Ordinary/intermediate moment frames in ACI 1

[WARose]

I’ve been forced into a situation where I’m probably going to have to use some Ordinary/intermediate moment frames in a seismic situation. As far as I can tell, the strong column-weak beam requirement doesn’t kick in until you get to a “special” moment frame for ACI. Is that how everyone else reads it? I wanted to be sure of it because I have the reverse. (I.e. weak column-strong beam.) I have rarely used ACI for this purpose over the years. Thanks.

 

[KootK]

How many stories will the frame be?

 

[WARose]

Just one. But to be more precise: some caissons are going to socket into bedrock and then go up (at heights varying from 25 to 50 feet) to have full moment attachments to some very big reinforced concrete beams/girders.

 

[KootK]

Well then you're in luck. Strong column / weak beam doesn't apply to one story moment frames. No multi-story yielding mechanisms to encourage and no alternative but to have a single story mechanism. So make your frame as special as you like and put your plastic hinges in your columns if you got 'em.

 

[WARose]

Thanks Kootk. I kind of envisioned the same thing as any other moment frame.....but in this case the hinges forming somewhere in the columns. I am a bit worried about the fact that it may be just one per column....but there are a bunch of them.

 

[KootK]

I don't think that you have a choice but to have two hinges in each column. No mechanism without 'em

 

[WARose]

Yep. I just wasn't too wild about having a hinge near that socket. (Next to that heavy girder is no problem.) Thanks again for your input.

 

[KootK]

Always a pleasure. Stiff grade beams connecting the piles would have some merit in this case.

 

[WARose]

Kootk, was thinking about this some more this weekend.....and reading on it too....and it appears another reason for the strong column-weak beam approach is the P-Delta effects.


The proportioning and detailing requirements of AISC 341 are
intended to provide ductile inelastic response. The primary
goals are as follows: (1) achieve a strong-column/weak-beam
condition that distributes inelastic response over several
stories; (2) avoid P-delta instability under gravity loads and
anticipated lateral seismic drifts; and (3) incorporate details that
enable ductile flexural response in yielding regions.

https://www.nehrp.gov/pdf/nistgcr9-917-3.pdf

Granted that is for steel.....but I would think similar concerns would be there for concrete. If I did a P-Delta check (as per the dynamic amplification factor) and the impact was minimal on overall strength.....do you think that would satisfy that concern?

 

[KootK]

...do you think that would satisfy that concern?

I certainly do. Having a significant number of floor levels participate in the inelastic drift of the structure is the mechanism by which P-delta problems are addressed. Since you've got 100% of your floor levels participating, you're all set. It's a single story mechanism but that's only a problem if there are multiple stories available to pancake down on the unfortunate one absorbing all the inelastic drift.

 

[WARose]

Thanks again Kootk.