Increase of Cross Sectional Area of Existing Concrete Column
Increase of Cross Sectional Area of Existing Concrete Column
(OP)
I am trying to increase the cross sectional area of a concrete column by increasing its diameter from 0.90M. to 1.20M. The current column has a diameter of 0.90M. and a second pour of 0.30M. around the column is to be made. The additional longitudinal steel and stirrups are to be located in the second pour. The longitudinal steel is to be connected to the foundation. The slab above is not yet poured and hence the column is not subject to any superimposed loads at the moment. What could I use to ensure that the two pours work together as one.






RE: Increase of Cross Sectional Area of Existing Concrete Column
RE: Increase of Cross Sectional Area of Existing Concrete Column
RE: Increase of Cross Sectional Area of Existing Concrete Column
Some caveats:
1. Unless this has actually been tested to confirm the theory used, you will have a hard time convincing the building official that this is proper - and you will have to use a pretty large factor of safety.
2. You will probably want to increase the number of rings to keep the spacing tight to ensure that the friction surface is properly tight for the full area
3. You will have to address what happens when the outer concrete shrinks and cracks - how does this affect the friction forces and the area used for friction?
These unknowns might make pins or dowels naturally move forward as a better choice.
RE: Increase of Cross Sectional Area of Existing Concrete Column
I would still consider roughening the existing column, but more for a servicability concern than structural.
If someone else can explain what the failure mechanism will be I'd appreciate it.
RE: Increase of Cross Sectional Area of Existing Concrete Column
The bending is due to either applied end moments on the columns or on the column resisting buckling effects.
In the bending - you must have a contiguous section that behaves as a unit to be able to properly engage the reinforcement in the C = T methodology.
With a donut section outside of an interior section, you would get slippage, and thus the two portions would tend to act as two independent columns taking load only to the extent that their individual sections can resist.
My guess is that the OP is trying to create a new "composite" section and not two distinct columns - one wrapping around the other.
RE: Increase of Cross Sectional Area of Existing Concrete Column
Ozgur Selmanpakoglu, PE
Greater New York Area
www.ozgurengineering.com
RE: Increase of Cross Sectional Area of Existing Concrete Column
Can anyone else think of another failure mechanism that needs to be checked?
RE: Increase of Cross Sectional Area of Existing Concrete Column
RE: Increase of Cross Sectional Area of Existing Concrete Column
RE: Increase of Cross Sectional Area of Existing Concrete Column
Just a suggestion.
Great spirits have always encountered violent opposition from mediocre minds - Albert Einstein
RE: Increase of Cross Sectional Area of Existing Concrete Column
Now practically I'd agree that there is some shear transfer, but you can't know for sure how much. I might agree with enginerding above with the suggestion that the hoop stress around the outer donut creates a compression of sorts on the interface. Not sure though.
Yes - perhaps much ado....and the thing is a column, not a beam. But if it is a slender column, or one with significant bending, not sure I'd count on a fully composite section there.
RE: Increase of Cross Sectional Area of Existing Concrete Column
RE: Increase of Cross Sectional Area of Existing Concrete Column
There is an example of increasing ultimate load capacity of reinforced concrete circular column with FRP wrap. (about 20% increase with 2 plies)
@ishvaag: Like you mentioned in your last post, Dr. Newman notes that for section-enlargement method to be successful, the new concrete must have an extremely low rate of drying shrinkage or be made with shrinkage-compensating concrete. He further adds that preplaced-aggregate concrete usually offers the lowest rates of drying shrinkage.
RE: Increase of Cross Sectional Area of Existing Concrete Column