Bending moment released & fixed Columns 1

[eit09]

I usually model my columns as one end bending moment released and the other end fixed. The fixed end is the top of the column where at least one beam frames in at each orthogonal direction of the column. The bending moment released end is at the bottom of my column which the column is welded to the base plate and four anchor bolts attached to the base plate by nuts and washers ( ie. no stiffeners). Is this how others model their columns? I just started thinking that at the bottom of the column would be somewhere between fixed and bending moment released so I have not been conservative in previous designs neglecting some moments. I appreciate any comments or feedback on how others model columns!

 

[msquared48]

That depends on the connection typ you have.

I will fix the base too to try to control lateral deflections better with less steel if I can justify the fixity through the foundation.

If the column is not part of a moment frame, then there is no need to fix the base. Just leave it broken.

Mike McCann
MMC Engineering

 

[JAE]

If we don't purposely intend to have moment connections in our beams, we assume that the column is pin-pin.

 

[eit09]

JAE,

If you assume the column pin-pin you would get instabilities. Wouldn't you consider the top of your column fixed if you have a beam framing into it in both directions?

 

[JoshPlumSE]

There is a lot of personal preference with how to model the columns. But, this is how I tend to do it.

At the Base of the moment frame column:
I will tend to use fully fixed end releases at the base and then use the boundary conditions to dictate what forces are capable of being transferred into the foundation.

At the Base of a braced frame column:
I'll model the boundary condition as fixed and then use the end releases of the column and brace to dictate what forces the column and brace can transfer into the foundation.

At the top of a column:
I will almost always model the end releases as fully fixed. Then I can set the end releases of the beams and such that frame into the top of the column to dictate what forces can be transmitted between the members.

Josh

 

[JAE]

eit09 - I'm assuming that the overall frame is braced by other means. Pin-pin on the column top would mean all moments are released but lateral translation is resisted.