Steel column - pinned supports

First thing to recognise is that your 'pinned' supports were never 100% moment free to begin with. This is a engineering slight of hand that is helpful for modelling but assuming zero moment can be dangerous. (Though much of the time there may be sufficient moment resistance even if it wasn't designed for it.)

mats12 said:

Since footings are not sufficient for fixed connection, how will this act in real life? Will footings rotate and the system will act as pinned supports?

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If the footings are generally not sufficient to resist the moment then yes, by your own assessment they will rotate if faced with design loads.

I've certainly seen it in the 'real world'. In a building I inspected, a large industrial portal frame building that I suspect was built and then 'engineering' showed evidence of footing rotation. The end walls of the portal did not have compression struts to transfer wind load to the lateral bracing, the best it had was the roof which wasn't enough. During a storm the roof purlins bent, the columns rotated and the large roller doors were blown out relieving some of the wind pressure.

Bent roofing members were evident, distinct cracking of the slab around the front 'pinned' columns was also evident. It was pretty clear there had been rotation in the footing.

In real life, there is no such thing as a perfectly pinned support. Likewise there is no such thing as a perfectly fixed support. You're always somewhere in between.

To determine how much moment gets into those frames, you could model the support as a rotational spring. What I've done in the past is look at a "unit" rotation of the footing and seeing what sort of pressure that creates on the footing (based on the soil's subgrade modulus). In this case, I assume the soil capable of resisting both tension and compression.... not reality. But, it should still give me a pretty good starting point for my rotational spring. Maybe it comes out a little closer to "fixed" than it should.

I personally don't like to look at footing / soil "failure" as a limiting factor in design. Yes, it can limit the maximum moments that can develop. But, I feel like I'm delving too far into the non-linear range and I'm not comfortable that I understand the consequences of relying on that type of damage. How ductile is it? Does it result in permanent deflection problems?

With pinned ends on a moment resisting frame you tend to have lateral shear forces at the column base that tend to rotate your foundation.

Look at it this way: your members are bigger because you designed them as pinned. So your system is stiffer as before. Base connection is not rigid, foundation can rotate... But still stiffer as calculated in the beggining. I would model my foundatin with like a slab with a small soil subgrade modulus. You will see how it behaves. Watch out for tension under foundation or run non-linear analysis with tension disabled.

In my view, your frame design and presumably its associated sways, are all based on a pinned base analysis. If the contractor makes these bases far stiffer than ‘pinned’ then fine..I don’t believe the base moments will ever mobilise because the frame doesn’t ‘need’ them to..

Hard to explain.. I think, if you’ve designed your pinned base frame correctly, your frame is stiff enough not to sway sufficiently to cause the base moments to come into effect.

Regardless, I don’t believe that there’s any such thing as a truly fixed base on springy soil..

In reality, everything in this world is somewhere in between pinned and fixed. You just pick one clear load path and stick with it.

mats12 said:

Since footings are not sufficient for fixed connection, how will this act in real life? Will footings rotate and the system will act as pinned supports?

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Footings will rotate but not enough to act as pinned supports. Footings need to be checked to ensure they are adequately sized and reinforced to resist a reasonable moment based on probable soil stiffness. This is not an exact analysis, so engineering judgment is required.


BA