RC moment joint 1

[greznik91]

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)

 

[jayrod12]

Technically it still works, just not as efficiently.

There's some information in this thread regarding efficiencies of joints using different rebar orientations. It is a good place to start.

thread507-401855

 

[WARose]

You typically only see the diagonal bar with water retaining structures. In those cases, such detailing minimizes undesirable gaps and is more efficient as far as reinforcement goes. (It can also deal with shear across the joint.)

Usually congestion isn't an issue with it as long as there is proper detailing.

 

[greznik91]

Aha, thank you both for reply. I havent seen diagonal rebars in joints for RC frames in the field yet or maybe I havent notice...
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?

 

[WARose]

Since i have only ground floor, do you think I can go without diagonal rebars?

I would think so. (Since we are talking just resisting seismic forces.....and not water retention or some other environmental concern.)

 

[greznik91]

Another thing im asking is would you bend all rebars from column in to upper side of beam (A)or just the outer bars (B)?
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?

 

[WARose]

Typically you wind up running the beam reinforcing into the column....not the other way around. Usually it's less re-bar in the beam so that cuts down on the congestion.

 

[mats12]

I kinda doubt that a joint without a diagonal rebar is a good way to go... I haven't done that kind of projet myself yet (RC FRAME), but can anyone confirm this is an OK way to do this? There must be some guys here who have been doing this kind of detailing/reinforcing for some time.

Any photo from the field would be interesting. Google didnt provide much for me.

 

[KootK]

This is what ACI recommends for these joints. No diagonals, standard hooks turned into the joint, and hairpins from the top. It's a good blend of performance and constructability.

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.

 

[greznik91]

Kootk TNX for this detail. However I havent decided yet what to do with my rebars in column since I have 8 of them. In a picture you posted there is some kind of hook at the top but since I have beams from 2 directions (one perpendicular to other) it gets confusing. I cant believe I havent found a single good photo of this example on internet yet.

 

[hokie66]

Maybe you need less bars or larger members. 300 x 300, and both column and beam the same size, makes for entirely too much congestion. Consider constructability in your designs.

 

[WARose]

[blue](greznik91)[/blue]

However I havent decided yet what to do with my rebars in column since I have 8 of them. In a picture you posted there is some kind of hook at the top but since I have beams from 2 directions (one perpendicular to other) it gets confusing.

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).

 

[KootK]

However I havent decided yet what to do with my rebars in column since I have 8 of them.

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.

 

[greznik91]

thanks for elaboration.

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.

 

[hokie66]

It sounds like you need to deal with a better class of architects, ones that are practical.

 

[KootK]

Your criteria:

- 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.

 

[greznik91]

Kootk thank you!

 

[KootK]

You're most welcome grezik. I hope that it works out.

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.