Composite Beams with Ordinary Moment Frame 3

[EngineeringEric]

Working on a project with an 80ft clear span building that is supporting heavy upper floors with concrete deck and limited room for structures and windows all around. Great!

So looking at a composite girder lines spaced 15/20/25ft apart with infill beams. The girders that span 80ft will need to be a moment frame (R=3). Seismic may govern due to the weight vs projected area per line.

I can size the girders for gravity as pin-pin. I was then thinking of making them fix-fix for a lateral design. the connection would be sized for the maximum shear/moment of the envelope of reactions. To detail and evaluate the connection i am currently unsure.

Question:
1) would you do something else?
2) would you ever make the moment connection after/post composite action? so we don't load the column with moment due to deflection and loading of the girder. the only moment transfer is live and wind.
3) does this require a flexible/partial moment connection as opposed to a standard full moment so my girder never sees negative moment?
3a) pending the amount of moment from DL only, would a full moment after DL is engaged work?

Any references to AISC or industry references would be appreciated, especially if they answer the above questions to save you time.

 

[KootK]

Thank you so much. You've saved me from a bucket load of embarrassment.

Well, if I did that I did it entirely by accident but, regardless, you're most welcome.

So what I found out is that, even without any end release, beam joints are not really stiff enough to act like a frame for a lateral load.

Can you not improve this issue by altering the connection type rather than the beam size? Pass through diaphragm plates or something? I know that HSS moment connection are extensively used in Japan for high seismic situations. They seem to have it figured out somehow.

(M=800 kN-m at top, M=2000 kN-m at support). I don't know is this enough to justify using an R = 5.5 for IMRF?

To my knowledge, you don't need any particular moments to justify the use of an IMRF. Rather, you just make the decision and provide the necessary ductility detailing. That said, it may take a certain level of demand to justify the cost of switching to an IMRF. I'm afraid that I don't know enough about the project to help you on that front however.

It can sometimes be difficult to integrate a hard working gravity beam into a high ductility moment frame. Firstly, you need to ensure that hinges don't form in the middle of the beams. Secondly, the very beam stiffness that you need to provide for successful gravity load serviceability design forces you to deal with high plastic hinge moments at the beam column joint. I'm not suggesting that any of this can't be done; only that you sometimes start to feel as though you're chasing your tail.

P.S. Should I make a separate post for this problem? I know I'm stealing the limelight here from EngineeringEric questions.

I am 100% willing to follow Eric's lead on this. I almost didn't respond to Blackstar123's question about his own project for that reason. That said, the questions are somewhat related and I hate to leave someone's question unanswered. 'Twas the lesser of two evils to respond I felt.

 

[Blackstar123]

I don't mind you expanding this thread, but it may help to make a new one and link them... but also I don't know (Edit: feel free to expand the knowledge in this thread!!!)

Thank you for being such a good sport.

I am 100% willing to follow Eric's lead on this. I almost didn't respond to Blackstar123's question about his own project for that reason. That said, the questions are somewhat related and I hate to leave someone's question unanswered. 'Twas the lesser of two evils to respond I felt.

I had no idea I have put some people into a moral dilemma. I sincerely apologize and now that I know, I am doubly appreciative of the help you provided.
At first, I was mostly confused about the fuss with pinned-pinned and fixed-fixed moment connection for before and after composite behavior. Since most textbooks only covers simply supported composite beam, I started to wonder if there is some prohibition of negative moment connection in composite construction, which I am ignorant of? I still do not perceive the intended behavior Eric is trying to achieve by doing so.

Well, if I did that I did it entirely by accident but, regardless, you're most welcome.

You helped by putting a nagging doubt in my brain (and might I add, on my day off) about the joints undergoing plasticity under gravity loads and that, my lateral system might not be behaving as I originally designed it for.
This is the point where Eric’s and my question have changed paths by 90 degrees. I read somewhere in this site that discussion should be relatable to the post and it starting to feel like I’m committing some sort of crime by further discussing my problem in this post.

Can you not improve this issue by altering the connection type rather than the beam size? Pass through diaphragm plates or something?

I was thinking along the same line, but instead of diaphragm plate, I was thinking of a hunched type connection since it has the added benefit of moving the plastic hinge further away from the joint. I’ll explore merits of both type of connection.
Another option I’m contemplating, but which I’m not thrilled to use, is to model the composite action of slab by using shell joint offset. So far, I’ve not considered it in the analysis due to the reason that concrete will be cracked near supports at ultimate loads. But this has become an “all hands on deck situation” for me. I’m willing to consider even a 25% stiffness provided by cracked slab. Now, I know what they mean when they say, “Ignorance is bliss”.

To my knowledge, you don't need any particular moments to justify the use of an IMRF. Rather, you just make the decision and provide the necessary ductility detailing. That said, it may take a certain level of demand to justify the cost of switching to an IMRF.

Yes, that’s what I’m thinking. What use is a ductile joint if it will not attract any moment?

Secondly, the very beam stiffness that you need to provide for successful gravity load serviceability design forces you to deal with high plastic hinge moments at the beam column joint. I'm not suggesting that any of this can't be done; only that you sometimes start to feel as though you're chasing your tail.

Exactly.

Are your columns extending to upper levels as well?
Can you increase their stiffness or make them W-sections?
I'm assuming you also don't have adjacent structures to utilize

I'm afraid that I don't know enough about the project to help you on that front however.

It’s a fairly simple structure (see attached image)

Some heavy equipment’s will be placed on top of slab. 20m span is provided to facilitate vehicles movement and other plant operations below the slab. This is why I can’t provide any columns or bracings in the bay width.
Along the length of building, structure is behaving like a perfect moment frame.
Along the width, it’s behaving more like a cantilever column than a moment frame. Drift is not an issue. Even for a cantilever behavior, it is within the allowable limit. Problem is, moment in columns and foundations will be highly increased if I use a R = 2.2 (for cantilever columns).

 

[Blackstar123]

For what it's worth, I've made a query related to this topic at this Link