Ky Factor for Moment Frames/Bi-Axial Base Plates

[CNCLLC]

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.

 

[JoshPlumSE]

Why use K factors at all?! Just set them to 1.0 per the AISC 13th edition.... It's actually a pretty good argument for using the new design procedures as calculating K values can be problematic at times.

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....

 

[paddingtongreen]

The rigid floor diaphragm does not affect your k values for the columns, only the column above and the beams framing in do that. It sounds to me as if you are doing well with a 3.0 for a moment frame.

Timing has a lot to do with the outcome of a rain dance.

 

[StructTaco]

How are you getting moments in two directions, do you have skewed lines of resistance, i.e. not orthogonal?

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.