column under designed, what now?

[sybie99]

Foundations to columns have been cast anf column starter bars are in. The height of the column must now increase but the size must stay the same. So the steel in the column must increase, and I must add another 4 20mm bars. The column starter bars were cast into the pile foundation so we cannot drill additional bars in.

Am I right in my understanding that these additional bars can be added to the full height of the column without being anchored into the foundation as long as at the bearing point the existing bars and concrete can take the force being transmitted. The column is just classified as slender so the addtional rebar is more to give it more bending capacity.

If I take 40% of the concrete area times the concrete strength and 80% of the steel area times the steel strength as for a short or stocky column the column is fine. In other words it can transmit the load at the bearing point, so I would think it is okay to simply rest the additional bars needed on the foundation and take it through to the slab above. What are your thoughts?

 

[jayrod12]

If the connection at the base, i.e. the current dowels that are installed, can handle the shear (and possible moment) at the base of the column than I cannot see you requiring to change it. It all depends on what you were expecting at the base for support conditions, if pinned then you really only have to make sure that you're ok for shear, if fixed then this may become a larger problem.

 

[HermanLJ]

I've been through this.

What I ended up doing was increasing the size of the column (ie casting an extra section onto the side of the existing column).

Some important things to consider:

- Increasing column size will increase the stiffness of the column and the column will therefore 'take' a larger moment.
- You need to make sure the bars are sufficiently doweled in. This will require a rational design in order to calculate the tensile forces in the bars.
- You need to consider the eccentricity of the reinforcement.

 

[sybie99]

Brings me back to another question, if an edge column us not designed to take any moment, but the slab it supports is designed to be simply supported at column locations, will it actually fail or simply crack and redistribute the moments to the slab?

Herman you think the bars must be dowelled into the base? There will be no shear (or little) at the base and only nominal bending which can be handled by existing rebar anchored into foundation.

 

[hokie66]

No, you don't have to have the same number of bars as starters. As long as the strength conditions at the column to footing joint are satisfied, you are fine.

Columns must always be designed to take some moment, even if it is only from nominal eccentricity. Codes require this. Redistribution will occur if/when the column cracks, as that reduces the column stiffness, but the column will still take a portion of the joint moment. Cracking is not failure.

 

[Worldtraveller]

Depending on the mode of potential failure, I've seen some designs where composite material strips are added to the outer 'fiber' of a column or even building wall. Not sure if that kind of design would help with what you are doing, though. It does rather increase the bending capability, since you now have a outer fiber that is very good at taking tension load (unlike your concrete).

 

[HermanLJ]

Sybie; I cannot really say whether doweling will necessary without seeing a BMD first. If you don't dowel, the bars will only start 'working' after their development length. And since most columns' max moment is at the top and bottom it will render the rebar practically useless.

I would would also consider doweling the new stirrups into the existing column to prevent the new piece from breaking away under axial force.

 

[dik]

Hokie's got it... it's nice to have the same number, but, not necessary, in particular if designed as pinned at the base, or if the base has a small moment.

Dik

 

[rowingengineer]

Agree with Hokie

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[TDIengineer]

Agree with Hokie. Unless this is a moment frame column that is part of the lateral system, you should be able to design your column with a K assuming a pinned base. You're preventing the column from buckling at midspan, so you'll get those added bar moment capacity where you need it assuming the column isn't super short and the bars are developed at mid span.

 

[sybie99]

Unfortunately the column has been cast... would it be possible to add the additional steel area required by providing a steel plate to the outside face of the column, bolted into the column at the top and bottom that will provide additional tensile resistance and prevent the column from buckling outward?

 

[rowingengineer]

The column can easily be recast. I would prefer to knock it down and start again rather than do a strengthening unless the slab over has been cast.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[hokie66]

rowingengineer's way is the answer. Do it right. How hard can it be to knock out a newly cast column and do it again?

 

[sybie99]

Hokie, not hard but will have effect on contractor program as they want to deck for the slab, so will set them back quite a bit.

I was thinking, what about providing load bearing brickwork (40MPa bricks)to the inside (compression face) tied to the column with hoop irons at regular intervals, or even 8mm U-Shape Bars chemical anchored into the column face at say 400 vertical centres so the column and brickwork act together at mid height of the column, thus increasing the column bending capacity by giving a longer lever arm where it is at its most critical. Then I will also provide straps to strap around the brickwork and column to tie the two elements together.

If I do this I will not effect contractor program as this can be done at a later stage as long as its done before major loadings on the columns are applied

 

[rowingengineer]

I'm. still voting for recast, i really dont like the strengthing idea we ate onlytalking about two days if we use quick set concrete

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[hokie66]

Doesn't have to be accelerated concrete. The load will still be on the formwork. Should only take a day at most, meanwhile the contractor can be working in other areas. They like to complain. Your proposed methods of strengthening would be unacceptable to me.

 

[rowingengineer]

true

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[TDIengineer]

A welded steel jacket is a possibility. See these retrofits done all the time on freeway columns. I suspect it is not cheap, lots of field welding.

Assuming this is all on the contractor's dime, propose a quick sketch and let them decide to keep moving and retro or knock down and work it into the schedule.

And yes they love to complain!

 

[DamsInc]

Out of curiosity, what would be the steps to knock down/replace the column? I would imagine it would go like this:

- chip away concrete at the bottom of the column to expose the dowel bars (while ensuring it doesn't collapse on the workers somehow)
- remove column (knock over? saw cut?)
- clean concrete off the dowels (hydro demolition?),
- install new rebar, formwork
- repour concrete

I'm not familiar with the methods involved but it seems like more than a day or two's work?

 

[jayrod12]

I would think it would be,

Sawcut column above (but close to) top of dowels. Chip out remaining concrete around dowels. Clean dowels. Then form and reinforce and pour. It wouldn't be an incredible amount more time than two days.