column under designed, what now?
column under designed, what now?
(OP)
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?
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?






RE: column under designed, what now?
RE: column under designed, what now?
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.
RE: column under designed, what now?
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.
RE: column under designed, what now?
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.
RE: column under designed, what now?
RE: column under designed, what now?
I would would also consider doweling the new stirrups into the existing column to prevent the new piece from breaking away under axial force.
RE: column under designed, what now?
Dik
RE: column under designed, what now?
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."
RE: column under designed, what now?
RE: column under designed, what now?
RE: column under designed, what now?
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."
RE: column under designed, what now?
RE: column under designed, what now?
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
RE: column under designed, what now?
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."
RE: column under designed, what now?
RE: column under designed, what now?
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."
RE: column under designed, what now?
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!
RE: column under designed, what now?
- 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?
RE: column under designed, what now?
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.
RE: column under designed, what now?
What I suspect is a more delicate issue is who bears the cost, especially since an unscrupulous contractor could try to bundle in any hitherto-fore incurred delays into the change order this will likely generate.
RE: column under designed, what now?
RE: column under designed, what now?
RE: column under designed, what now?
RE: column under designed, what now?
RE: column under designed, what now?
If your concerned about the contractor charging an arm and a leg get a quote first, and then get a quote from another builder and ask for justification if the contractor if over charging by a fair bit, if not move on with your life. I handle all change requests as a quote, this way they may charge 20% rather than 10% over but not significantly.
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."
RE: column under designed, what now?
RE: column under designed, what now?
RE: column under designed, what now?
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.
I intend to increase the column size by pouring a 100mm sleeve around the existing column, so increasing it by 200mm.
I will provide closed stirrups that go around the column, which will act in hoop tension so to speak. These will keep the additional bars confined. Is it still necessary to have dowelled in bars into the column to connect the existing and new concrete? I would think not as the new concrete cannot go anywhere. What are your thoughts?