Pinned or fixed foundation on a 3 story building. 2

[smcp4blo]

Hello,

This question may sound a little bit odd (for not saying silly), but I had a discussion with a colleague and I would like to have other opinions. I'm designing a 3 level steel building with HSS columns and W beams. Since the land is not very good I made the decision of modeling it in Staad with a pinned base, and so designing the foundation with only axial and shear forces. However, my colleague (who has more experience) says that buildings can't be modeled as Pinned in their bases for buildings more than 3 levels. I find this wrong, as for my structural criteria states that I can model a building as pinned since it isn't unstable, and the only difference with a fixed model would be that my displacements are greater and so the stiffness of my columns must be greater. Right?

So, pinned = less foundation, bigger columns.
Fixed = more foundation, smaller columns.

Anyway, my colleague refuses to accept this design as acceptable, and he tells me that according to his experience he has never seen a building constructed as pinned foundation. However he hasn't presented 'technical' data to support this, just his experience.

Can I have any opinions?

Thank you!

 

[Archie264]

The problem with modeling something as having fixed bases is that it is difficult to achieve this in reality without going to the intentional (sometimes heroic) effort of doing so. So it can unfortunately become an easy way out for the lazy or misinformed. It chases the theoretical forces into the ground leaving little requirement that the structure be otherwise braced for stability. It is almost akin to solving a problem by first stipulating no gravity. I've seen some very flimsy structures designed and built this way, and in some cases they blew down in the wind.

On occasion I'm asked to perform peer reviews of calculations by some entities that, unfortunately, seem to be specialists at modeling in this manner. In fact, sometimes the fixed bases are the least of their sins. So much so that now when I get the results of STAAD calculations emailed to me in a pdf file I now go through an three-step review process consisting of the following:

1. I hit <ctrl> F to electronically search the file for the word "fail". About 50% of the time it's in there, usually many times over. Yes, that's right, their own modeling output predicts that the structure will fail, and they are either too ignorant to realize this or are too unethical to care, trusting instead on the presumption that no one will read the reams of modeling output.

2. If it passes Step 1 I then check to see if the model is founded upon fixed bases (or, in some cases I've found, elements pinned to the sky). Pinned-to-the-sky gets rejected out of hand, of course, but fixed bases can be at least somewhat justifiable in some cases. But, I check if the drawings show any provisions for the base to be anchored in a way that might reasonably be argued could carry some significant percentage of moment. If not I ask them to either pin the bases or provide a moment-resisting base detail and resubmit. When this is done usually the next submission is of a redesigned model with pinned bases and some type of lateral bracing.

3. At this point I review the submission in earnest.

My two cents, for what it's worth.

 

[BAretired]

I don't think anyone can claim great precision in this regard.

In Alberta, foundations are frequently piles. Piles may be considered an additional frame member, laterally braced at the top and hinged at the bottom or whatever other support condition is deemed realistic. Soil, as an elastic foundation can also be included if desired although I typically ignore it, tending to make the pile more flexible for purposes of analysis than it actually is.

In this way, the column base plate may be designed for a moment found on a somewhat rational basis. The foundation is neither pinned nor fixed. But the moment found is not particularly exact.

For pad footings, it should be possible to develop a rotational spring representing the rotational resistance of the footing.

BA

 

[wannabeSE]

By the way, Chapter 19 of ASCE 7-05 provides a method to account for soil structure interaction in seismic design for analysis models with a fixed base.