Column base plate for moment frame

[bruinboy]

Hi,
I'm trying to design the column base plate for a residential moment frame. The foundation is an 8" by 29" grade beam with 3-#5's top and bottom on piers. The column is subjected to a 13 kip axial force and a 17 kip moment which seems to have a pretty high eccentricity of 15 inches (this is from a lateral EQ load using a 1.0 factor). I'm using an example from page 22 of the Errata for AISC Design Guide 1: Column Base Plates that I downloaded for free off the internet. Unfortunately, I couldn't get access to the whole thing. My question is, to find the length "A" in that example, what do the different numbers in that equation refer to (specifically the 4(2.1 x 14)/6 and also the (2.1 x 14)/3))? I'm getting some wierd numbers for my tension force. If increasing the size of the grade beam width is not an option, is shaping the base plate into an upside down "u" and bolting through the grade beam acceptable? Problem with that is there still may not be enough concrete area for bearing. Does everyone use UBC Table 19-D for the bolt strengths? Thanks for all the help.

 

[Ron]

Something you need to check before you get to the baseplate stage is the direction of that moment. If it is in the lateral plane of that grade beam, you have a torsion problem in the grade beam. If longitudinal, perhaps OK but you need to check.

 

[bruinboy]

Moment is in the same direction as the continuous grade beam runs. No torsion involved.

 

[Lutfi]

First, you do not have to fix the base of the moment frame. It can be simply pinned!

Second, I fix the frame base only if the column size needs to be reduced.

Regards,

 

[bruinboy]

Lutfi,
So you are saying to just model the frame as pinned instead of fixed and I'll get a larger axial but no moment to deal with? Am I understanding this right?

 

[dawn836]

yes i think that is what Lutfi means and i agree with him totaly.
fixed base in steel results in much work for fabrication and detailing and unaccepted shape and may be at last you do not find much save in cost.
you better using bracing system for lateral load resistance

 

[bruinboy]

Dawn,
I can't use a braced system as a large window is in the way. But I can model an ordinary moment frame as pinned instead of fixed at the base?

 

[bruinboy]

Hey everyone,
Thanks for the help...I pinned the bases and now have no moment to deal with.

 

[apsix]

Of course the sway deflections will increase substantially with pinned bases if the frame is resisting horizontal forces. This should be checked as part of the design.

 

[Lutfi]

Yes bruinboy and thanks for the response dawn836. I run into this a lot in PEMB and steel frames. I would use simple 2-D model (I like STAAD and ETABS) and run the lateral load at top of the frame.

Good luck