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New residental building right next to existing
3

New residental building right next to existing

New residental building right next to existing

(OP)
I have a small new residental building right next to an existing.
The structures are completely seperated - dilatation 50 mm.
Existing building has a basement, but new building doesnt.
New foundation slab is 1,50 m higher than existing foundation.

Max contact stresses at the edge of a new found. slab are 80 kN/m2.

I have no data about existing building's footings and basement wall.

Im wondering how much effect does new building has on existing basement wall (horizontal pressures) since its right next to it and 1,50 m higher.
Im also wondering about additional settlments of an existing building because of the new one next to it. Can this be a problem?

What do you suggest?


RE: New residental building right next to existing

Do you really want to survharge the adjoining basement? I would either trench fill or pile to get down below existing formation level.

RE: New residental building right next to existing

You can simplify things immensely by making the footing elevation for the new building match the elevation for the existing building. Extend them down and then backfill up to where you want your slab. Where you are proposing the new foundations is very likely disturbed fill that was excavated to build the old foundations and will need to be removed anyways.

RE: New residental building right next to existing

Don't forget the backfill to the old building is still loose. You would have settlement problems with the new one also. Lower the new wall to match the old.

RE: New residental building right next to existing

I agree with lowering the foundation of the new to match the existing footing.

Mike McCann, PE, SE (WA)


RE: New residental building right next to existing

If you are concerned with any differential settlement it might be an option to discuss with the geotech engineer whether the new footing is tied into the old assuming they are at similar depths.

50mm is also very close, existing foundation will possibly extend beyond 50mm past the face of the existing wall. So in all liklihood new footing will sit over the existing so you might have no option but to join them. I'd do some localised exploratory test hole to see what you are dealing with, so you can address it appropriately.

RE: New residental building right next to existing

(OP)
I appreciate your help, but I cant lower new foundation slab.

The backfill of old building will be completely removed and replaced with new compacted fill.
What do you think about this solution - green is new concrete that connects existing footing and new foundation slab along the whole lenght.

RE: New residental building right next to existing

Your ground floor slab sits generally on fill but is end supported on the rising wall which acts as a stiff point. I would expect some reflective cracking with that detail and suggest an isolation joint between slab/rising wall.

RE: New residental building right next to existing

(OP)
Good point!

You think something like this:

RE: New residental building right next to existing

Instead of the compacted fill against the foundation wall, use a very lean gravelly concrete mix and pour slowly, or in lifts so cement hydration inhibits lateral pressures.

RE: New residental building right next to existing

And do not impart new load to the existing footing.

Mike McCann, PE, SE (WA)


RE: New residental building right next to existing

I suggest not connecting to the old and place load inside the new area not on the footing.

RE: New residental building right next to existing

(OP)
@oldestguy

thank you but Im not really sure what you draw.

Green is new compacted fill under new RC foundation slab.
What is marked as yellow? concrete?

Im also wondering what is that bulge under new RC slab (red question mark).

Thanks for elaboration.

Regards


RE: New residental building right next to existing

Yellow is compressible fill like Styrofoam. Green is concrete filled after the compacted fill is placed.. To the right under the slab is your compacted fill first placed. The question mark is just part of your floor slab. That helps direct loads off the old footing onto the concrete fill. I suppose you could place the green concrete along with the slab pour.

Edit: You need some re-bar in the slab to resist the bending moment due to the eccentric loading at slab edge. Run it well back to the right in dwg.

RE: New residental building right next to existing

(OP)
Thank you for elaboration.

There are 2 things I dont really fancy about this solution:

- the edge of a new RC foundation slab is not supported + it is laoded with a bearing wall along the whole lenght of the edge. So the edge of a RC slab acts as cantilever.

- existing basement wall is not additionally reinforced / thickened by few inches of reinforced concrete even tho there are additional pressures acting on it.

RE: New residental building right next to existing

I'd estimate the center of new "footing" would be about in the center of that deeper concrete. Thus the eccentricity is not much, some re- bar in the slap section would not be taking a big moment.

RE: New residental building right next to existing

Check where the property line is. Without permission or a permanent easement, your new building should be entirely on your own property. Check, by test hole and surveyor, if the existing footing is on the other side of the property line or on your side. I would not tie the new building into the existing building. I would not want problems later on if (when) your building settles.

www.PeirceEngineering.com

RE: New residental building right next to existing

(OP)
This is what I came up with...

Blue material is EXTRUDED POLYSTYRENE - XPS 400 - thermal insulation that is placed under new RC slab (common practice in my country). It has 400 kPa kN/m2 of compressive strenght.

YELLOW - Expanded Polystyrene (EPS)
BLUE - Extruded Polystyrene (XPS)


Design load of new bearing wall on the edge of RC slab is 70 kN/m
Design contact pressures under slab are around 80 kn/m2.

What do you think?


RE: New residental building right next to existing

Looks great. A little more complicated than I'd do, especially with very thin "concrete base". I do not use any such "base" since insulation can take some distortion.

RE: New residental building right next to existing

(OP)
@oldestguy tnx for help. I appreciate it.

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