Isolated Footings - Fixed or Pinned 1

[rushitbvm]

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

 

[KootK]

Do you agree with this now?

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.

 

[wilberz]

1) The brace will be stiffer and will attract more seismic load.

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?

 

[KootK]

Is this a typo.

Yup, good catch.

ON an unrelated inquiry. I understand why you said only intermediate frame has significant effect with fixed colum

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.

 

[wilberz]

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.

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?

 

[KootK]

So in soft storey. Fixing the base would be advantageous isn't it?

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.

 

[rushitbvm]

Whatever with the super structure it is. I want to be clear about the restraint we provide at the column base for different type of foundations.

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.

 

[wilberz]

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.

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.

 

[KootK]

I generally prefer to design foundation by presuming that columns are fixed at the base.

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.

 

[KootK]

@wilberz: our discussion has drifted pretty far from rushitbvm's original question. We may be in danger of hijacking a thread here. If you wish to continue the conversation, and I'd be happy to, consider posting your latest comments over to a new thread.

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.