Isolated Footings - Fixed or Pinned
Isolated Footings - Fixed or Pinned
(OP)
In structural analysis on CASAD software, there are two types of working systems, we keep column end node either fixed or pinned with the footing.
1) What are the site conditions or any detailing privileges to consider them (column footing joints) as fixed or pinned.
2) are there any benefit in economy on either side
3) which type of commencement is more realistic and safe as well
1) What are the site conditions or any detailing privileges to consider them (column footing joints) as fixed or pinned.
2) are there any benefit in economy on either side
3) which type of commencement is more realistic and safe as well






RE: Isolated Footings - Fixed or Pinned
RE: Isolated Footings - Fixed or Pinned
Stenbrook.. let's say the column is not only rotationally restrained to the foundation but the column is designed such that it won't deflect even during lateral loads (say the foundation and column is solid steel that is large (1 meter solid steel column). If a beam is connected to it.. would the beam rotate? The way I understand it.. it is the moment or the drift of the column that produced rotation and probable moment strength in the beam.. with fixed column.. the beam wouldn't reach Mpr and shear from that Mpr. Am I correct?
RE: Isolated Footings - Fixed or Pinned
If that wasn't clear, you can always run some calculations using the matrix method of analysis to determine the exact rotation at each node as well. Or input a simple frame into a program like RISA or STAAD which will determine your beam deflections and what not for you.
RE: Isolated Footings - Fixed or Pinned
Once I designed a foundation where two separate footings were made to combined into one large footing (for rotational restrained).. then I made the column twice bigger (to prevent moment).. I guess the beam would be more resistant to rotations with this design.. isn't it hence it wouldn't reach Mpr and no shear failure would develop. Is this correct?
RE: Isolated Footings - Fixed or Pinned
RE: Isolated Footings - Fixed or Pinned
There are all kinds of arcane exceptions but, in general, you need:
1) A footing reinforced on both top and bottom (top rebar adds cost).
2) A footing sized to resist column base moment (extra width adds cost).
3) Soil that is stiff enough that the footing doesn't rotate appreciably when moments are applied.
4) Anchor bolts that won't pop out of the concrete under tension (longer, bigger bolts add cost).
5) A base plate thick enough that it won't yield or rotate excessively under the applied column moments (with large moments, the extra detailing can add lots of cost).
As you've surely gathered from my previous comments, I feel that fixed base columns are very expensive. In practice, they are only used when they are required. This includes columns that cantilever from the foundations and sometimes moment frame columns. You might be able to use smaller columns assuming them to be fixed at the foundation but that benefit is likely to be offset in spades by the other issues.
In my opinion, a building with a bunch of fixed based columns is safer. It provides a multitude of redundant load paths for lateral loads and stability. However, I don't recommend this approach. As structural engineers, our job is not to design the safest possible building. Our job is to design the cheapest possible building that is safe enough.
I agree with Stenbrook that, in reality, all column bases are fixed to some extent. Even without the extra detailing measures, you will get some fixity from the fact the the joint is somewhat prestressed by the column axial loads. In renovation work, I will sometimes take advantage of this fixity to avoid reinforcing overloaded columns.
I think that your real question here is "what is standard modelling practice for column/footing joints". The answer to that question is that they should always be pinned unless you have a very good reason for fixing them.
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: Isolated Footings - Fixed or Pinned
If costs is not a factor.. mat foundation reinforced top and bottom would produced the rotation restrained of the columns.. isn't it? Also this means large building with mat foundation is naturally more earthquake resistant? I guess the columns need to be oversized too to avoid larger moments (you didn't mention oversizing columns)?
RE: Isolated Footings - Fixed or Pinned
For moment frames, some fixity at the base can be added with a rotational spring. But I would not count on only the footing connection for stability unless it's not subject to any serious lateral loads.
RE: Isolated Footings - Fixed or Pinned
RE: Isolated Footings - Fixed or Pinned
I have used fixed columns on footings only when I am limited in what I can do for the bracing. For instance, if I were to be designing a carport , I can't throw a brace at the end because the client wants it to be an open frame. Sometimes a moment connection at the top with pinned bases is not enough to resist the lateral movement. In that case, fixing the base is the best option for the client to maintain the look they want.
As for designing a footing for a moment, there are several programs out there that will do this, but the calculation is not too terribly difficult to do by hand. P/A+M/S and P/A-M/S will be the pressures on either side of the footing. There is a little more to it involving the eccentricity of the footing with a minimum length based on the M/P ratio. There is a good example in the Structural Engineering Reference Manual by Alan Williams.
RE: Isolated Footings - Fixed or Pinned
Similar to Stenbrook:
- flag pole colums
- moment frames if I need it.
- renovation if I'm desperate.
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: Isolated Footings - Fixed or Pinned
A mat/raft footing would probably improve column base restraint and, in very general terms, also improve seismic redundancy. If one intentionally used the generic building columns for seismic resistance then, without question, they would need to be sized to accommodate the anticipated moments.
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: Isolated Footings - Fixed or Pinned
In special moment frames.. there is an R or Reduction of 8 of the base shear because of the flexibility of the structure.. if you make the foundation on rock stiff and the column totally fixed to the foundation with all longitudinal bars at full development and bent at bottom.. in your experience.. how much bigger you need to make the columns to resist the base shear? But at least the beams won't reach probable moments at the ends as when it's flexible structure.. what can you say about this?
Why does one need (in your observation) to go for fixed foundation in moments frame.. maybe because of the lack of bracing or shear wall (when the ground need no walls such as parking)?
RE: Isolated Footings - Fixed or Pinned
I would expect the columns to be smaller with fixed bases. There would be more base shear but, probably, lower maximum moment. Shear in columns is usually pretty easy to handle.
Fixed bases don't eliminate plastic hinging in beams. Special moment frames must form mechanisms which means eventual hinging at column bases and beam/column connections.
1) Drift control
2) Inability to create a convincing pin connection.
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: Isolated Footings - Fixed or Pinned
You were referring to the Vc and Vs of the column concrete shear capacity and stirrup rebar capacity in resisting base shear? And it uses the same formulas as beam shear capacity of Vc and Vs?
Since there is very big reserve of shear capacity in columns. If you have certain fixed column base and fixed big foundation on stiff soil/rock. Then why not increase the moment resistance of the columns by making it bigger and putting more rebars.. this would make it stronger in moment hence causing less rotations in the beam-column joint.
In other words. In your experience.. how big have you made the columns and foundations to create seismic resistance totally on the elastic limit (without yielding and plastic hinges forming)?
Since it is easy to create shear resistance in columns (by enlarging the concrete Vc shear capacity and putting more transverse ties (increase Vs).. what is the reason why you need to go for longer period and lesser base shear and making the frames more flexible. Why not make fixed foundation and fixed column bases and creating bigger columns that would resist bigger base shear and cause lesser rotations by increasing moment rebar capacity? (in other words, causing less period, more base shear and making frames more rigid) What is the cons in this method?
RE: Isolated Footings - Fixed or Pinned
Precisely.
Clients tend to frown upon unnecessarily ginormous columns. Besides, making your column huge still wouldn't eliminate the need for plastic hinging at the beam ends. The only way to get around the plastic hinging would be to accept a greatly reduced R value consistent with essentially elastic level earthquake forces.
I can't comment as I've never done this on a building located in a seismically active jurisdiction. Such a solution would be economically irresponsible, in my opinion, for any structures other than perhaps essential service facilities which are sometimes required to remain undamaged post-earthquake.
You don't need to go for a longer period, lesser base shear, and making the frames more flexible. Those are just a subset of the many parameters that a designer can play with to optimize cost while preserving safety and architectural intent. Consider that:
1) For very short buildings, it's pretty hard to get beyond the constant velocity plateau of the response spectrum whether you fix the column bases or not (i.e. little change in base shear) and;
2) For very tall buildings, the effect of base fixity on stiffness and period will be negligible (i.e. little change in base shear).
So it's really only intermediate buildings that might have their base shears appreciably affected by column base fixity. And even then, the impact is unlikely to be very great.
Why not cast buildings out of solid blocks of concrete and just core out little tunnels for folks to move around in? Again, the answer is economy. Imagine a parabolic curve pointing downwards that represents overall cost plotted as the ordinate and frame rigidity plotted as the abscissa. At one end, you've gone out of your way to make your structure super flexible... and it's very expensive. At the other end, you've gone out of your way to make your structure super rigid... and it's very expensive. Somewhere in between, cost is minimized. That's where we want to be.
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: Isolated Footings - Fixed or Pinned
How many storeys do you consider "very short buildings"? Does 3 storey fall under it?
But even if there is little change in base shear by fixing the column and foundation bases.. it still is helpful.. are you saying it is not helpful? I'm saying helpful because if there is less rotation in the column base.. there is less rotation in the column-beam joint.. hence lower moment and lower shear in the beams? Do you agree with this?
RE: Isolated Footings - Fixed or Pinned
It's tough to say. Like the response spectrum, it depends on the location, soil conditions, and occupancy. and a bunch of other factors too such as seismic mass. Also remember that moment frames are often sized for drift. Consequently, frame stiffness may well be the same regardless of whether or not the column bases are fixed.
I'm saying a) that it is helpful is some regards and not others and b) that it is just one of many factors that goes into optimizing a design.
In a seismic, plastic design context, I definitely do not agree with this. As I mentioned a couple of times above, your beams will develop plastic hinge level moments and shears no matter what you do with column/base/footing stiffness.
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: Isolated Footings - Fixed or Pinned
We know the reason the beams will develop plastic hinge moments is due to the energy of the seismic force being dissipated in the ductility of the frames (yielding for example) which is beyond the mere elastic capacity of the frames. But in braced frames, the plastic hinge will develop at much greater seismic energy because it's first absorbed by the braced frames.. so Im thinking if you can similarly make the frames stiff by fixing the column/base footing, it can have greater resistance. And plastic hinge can develop at greater seismic force (so instead of your structure failing at magnitude 6.. it fails at magnitude 7) by fixing the base (just like braced frames making the frames rigid). Do you agree with this now?
RE: Isolated Footings - Fixed or Pinned
Sort of. If you design yourself a moment frame with pinned column bases and then fix the bases without changing anything else, the following ought to be true which is in line with your thinking I believe:
1) The brace will be stiffer and will attract more seismic load.
2) Some of the seismic moment previously developed in the beam / column joints will redistributed to the fixed column base joints. Whether or not there is a net decrease in moment at the beam / column joint will depend on which effect dominates (#1 or #2). I would expect a net decrease in most scenarios.
3) The plastic hinge moment that needs to be developed at the beam / column joint will remain unchanged because it depends only on the cross section and material properties of the beam which also will remain unchanged.
4) The seismic load at which a full frame mechanism will be formed will be higher because mechanism formation now requires plastic hinge formation at the column bases as well as the beam / column joints.
Unfortunately, the list above does not reflect the normal design process wherein fixing the column bases would likely result in changes to beam and column sizes and difficulty in assessing the benefits / drawbacks of introducing base fixity.
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: Isolated Footings - Fixed or Pinned
Is this a typo. Do you mean "frame" instead of "brace"? Because brace frames are X shape.. and when you have brace frames, it can prevent rotation of the column-beam joint.. and hence you don't need moment frames when you have brace frames.. Unless you mean by "brace" is the columns fixed to the foundation?
ON an unrelated inquiry. I understand why you said only intermediate frame has significant effect with fixed column.. and very short building has much less effect because the base shear is *almost* constant. In other words, you mean very short building are very stiff.. hence there is minimal rotation of the column-beam joints.. so in seismic movement.. shear of the columns dominate more than plastic hinge formation at the beams? Because if it is so rigid.. less rotations.. and where there is less rotations of the column-beam joints.. less moments at the beams hence in strong seismic activity.. the columns would take more shearing load. Or do you mean beam plastic hinge would form in very short buildings only with very strong seismic movement that can overwhelm any advantage of fixed columns which has tiny positive effect compared to intermediate buildings where tiny seismic effect can make it drift and so column base fixity has more advantage?
RE: Isolated Footings - Fixed or Pinned
Yup, good catch.
Nope. Short buildings have short periods that often put them on the upper plateau of the response spectrum curve where it takes a pretty big change in stiffness to affect seismic load. That's all.
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: Isolated Footings - Fixed or Pinned
Soft storey is very common in very short building. For example. In many 2 storeys. The ground floor has longer columns and open space than the 2nd floor which has shorter columns and with infill wall. In seismic activity, the ground columns can fail as you see in many illustrations and pictures. So in soft storey. Fixing the base would be advantageous isn't it?
RE: Isolated Footings - Fixed or Pinned
That might be a clever way to even out he stiffness disparity.
It's a bit of a moot point in practice. All moment frames expected to respond to earthquakes in a ductile fashion should be detailed with fixed column bases. There really aren't any other ways that I know of the promote the development of the required hinging at the bottoms of the columns.
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: Isolated Footings - Fixed or Pinned
1. Isolates pad or spread footing
2. Combined footing
3. Raft or mat foundation
4. Pile group with pile cap
5. Individual pile
I generally prefer to design foundation by presuming that columns are fixed at the base. But my colleagues design by taking them as pinned, by their convention. I strongly believe that column base will not allow any type of rotation at the foundation base level. So, for the restrained degree of freedom I design foundation for PMM; so far as isolated or combined footing are being discussed.
RE: Isolated Footings - Fixed or Pinned
Please refer to the figure below.
You mentioned above that in few scenarios, there is no net decrease in the moments of the column-beam joints (even when the column base is fixed).. something about #1 where the frame can be stiffer and attract more seismic load. We know that increase in base shear just needs more tranverse ties in the columns.. so what specific scenerio do you mean where there is no net decrease in the moments at the column-beam joints? The figure above shows the moments decrease in the joint so please show how it can remain the same.. unless you mean the load above beams is increased due to increase member sizes or vertical components of seismic movement.. or what specific scenario are you referring to? Thank you.
RE: Isolated Footings - Fixed or Pinned
I shared my thoughts on this at the top. Obviously, most columns will have some degree of fixity at the foundation level and I doubt that anyone will deny you that. And what member you're looking at will dictate what assumptions are conservative for the design of that member.
You may be interested in this thread which delves a bit deeper into what it actually takes to "fix" a column base such that it could resist flexural tension. Link
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: Isolated Footings - Fixed or Pinned
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.