If the footing could rotate (which they almost always can), you design as pinned. The nature of pinned v fixed is about whether the connection is designed to take the full moment of the attached members. If you plan to fully fix the footing and column, and then model the footing on soil springs, it may be unnecessary to use a pinned connection. The conventional analysis process is about notional structure rather than actual. When using FEA and complicated analysis methods, the need to model actual behavior becomes more important, but also then demands an evaluation of all the underlying assumptions.
thanks a lot..i think it is more economical to design the base pinned rather than fixed..is it right? So far i have design only pinned bases..In pinned bases, there are only bottom reinforcement for tension, and for fixed bases, top reinforcement have to be provided? the same logic applies to beams also?..
when i design a pad footing as pinned, the linear pressure beneath the footing is linear making the design more simpler than a fixed one..i have designed a simple house for an assignment where i have used pinned supports and not fixed. Now i have to explain why i have preferred to use pinned and not fixed..The reasons that i have given as that it is more conservative to design pinned and it is simpler but these reasons are not enough.
If you design as fixed, the actual foundation must closely approximate that fixity. You still have a linear pressure distribution but sloped. the problem is designing a pad that will not rotate by compressing the soil more on one side and possibly lifting on the other.
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
I don't think I have ever designed a building footing as fixed, even when the degree of restraint seems to warrant it, if that helps. Obviously that is not a universal principle, and does not apply to things like mass foundations for signs or wind turbines.
We are all a bit conservative about this, but the amount of savings by fixing the base of concrete building columns is relatively small compared to the cost of the structure with pinned bases. For steel, I would be concerned about fixing a base to a foundation element that could settle and induce unwanted bending.
I don't understand the question. When is a pad footing pinned or fixed? I can understand how a column attached to the footing is pinned or fixed but the footing itself is neither pinned nor fixed. If it rotates in the soil, variable pressure results on the faces of the footing which tend to resist rotation.
A steel column may be considered pinned to the foundation, but in the case of concrete columns, it is usual to connect rigidly to the foundation. The rotational restraint of the footing then becomes a function of the soil properties.
Please explain what is meant by the question in this thread.
The fixity is between the column and footing only. If the column is fixed to the footing, it transfers axial load and moment to the footing (along with any horizontal shear). If the column is pinned at the footing(which is normally done in steel columns)the footing will be subjected to only axial force and horizontal shear only.
