×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Reinforced Concrete Column Transition Transmission of Axial Force and Bearing Surfaces

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.

RE: Reinforced Concrete Column Transition Transmission of Axial Force and Bearing Surfaces

You'll find some relevant information in this thread: Link. I believe that strut and tie is thoroughly appropriate in these situations. The trouble is that often the strut is unreinforced and therefore does not have sufficient capacity. Many designers will overcome this by assuming that the the strut is vertical and only made up of the concrete in the zone of overlap between the upper and lower columns. This strut is then reinforced with longitudinal bars and ties just like a short column. Note that if your overlap zone is not concentric with your columns, you will have moments to deal with in your columns as a result of axial load eccentricity.

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

Your OP says there is a transfer beam 900 deep, but doesn't say how wide. The sketch shows a 900 deep transfer slab. If it is a slab, I can't see how it would not satisfy Paragraph 10.8. And I think it satisfies the bearing provision as well. Perhaps if you post your numbers, we could better critique.

RE: Reinforced Concrete Column Transition Transmission of Axial Force and Bearing Surfaces

(OP)
Thank you for the replies!!


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

I do not see a problem, other than the concrete strength difference between floor and columns. Normally a difference of 1 grade is regarded as acceptable. 65 to 40 is too much!

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

(OP)
Rapt,

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

Might be worth having a read of this thread also...

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

Also - chiano91, There is a separate forum for all things Strayan'

RE: Reinforced Concrete Column Transition Transmission of Axial Force and Bearing Surfaces

I disagree with rapt on this occasion. 65 to 40 (columns to floors) are done all the time. And I also disagree with your colleagues on the bearing areas. The column sizes are the bearing areas A1, and A2 is much larger. Just read the definition. Your check for bearing needs to be on the slab, not on the column. The column load includes the steel, so the bearing of a column is by definition satisfied if the column compression capacity is adequate.

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

Sorry, I used your numbers, but the column is actually 300 x 1500, so the bearing is quite all right without any special reinforcement or puddling.

RE: Reinforced Concrete Column Transition Transmission of Axial Force and Bearing Surfaces

You really should consider the method that I pitched above Chiano. It's technically sound, leads to rational, constructable detailing (see below), and only takes about 5 min for a concentric case.

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources