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Precast column offset issue

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Tim_777

Tim_777

Structural
Mar 4, 2021
6
Due to architectural reason, the below column offset cannot be avoided. Can anyone please help me figure out how to analyze this?
1. What the connection is gonna be like since I cannot have dowel bars extended from bottom to top?
2. I have seen comps using the shared area only to check the compression. Is it necessary? I would expect the load transfer to the column below through the slab to its full area. What extra comps that I need to do here?

Thank you.

Screenshot_2021-03-04_190607_fgqmsp.png
 
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Looks like a full transfer design, using whatever appropriate program you have available.
You would have to carry out all the normal checks bending, shear, deflection etc. You would get a bit of a bonus for punching shear, given that you're partially bearing and inside the critical punching shear perimeter. Without knowing more about the load and the rest if the structural system, e.g. pt band beams vs for plate etc, I can't comment further.
It's a real shame you can force the architect to be sensible here. This will result in added cost to the structure and added risk that could otherwise be avoided.
 
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Thanks for your comments. Appreciate that.
Currently it is simply taken as transfer loads and the PT slab is design by others. So my main concern here is for the precast panel and connection design. ie. Do I design the column as normal one with N-M iteration using full area etc or is there anything else that I miss? What about the column-column connection? How do I arrange dowel bars since they cannot go from column below to above? Do I just simply connect to slab? I check the joint capacity in compression here?
 
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I can’t say I’f be terribly happy. How does your rebar continue through for vertical tie/robustness requirements alone??

Its a horrible detail and I would be putting in a call to the architect.
 
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Can you detail a offest head on the lower column? It is a pain for the precaster, but it is possible. I too would be talking to the arch and if you have a frugal owner I would tell them about the cost reality/problem with this. Those owners tell the arch to fix problem, whereas many archs have silly explanations why it must be that way (not wanting to change dwgs being the most common). I once had to detail an offset column from main to second due to a desk layout. It was the dumbest thing.
 
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So this is precast columns above and below with a cast in place, post-tensioned slab passing through?

Will there be dowels from the columns into the slab, even if most of the dowels will not pass from the low column into the high column?

Most of the difficult parts of this will be in the PT slab designer's court.

OP said:
I have seen comps using the shared area only to check the compression. Is it necessary?
Click to expand...

It is common, expeditious, and somewhat reasonable, to assume that the axial load is passed between the columns over their overlapping area in plan. That, after all, is bound to be the stiffest load path.

OP said:
I would expect the load transfer to the column below through the slab to its full area. What extra comps that I need to do here?
Click to expand...

I would not expect that unless there is s very thick slab or beam passing between columns and you've detailed the connection with strut and tie methodology. There's an obvious eccentricity to deal with here that's going to produce a moment in the connection and therefor, and eccentric load in the columns.
 
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I had a similar problem about 40 years back... about a fifteen storey building and they wanted the column on the second floor moved a couple of feet. I used the columns on the one or two floors above (don't recall now) to transfer the eccentricity, and the columns on these floors were increased in size to match the offset. Fortunately the column was the first one in front of the elevator shaft and the large horizontal forces were directly tied to the shaft. I was taking a look in the area about a year ago (my son's lawyer works in the building and I went there with him) and no distress, so seems to have worked OK.

The floor system was a flat slab with drop panels and the drop panel was increased in size, but not in depth... I recall that. The one column depth may have been 4' or 5' and concrete strength was very high (maybe 8Ksi, which was high at the time for me, and when the forms were stripped, you could feel the heat.)
Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
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Thank you everyone.

Yes, precast panel with PT slab.

Something like a drop panel? And what for if so?

Does that mean the dowel can run through slab from column below to column above?

I have no clue about the connection and hence my post.
That said, what if the column is totally offset as below. We normally just provide dowel bars to slab right? Can we do something similar here. ie. instead of running the dowels from column below to above, connect them to slab instead?

Screenshot_2021-03-05_093535_cjptpf.png
 
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How are you achieving vertical tie requirements with no continuity of vertical bars here?
 
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1
You have to provide the "PT slabs by others" a layout that works the way you want it to. You are responsible for the overall structure.​

2
Yes, you have to check for bearing compression effects on the overlapping parts. Any solution such as strut-tie to resist forces only works after the bearing works! That was one of the problems at Opal Towers.​

3
if you have no clue about it, you should be asking your supervising engineer.​
 
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Something like... for precast, you almost need a transfer beam at the low level. Depending on the loads, it could become substantial... I was transferring over a dozen floors above... and the horizontal forces are substantial.
image_e8v9te.png


Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
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Rapt, is there a report available on the cause of issues at Opal towers? Or was the discovery discussed here at one point? I found the initial thread but it’s all speculative. Would love to know more about it.
 
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I was thinking provide dowels to slab for both top and bottom panels. Like the sketch I showed above for columns that are totally not overlapped.
 
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And that's why I am here looking for opinions...
I will check the bearing at these local areas.
Thanks.
 
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Thank you dik. Are these precast and how do you arrange the dowels if so?
 
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I doubt that your dowels will work as either dowels (there is insufficient development) or to provide vertical tie requirements (they do not provide a vertical tie).
 
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This was CIP concrete... I'm thinking precast, without transfer beams would be very difficult... either that, or you have to substantially bear on the column below, and your horizontal reactions can become substantial... this was an oddball case... the building owner wanted the column translated several feet... and picking up over a dozen floors above. I did it when I was young, and before I learned that these things could be difficult.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
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Thanks rapt! Will check it out!

Tim - I really think you’re missing the vertical tie requirement here. This detail is inherently wrong.

Dik’s example still has some overlap of the column to allow continuity of tie bars. Your proposal does not, so fundamentally the architect’s proposal is nonsense in my view. Some architects need to be told NO!
 
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