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Reinforced Concrete Beam-Column Moment Joint Design & Detailing 1

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abusementpark

abusementpark

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Dec 23, 2007
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I am using RAM Concrete to design two stories of conventionally reinforced concrete beam and column frames. The beams are 2'-0" deep and range in width from 2'-0" to 3'-10". For the analysis in RAM Concrete, we are considering almost everything fixed at the joints, as this seems most realistic and allowing the columns to take some moment really helps with the beams design and distribution of load. RAM Concrete is showing some pretty significant moment transfer occurring between the beams and the columns, especially at the top floor where there is a lot of load. Can anyone provide some guidance for the primary design and detailing checks that should be done to ensure this transfer can occur at various conditions? I should note that this is a low seismic zone (SDC B) and the beam and columns are not part of the LFRS (we have shear walls for that).

For the beam over column condition at the upper floor, I know I need to hook the column bars and check their development length into the beam. Do column ties need to continue into the beam? Similarly, I assume beam bears need to be hooked and developed at any end joint where a beam terminates?
 
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1. Is the high moment at beam-column joint caused by wind load case? You have to identify the source of problem. Does it causing design difficulties?
2. Does the beam depth is adequate to develop the column bars without hook? You may not require to hook the vertical bars into beam. Though many engineers prefer that way, I don't, especially for interior columns.
3. No, stop the tie just below the beam.
4. It seems ACI has such requirement, but I am not sure it is required for structure with low seismic concerns.
 
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You want to take a look at ACI 352R

In short, my practice in a non-seismic region is to develop both beam and column bars, generally maintain good joint geometry (or take a closer look), and I do ask for two column ties within my joint core region.

For the top of a multistory column, I'll usually lap the column bar with a small diameter hooked bar (instead of bending the column bar itself) to make bar placement easier.

----
just call me Lo.
 
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This is a topic of particular interest to me as I've struggled with it in the past with respect to how codes treat things. Hopefully you're game for some theory in addition to just the raw "what should I do" answer.

abusementpark said:
For the beam over column condition at the upper floor...
Click to expand...

1) By upper floor, you mean roof / topmost level, right? I"ll assume so.

2) Agree 100% on the utility of ACI352 for this as well as Lo's proposed corner bar detailing at the roof level.

abusementpark said:
Similarly, I assume beam bears need to be hooked and developed at any end joint where a beam terminates?
Click to expand...

3) The beam bars do have to be developed within the joint but recognize that, from a theoretical standpoint, development really only gets part of the job done. What you need to do the whole job is:

a) Development and;

b) Anchorage into the joint and the ability to transfer flexural tension from the beam bars around the corner and into the column bars / compression block.The codes, and good designers, generally provide one of two ways:

c) Some kind of lap between the beam bars and the column bars at the the uppermost level of which Lo's solution is one permutation (and a very constructable one) OR;

d) Development combined with the handy clamping mechanism available at lower level joints and shown below.

Because the code often lets you make do with clamping + development at the lower levels, I feel that it kind of creates the erroneous impression that development is all that is required.

4) There can be a few good reasons to run the column ties through the joints, particularly where general confinement and/or confinement of hooked ends is an issue. I favor typical detailing that shows the ties running through at exterior joints, moment frame joints, and in high seismic scenarios. Where beams are deep relative to the columns coming into a joint, as shown below, the simple mechanics of controlling the diagonal strut angle will produce a need for such ties, irrespective of confinement concerns.

C01_sb0p26.jpg


c02_g4cryr.jpg
 
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KootK said:
d) Development combined with the handy clamping mechanism available at lower level joints and shown below.
Click to expand...
Out of two options(c and d) given by you for clamping mechanism additional to development, option D is only applicable for lower story columns(in multistory construction) and the designer is only left with option C for roof floor columns right?
But I have not seen such detailing in general where I work and they just bend the roof column bars inside beams/slabs for a length of 300mm or so.
How would STM at roof column and slab joint looks like(just without the strut of the upper column?)?
 
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MSUK90 said:
option D is only applicable for lower story columns(in multistory construction) and the designer is only left with option C for roof floor columns right?
Click to expand...

Right.

MSUK90 said:
How would STM at roof column and slab joint looks like(just without the strut of the upper column?)?
Click to expand...

Right again which, of course, causes one to raise an eyebrow at this...

MSUK90 said:
But I have not seen such detailing in general where I work and they just bend the roof column bars inside beams/slabs for a length of 300mm or so.
Click to expand...

C01_cssahi.jpg
 
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Not sure if this is relevant, but wouldn't you also need to hook the bottom bars (or otherwise develop them within the joint) to deal with bending in the opposite direction? That could come with some additional congestion issues. If your frames are part of the LFRS you may need to take this into consideration.
 
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retired13 said:
1. Is the high moment at beam-column joint caused by wind load case? You have to identify the source of problem. Does it causing design difficulties?
2. Does the beam depth is adequate to develop the column bars without hook? You may not require to hook the vertical bars into beam. Though many engineers prefer that way, I don't, especially for interior columns.
Click to expand...

1. It's basically only caused by dead load and live load cases.
2. The column bar sizes range from #7 to #10 and beam depth is typically 24 inches. The column bars will definitely have to be hooked to develop the full strength.
 
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KootK said:
1) By upper floor, you mean roof / topmost level, right? I"ll assume so.
Click to expand...

Correct.

KootK said:
c) Some kind of lap between the beam bars and the column bars at the the uppermost level of which Lo's solution is one permutation (and a very constructable one)
Click to expand...

How would this would work at an interior beam over column joint at the top level? The beam beams would be continuous across the joint and the column bars would extend up to just under the beam top bars with a standard 90 degree hook.
 
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By the book, I believe that it should be something like this if the intent is to share unbalanced moment with the column. That said, I fully acknowledge that the detailing that you've described is the most prevalent in the markets in which I have practiced.

C01_a9b6n7.jpg
 
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