
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






RE: Footing: pinned or fixed?
RE: Footing: pinned or fixed?
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: Footing: pinned or fixed?
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.
RE: Footing: pinned or fixed?
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.
RE: Footing: pinned or fixed?
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.
RE: Footing: pinned or fixed?
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.
RE: Footing: pinned or fixed?
RE: Footing: pinned or fixed?
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.
RE: Footing: pinned or fixed?
Dik
RE: Footing: pinned or fixed?
RE: Footing: pinned or fixed?
RE: Footing: pinned or fixed?
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