transfering moment from a steel column to a single foundation?

mats12 · Oct 10, 2017

I have a steel moment frame (beam welded to columns)as shown below.
I have designed this frame as fixed at supports. Then I have desing connection (base plate + anchors) between RC foundation and column.

If I consider my support reactions while Im designing/calculating foundation I get an enormous dimensions since a moment at the base of column (fixed support) is pretty big.
But if i make a 3d model in FEM software I get much smaller foundation (600 x 600 mm with depth of 900 mm) because I think it takes into account that foundations will rotate a bit (I have modeled soil as springs under foundation. I checked forces/moments in columns and it looks like the software calculated them as the supports were PINNED). In reality does that mean, that there will be some rotation of foundation which means I could consider my supports as pinned not fixed? Which model should I use since I get so much different results in terms of foundation dimensions?

WARose · Oct 10, 2017

But if i make a 3d model in FEM software I get much smaller foundation (600 x 600 mm with depth of 900 mm) because I think it takes into account that foundations will rotate a bit (I have modeled soil as springs under foundation. I checked forces/moments in columns and it looks like the software calculated them as the supports were PINNED). In reality does that mean, that there will be some rotation of foundation which means I could consider my supports as pinned not fixed? Which model should I use since I get so much different results in terms of foundation dimensions?

I think it would depend on how much confidence you have in the spring constants you are using. You being a geotech are in a unique position to evaluate that. If you are using ultra conservative constants (i.e. it can't be any less flexible than that)....then you might be able to justify it.

That being said, I'm not sure if I have ever used a flexible footing foundation model to justify cutting the moments down. In such a case (depending on the soil type) I'll model it both ways and run with the worse for each part.

You might be able to take the guess work out of it by making the base a pinned detail.

KootK · Oct 10, 2017

It's usually something of a judgement call. Either you make an attempt to model the true flexibility of the base assembly or you make some assumptions and run with it. I might do something like this:

- Design the superstructure as though the bases were pinned.

- Design the base plate structure for 50% of the moment that would occur assuming full fixity.

- Design the footings assuming pinned bases but throw in some top steel to the tune of 1/3 the bottom steel.

This is not an entirely conservative approach of course.

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.

n3jc · Oct 10, 2017

I got a lot of questions about this too - what happens if foundation has insufficient dimensions? Does that mean the part of foundation under compression moves - settles too much and foundation rotate? in this case, doesnt foundation press against soil (vertically)?
since foundation rotates, has that the same effect on steel frame as pinned supports?

Tomfh · Oct 10, 2017

A lot of people just say they're pinned and use a thin flexible baseplate.

DETstru · Oct 10, 2017

I have modeled soil as springs under foundation

Did you model these as two way springs or compression only? Remember the "connection" between footing and soil cannot transfer tension.

If you can, I would simplify the whole thing and design the frame with pinned bases.
Design your footings and base plates for the axial loads only. If you want to be somewhat conservative, design for some arbitrary moment in those footings.

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transfering moment from a steel column to a single foundation?

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