Ky Factor for Moment Frames/Bi-Axial Base Plates
Ky Factor for Moment Frames/Bi-Axial Base Plates
(OP)
I have a two story moment frame that I am modeling and I do not agree with what the program is using for the Ky factor on the columns. The building has a rigid floor diaphragm and a flexible roof diaphragm and my program defaults are set to choose the K factors is the AISC nomographs.
If I were to pin the bases in the Y direction I am getting a Ky factor of 3.0 and my column sizes are being blown up. I understand how this is being calculated, but I do not agree with it, at least on the lower level, since the frame should be braced by the floor diaphragm.
If I were to fix the bases in the Y direction I am getting a Ky factor on the order of 2.0, which is more reasonable, however, there is no clear cut way of designing bi-axial base plates.
I would actually prefer to fix the base plates, so I was curious to know if anybody has a method of designing a bi-axial base plate that I can incorporate into a spread sheet, since there are quite a few load combinations. Otherwise, would it be reasonable to design as a pinned base and force a Ky into something less then 3.0.
Any opinion would be appreciated.
Thanks in advance.
If I were to pin the bases in the Y direction I am getting a Ky factor of 3.0 and my column sizes are being blown up. I understand how this is being calculated, but I do not agree with it, at least on the lower level, since the frame should be braced by the floor diaphragm.
If I were to fix the bases in the Y direction I am getting a Ky factor on the order of 2.0, which is more reasonable, however, there is no clear cut way of designing bi-axial base plates.
I would actually prefer to fix the base plates, so I was curious to know if anybody has a method of designing a bi-axial base plate that I can incorporate into a spread sheet, since there are quite a few load combinations. Otherwise, would it be reasonable to design as a pinned base and force a Ky into something less then 3.0.
Any opinion would be appreciated.
Thanks in advance.






RE: Ky Factor for Moment Frames/Bi-Axial Base Plates
That requires, of course, that you comply with other analysis requirements (P-Delta and such). But, those provisions aren't so tough.
Many programs will account for most of those new requirments automatically. I know RISA does.... :)
RE: Ky Factor for Moment Frames/Bi-Axial Base Plates
Timing has a lot to do with the outcome of a rain dance.
RE: Ky Factor for Moment Frames/Bi-Axial Base Plates
If you fix the base, be careful, the moments and subsequent tensions in the anchor bolts get very large by doing that, and with ACI 318 App. D, everything gets bigger (anchor bolts, base plate, concrete that you are trying to anchor the bolts into)...but your deflections/drifts will be a lot lower.