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New continous footing overlaps existing isolated footing

New continous footing overlaps existing isolated footing

New continous footing overlaps existing isolated footing

(OP)
Hello,
I have a problem regarding the solution for the extension of a 4 story building.
The new columns are very close to the existing columns and the foundations overlap according to the drawing.
The new foundation is a continous footing that is mounted over the existing footing. The reinforcement is not connected with the existing isolated footing.
Can this type of details cause problems in the future for the structure? I've calculated the new foundations as 2 way continous footings and it is a seismic area.
Thank you.

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RE: New continous footing overlaps existing isolated footing

Is there any reason that you decided to not connect the existing isolated footings to the new continuous strip footing?

While I am not a structural eng, as they are founding at the same depth I would think that it would be better to connect the two. Dowel some bars in with some hilti resin (the more knowledge structural engs here can advise better on connection details). This would reduce the potential for differential settlement between the two.

A 1m x 1m column is pretty beefy. I imagine you are taking some large loads? Have you looked at settlement? What are ground conditions?

RE: New continous footing overlaps existing isolated footing

(OP)
Yes there are pretty big loads, about 4000 kN axial load.
The chief engineer suggested this solution and now I started having doubts about the solution. I think it was because the existing structure is already settled on the ground and the new structure has bigger loads.
The ground conditions are good and the building is located between 2 existing buildings (shopping destination). The existing foundations are currently uncovered and the middle part of them is currently demolished.
I could probably check the continous foundation connected with the isolated footings but should I'm afraid that the isolated footing needs reinforcement.
Any help will be appreciated.
Thanks

RE: New continous footing overlaps existing isolated footing

(OP)
Hello,
So no suggestion whatsoever?

RE: New continous footing overlaps existing isolated footing

Consider:

1) Sizing the addition to the existing footing so that the total soil contact pressure (kN/m2) from the two columns is the same, or slightly lower than the soil contact pressure from the original column on the original footing.

2) Shaping the addition to the existing footing so that the resultant from the two column loads has zero, or near zero eccentricity.

3) Constructing the addition to the existing footing on flowable fill to minimize the importance of subgrade compaction. During excavation for the footing addition take steps to prevent undermining the existing footing.

4) Do not construct what appears to be a 15 cm thick concrete slab (and subgrade) above the footing addition. Stop the proposed slab/subgrade at the edge of the footing addition.

5) Securely connect (dowel) the footing addition to the original footing.

6) Do not use a continuous footing. After the addition, each footing remains isolated. The original footings are isolated... leave them that way.

The above comments are my opinion for a reasonable compromise design. Doubt there is an "ideal" answer.

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RE: New continous footing overlaps existing isolated footing

(OP)
Thank you very much.
I did not connect the continous footing with the isolated one.
I will put polystyrene or something similar between the slab and the continous footing on top of the isolated one.
Thank you again for the advice.

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