Moment Frame Base Plate Calculation

[RM87]

Hello folks,

I modeled an ordinary steel moment frame with pinned bases and now I'm coming to you all with a question that has been gnawing at me for some time.
Should I design the moment frame base plate as the more conservative 'fixed' condition or can I stick with the 'pinned' base condition?

In addition, does anyone have any suggestions/theory/raw calculation procedures to help design a moment frame/base plate on a concrete column where concrete breakout (tension) is the governing restraint. I was hoping to weld the base plate to rebar and further confine the entire member with stirrups, instead of anchor bolts. Does anyone have any suggestions?

Do I need to be conscious of electrolysis or another corrosive aspect between the rebar and the baseplate?

Thank you all,
R

 

[KootK]

Similar discussion: Link

Do I need to be conscious of electrolysis or another corrosive aspect between the rebar and the baseplate?

Nope. You ought to be fine.

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.

 

[RM87]

Thank you very much KootK.

Do you have any suggestions on how I should design the base plate? Pinned values or go with the more conservative fixed values?

 

[DaveAtkins]

Definitely pinned. So--all you must do is design the base plate for the vertical and horizontal reactions (no moment).

DaveAtkins

 

[KootK]

I design for the fixed base moment. If I really dislike the answer, I'll either try to provide a ductile failure hierarchy in the connection so that it can eventually yield into a hinge or intentionally create base flexibility to limit the developed base moment. Neither of the last two methods can be done with much accuracy I'm afraid. Apparently, I'm Europe, they've developed something called the component method that supposedly facilitates expedient evaluation of base connection stiffness for use in accurate modelling. That may be the way of he future.

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.

 

[zackIX]

I would calculate the supports only for the N + V forces. No moments. You may apply a rotational stiffness to your support of 10% the column above.

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[RM87]

@Kootk - How would you try to ensure base plate flexibility? If I want to 'guarantee' pinned behavior, how would you detail that?
@Dave - Thank you for your input.
@Teix - Okay. You would assume that the assembly has an inherent 10% rotational stiffness?

 

[KootK]

@Kootk - How would you try to ensure base plate flexibility? If I want to 'guarantee' pinned behavior, how would you detail that?

Guaratee is a pretty strong word. The only way to guarantee pin like behaviour is to build a pin. And that's pretty impractical for most column base situations. Bridge folks do it more often.

One strategy is to ensure that the connection could absorb moments safely until a part of the connection yields and the connection effectively turns into a pin. This usually means designing all brittle failure modes like concrete breakout and crushing so that they will occur safely after at least one reliably ductile failure mode like anchor bolt or base plate yielding.

So small, well developed anchor bolts and thin, expansive base plates will promote base connection flexibility. Trouble is, you often need these same elements to resist significant axial tension as well which works against you. Some engineers will place the anchor bolt inside the column flanges under the premise that a smaller lever arm to the bolts means more pin like behaviour. I disagree and think that a smaller lever arm usually just means that you'll just initiate a brittle anchor breakout failure or concrete crushing failure sooner in the load history.


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.

 

[msquared48]

A pinned assumption is more conservative for the frame, but not for the baseplate, anchor bolts and foundation.

Mike McCann, PE, SE (WA)

 

[KootK]

Here's a document on the component method that a friend here recently shared with me: Link. It looks to be pretty relevant to this discussion.

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.

 

[RM87]

Thanks folks for the help.