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?

 

[Archie264]

As long as the concrete's fairly fresh a hand-held electric chipping hammer could likely remove the concrete from the rebar in a matter of a few hours. In other words it would not even require a pneumatic jack hammer. (Again, that's presuming the concrete's still fresh.) But time is working against the situation...the harder it gets the harder it gets, so to speak.

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.

 

[sybie99]

Archie, as you said it, who pays and contractor very likely to claim for delay

 

[Archie264]

Well, unfortunately, that's the nature of the construction industry. As if the technical issues aren't challenging enough as it is...

 

[civilperson]

Wrapping the column with either high modulus fabric, (with epoxy matrix) or steel is a normal method of retrofitting for seismic requirements. This may work for you if bending is the critical fail mechanism.

 

[hokie66]

You have alreaady lost a week, talking about it. The delay situation won't get better. What about fire with the wrapping solutions?

 

[rowingengineer]

This is crazy, because of a concern about the contractor putting in a claim for extra costs we are considering poor solutions.

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.

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"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."

 

[ooox]

If you have a band beam over is it possible to increase the depth to reduce the effective length, or perhaps carry out an elastic buckling analysis?

 

[humanengr]

Can you increase the specified concrete compressive strength?

 

[sybie99]

HermanLJ, you posted:
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?