RC moment joint
RC moment joint
(OP)
I have studied some moment RC joints. I m reinforcing concrete moment frame (3D). My problem is that beams and columns are both 300/300 mm (small dimensions when rebars from column, beam and another beam perpendicular to other beam comes/meets together). Since im dealing with a moment frame (earthquake) every column - beam joint is an opening and also closing joints (it depends on direction of lateral forces).
Without red diagonal rebar the joint doesnt work when it comes to opening moment right?
is there any other options on how to reinforce this without diagonal rebar by using rebars from beam/column?
(there are just too many rebaras in small joints already)
Without red diagonal rebar the joint doesnt work when it comes to opening moment right?
is there any other options on how to reinforce this without diagonal rebar by using rebars from beam/column?
(there are just too many rebaras in small joints already)






RE: RC moment joint
There's some information in this thread regarding efficiencies of joints using different rebar orientations. It is a good place to start.
thread507-401855: Retaining Wall - Flexural Reinforcement from Stem Into Footing
RE: RC moment joint
Usually congestion isn't an issue with it as long as there is proper detailing.
RE: RC moment joint
So in this case i think detailing of joints also depends of scale of forces. Since i have only ground floor, do you think I can go without diagonal rebars?
RE: RC moment joint
I would think so. (Since we are talking just resisting seismic forces.....and not water retention or some other environmental concern.)
RE: RC moment joint
since I have so many rebars in column i dont know what to do with all of them.
Another issue is that i have beams connecting to a column from 2 direction. So what is the best case with bending columns rebars in case of so many bars?
RE: RC moment joint
RE: RC moment joint
Any photo from the field would be interesting. Google didnt provide much for me.
RE: RC moment joint
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.
RE: RC moment joint
RE: RC moment joint
RE: RC moment joint
It depends on the forces at the joint: if you don't need a lot of capacity, you might be able to just discontinue the vertical bars in the column (a few inches away from the end of it) with no hooks. If you need the capacity there (aside from hooks), another option is hairpins. In your case (with 8 vertical bars), you could have 4 hairpins (i.e. U shaped bars) and lap them with the vertical reinforcing away from the joint (i.e. down the columns).
RE: RC moment joint
In my market, I consider my options to really be quite limited:
1) Truly design the joint rigorously as opposed to simply detailing it. Strut and tie etc. To be honest, as important as these connections are, I've yet to actually see one "designed" in practice by anybody but me. And I only tried it once. The result was terrible and it took me a ridiculously long time. I won't be doing that again any time soon.
2) Follow suggested ACI detailing pretty much to the letter and execute the nominal checks that they recommend. I consider this situation to be a bit delicate in that, in several ways, the suggested ACI detailing really does not satisfy the moment transfer load path requirements in the sense that was discussed in that referenced thread on retaining wall joints. The outside bars are not continuous around the corner and there's a difference between the inside bars being "developed" and being "anchored". Since the whole thing doesn't really check out in a rigorous way, I find it difficult to know where I can take liberties and where I can't.
So I straight up follow the ACI detailing with two exceptions:
1) Where congestion issues make it sensible, I'll substitute headed bar anchors for the standard hooks.
2) If I'm truly confident that there will be no opening moment on the joint, I'll omit the hooks on the inside flexural bars.
As hokie intimated, you're joint is pretty small for it to be able to develop beam plastic hinges in both directions while maintaining strong column - weak beam requirements. 300 x 300 is no more than the size of a pair of my shoes side by side. Heck, once you factor in bar layering issues, your flexural depths may be as little as 9". If you can't make something like the stock ACI detailing work, tread carefully.
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.
RE: RC moment joint
arhitects dont like it when you chose columns more than 300 mm wide. they also dont like columns and beams to have different width. But once you start reinforce things you see how little space there is.
Since its a one storey structure, the plastic hinge can be developed in beams or at the top of columns (Eurocode 8). So there is no need for 'strong' columns or better said for columns that are stronger than beams.
RE: RC moment joint
RE: RC moment joint
- 300 x 300 column
- Column width = beam width.
- Strong beam : weak column.
Seems to me the answer is to use deeper beams so that bar layering can be accommodated without significantly compromising strength. I feel that the detail below would be fairly contructable and more or less consistent with conventional detailing practice.
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.
RE: RC moment joint
RE: RC moment joint
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.