Column bases

[SteelPE]

I usually try avoiding fixing the bases of columns in my buildings; however, I am faced with a project where it may be unavoidable. The building in question is a 3 story office building and has a brick/masonry façade and I need to limit the horizontal drift of the first story. The limit of the drift has to do with cracking of the façade and perception of the tenants (H/400).

When I run moment frames I have been taught to use 10% of the fully fixed case for “pinned connections or 0.4EI/L (a 10% fixity). Now I am looking to get more stiffness out of the bases. I know this has been a hot topic on this forum from time to time. You just can’t go into your programs and say that the bases of the columns are fixed because that is impossible. I am wondering if I use 4EI/L for my fixity of my base or say use 90% of this (0.9)*4EI/L if this would be more realistic.

Faced with similar circumstances I am wondering what others do (how they approach their computer models)?

As a side not, while I care about the foundations, I am not concerned that my foundations will be larger as that is a price that needs to be paid for the insufficient bracing the client has allowed.

 

[KootK]

You just can’t go into your programs and say that the bases of the columns are fixed because that is impossible.

Actually, that is what I do most of the time. It's an approximation, like everything. else. If I'm very concerned about underestimating drift, I'll use the method in the PCI manual for estimating the rotational stiffness of the foundations.

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.

 

[sandman21]

Model the grade beam to account for rotations in the beam.

 

[SteelPE]

KootK

I am aware of the PCA method. Seems like a reasonable approach, but I usually try to avoid it. Most of my reasoning has to do with this forum and everyone saying you should tell the program the base is fixed. In my instance my frames would be going around the perimeter of the building and have 4' deep foundation walls connecting the piers/footing together, so I am guessing that system is going to be quite stiff but the time we are done (although I design the footing as if it is an isolated footing 4' deep).

I am thinking I can go back into my notes and figure what stiffness the PCA method would give and see if that is enoug to "stiffen things up".

 

[Deker]

I agree with sandman. If you want to rely on base fixity you should provide grade beams between the columns and model them explicitly. Make sure you reduce your moment of inertia to account for cracking. In your case, the 4' deep foundation walls should provide significant fixity.