Reinforced Concrete Column Transition Transmission of Axial Force and Bearing Surfaces
Reinforced Concrete Column Transition Transmission of Axial Force and Bearing Surfaces
(OP)
Hi all,
I'm a graduate with 2 years experience, however the job I'm currently on is very unique (from what I've experienced), with many unconventional things due to a bazaar architect.
I'm having an issue understanding the methodology behind column transitions, where a column under may have a large change in size and geometry to the column over.
Because of this transition, I'm struggling to satisfy AS3600 Clause 12.6 - Bearing Surfaces and Clause 10.8 - Transmission of Axial Force.
The situation:
600 Dia. column concentrically over a 300 x 1500 column, with a 900 Deep transfer beam in between. The Column concrete strength, f'cc = 65MPa, the slab f'cs = 40MPa. N* = 10,000kN.
My manager is telling me that due to the deep beam which provides restraint, transmission of the axial force will be fine, however by my numbers, the axial load can't pass through the joint, and there's too much bearing stress; it doesn't satisfy CL10.8 or 12.6. I can somewhat understand the reasoning behind my managers argument, but we have opted to run column ties through the beam to satisfy MY own concerns (which makes me more concerned in fact; is my manager confident in their justification?). I thought strut-tie might be a solution, however I've been turned down as we don't want cracking to occur before any tension ties can be engaged. Maybe my manager is correct, but I would like a calculated proof of concept to satisfy myself, as this is the most basic of several transitions present in this project.
If someone could give me a clear explanation of these Clauses and how to apply them, and whats the best direction to take in this situation.
Please see the attached Document.
I'm a graduate with 2 years experience, however the job I'm currently on is very unique (from what I've experienced), with many unconventional things due to a bazaar architect.
I'm having an issue understanding the methodology behind column transitions, where a column under may have a large change in size and geometry to the column over.
Because of this transition, I'm struggling to satisfy AS3600 Clause 12.6 - Bearing Surfaces and Clause 10.8 - Transmission of Axial Force.
The situation:
600 Dia. column concentrically over a 300 x 1500 column, with a 900 Deep transfer beam in between. The Column concrete strength, f'cc = 65MPa, the slab f'cs = 40MPa. N* = 10,000kN.
My manager is telling me that due to the deep beam which provides restraint, transmission of the axial force will be fine, however by my numbers, the axial load can't pass through the joint, and there's too much bearing stress; it doesn't satisfy CL10.8 or 12.6. I can somewhat understand the reasoning behind my managers argument, but we have opted to run column ties through the beam to satisfy MY own concerns (which makes me more concerned in fact; is my manager confident in their justification?). I thought strut-tie might be a solution, however I've been turned down as we don't want cracking to occur before any tension ties can be engaged. Maybe my manager is correct, but I would like a calculated proof of concept to satisfy myself, as this is the most basic of several transitions present in this project.
If someone could give me a clear explanation of these Clauses and how to apply them, and whats the best direction to take in this situation.
Please see the attached Document.






RE: Reinforced Concrete Column Transition Transmission of Axial Force and Bearing Surfaces
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: Reinforced Concrete Column Transition Transmission of Axial Force and Bearing Surfaces
RE: Reinforced Concrete Column Transition Transmission of Axial Force and Bearing Surfaces
sorry, the transfer beam is 3600w with this column scenario at the center.
I've talked to a few engineers here and have been told that my A1=A2 as the only area that is concentric AND geometrically similar is the overlapping area of each column, which is approximately 600x300. Checking the bearing at the surface of the column below, using this area, I get an allowable bearing force of 0.6x0.9x65x600x300=6300kN which is far below the N*=10,000. This seems to be due to the fact that the bearing area is so small, but I see no justification to increase it because of the A1_A2 clause. Perhaps I am misinterpreting that part?
For Clause 10.8, using the same overlap area (600x300) as the joint between the two high-strength concrete columns, I get an effective strength of f'ce=54MPa.
With 10N32 possibly passing through the joint from the column below (Column below has 22N32, column above 13N32), I get a phiNuo of 7100kN which again is still far less that N*=10,000kN. (I'm assuming that's what I am meant to calculate with the effective concrete strength).
This is the first time I've ever had to analyse and design something like this, and from asking some of the other engineers here, its very unique, and a poor situation for me to be learning on. I've been told that the best solution is "to have concentric column with similar geometry", but obviously that is not up to me.
Please, any further help would be great!
RE: Reinforced Concrete Column Transition Transmission of Axial Force and Bearing Surfaces
Regarding the connection itself, bearing between the upper and lower columns is not involved, You have a 900 deep transition and a width difference of only 900 on 2 sides in the worst direction, so the slope is only 30 degrees. Calculate it as strut tie and put in the reinforcement for it. And confinement reinforcement would not hurt as you have provided.
The logic that you do not want to use strut tie as the concrete has to crack before the tie steel is engaged is silly. Any tension steel in any slab or beam is basically doing nothing until the concrete cracks. This is no different.
And I do not see the need to criticize the architect on this one. It is perfectly normal and happens in many multi story buildings. Comparing him to a Middle Eastern marketplace is a little severe!!!!
RE: Reinforced Concrete Column Transition Transmission of Axial Force and Bearing Surfaces
Thanks for your input! you say 65MPa to 40MPa is too much.. but its still acceptable? Just not recommended? Or are you meaning that due to more than 1 grade difference in strength, it is appropriate of me to calculate the effective strength?
I'll use the strut and tie approach as you have recommended (how i'll deal with the complex changing geometry of the compression strut is another question haha!)
As for the logic not to use strut-tie, as I said above, that was not my decision as I had initially thought STM was the way to go, however as a graduate it can sometimes be hard to argue with your superiors, and not simply accept their decisions.
And sorry if I came across as critical of the architect, that definitely was not my intent. I merely said that as everyone here has said these transitions (there are several others that are much worse) are silly. This is a small project, but we have 4 transfer levels, with one having over 30 points if transfer! Not sure how that rates, but its pretty crazy to my green eyes!
Thank you again for the help!
RE: Reinforced Concrete Column Transition Transmission of Axial Force and Bearing Surfaces
Obviously, I don't think puddling the high strength concrete is applicable to this transfer beam situation.
From what I understand that practice is used more in the lower stories of high rise buildings where the slab f'c is much much less than the column f'c.
RE: Reinforced Concrete Column Transition Transmission of Axial Force and Bearing Surfaces
RE: Reinforced Concrete Column Transition Transmission of Axial Force and Bearing Surfaces
My check for bearing yields .6 x 2 x 40 x 600 x 300 = 8640 kN < 10000 kN, so it looks like you do need "special confinement reinforcement", or otherwise higher strength concrete for the slab in this area.
RE: Reinforced Concrete Column Transition Transmission of Axial Force and Bearing Surfaces
RE: Reinforced Concrete Column Transition Transmission of Axial Force and Bearing Surfaces
In general, the argument about strut and tie involving significant cracking does hold some water in my opinion. I certainly wouldn't want to see large swaths of slab strained to the yield strength of the rebar. It's a moot point in the concentric case, however, as you don't have any ties in the STM model.
I agree with Hokie regarding the A1/A2 business. It's about confinement more than load spread. And you have some confinement. In STM terms, it's a bottle strut that gets fat between columns.
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.