New foundation overlaps to an existing foundation

[Gercivil]

Hi,
The strip foundation of my new structure overlies in the foundation of an existing strip foundation. I attached a file of my propose detail design.
I would like to ask for a second opinion for my design proposal. Any reference books I could refer to regarding connection of new foundation to existing foundation?

 

[msquared48]

I have a real problem with the large eccentricity of the new footing as well as the load imposed to a portion of the existing strip footing.

I would much rather see the new column footing underpinning the existing strip footing as much as possible to minimize the eccentricity to the new footing.

Mike McCann
MMC Engineering

 

[hokie66]

What do you mean by "strip foundation"? It looks like a column footing.

You have a 700 x 400 column, which indicates a substantial load, and yet you have little continuity in the bottom reinforcement of the footing. Do you even know how that existing footing is reinforced?

In summary, I think your detail is terrible. Your new footing needs to be new. If you can't move the column over far enough to avoid the existing, then as Mike said, you need to underpin.

 

[paddingtongreen]

Generally the same as these guys.
The exception: In a similar condition, I ran the footing back into the new building to pick up the first interior column, making it as two column foundation, and cantilevered over the existing footing. I placed a compressible material between the old and the new. It was cheaper than underpinning in my case.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[Gercivil]

The foundation of the new building is a strip footing/foundation.....longitudinal and transverse direction(longitudinal dir-6 bays and transverse dir-3 bays).... The section detail that I am showing is from a longitudinal strip footing. The depth required for the footing is 700mm which is not correct in the attached detail. The transverse strip footing (column to column) have a base width of 2000mmm and a depth also of 700mm.
hookie: I know the reinforcement of the existing foundation base on the As-built drawing of the client.
As hookie said, my concern is the continuity of the new footing to the existing foundation. I have read one article in an engineering web site which I think answered my concern.

"........The intend of the dowels are to pull together the old and new concrete together such that their bonding become inseparable. Some engineers require the edges of the existing footings and foundations where the new and old concrete meet be roughened. Even some require application of bonding agent to enhance the bonding.

Since most footings and foundations are coated with waterproofing, the waterproofing must be removed and scraped clean to allow concrete to concrete bonding. Failure to do this will yield weak connection and can lead to water leakage.

The longer side of the dowel also call the lap splice, is calculated by multiplying the bar diameter in inches by 40. For #4 bars, it requires 20 inches long lab splice.

During the pour make sure these connection points are vibrated really well to make sure there are no honeycomb or exposed reinforcements."

 

[hokie66]

That sounds like an answer given by a contractor. A little knowledge is a dangerous thing. I still don't know how you can call the column footings "strip footing/foundation".

 

[Gercivil]

Suppose a number of columns in a row are to be supported on a soil of such low bearing strength, that separate bases if provided would overlap,a practical solution would be to provide a column footing to support all the columns of the row. such a footing is called a "STRIP FOOTING"

 

[hokie66]

I suppose. But usually, a footing which supports two or more columns is called a "combined footing". The term "strip footing" is usually reserved for a footing which supports a linear element, e.g. a wall. Maybe just semantics, but using customary terminology avoids confusion.

 

[ukengineer58]

Maybe you could vertically dowel into the existing to create a base full depth with the two acting as one. Agree with others the overlap detail as it is looks a bit week going by the relative sizes. Could you cut back the existing as far as required maintaining reinforcement and then recast all back in.