connecting new and old foundation

[n3jc]

Hi, I hope you will help me.
Lets say we have existing foundation in the basement of the house - see attached file.
We want to add another story to building so existing foundations are not wide enough.
We add new concrete foundation and anchor it to existing foundation. How to calculate anchor bars?

Also? if foundation like this are under basement there is a problem because you can only add width to the foundation from the inner side of the basement wall (outer side is under ground). So we get eccentric loading on a foundation (new + old). is that a problem? what to do?

sorry for poor language.

tnx for answers.

 

[hokie66]

Your efforts could well be for nought and could make things worse. Better to spend your money on a geotechnical investigation of the effects of adding load to the existing footing. For a house, it will most likely be OK, but don't accept my thoughts as final advice.

 

[n3jc]

I dont see how can i make things worse? because of eccentric load?

 

[hokie66]

No, by digging down beside the footing and disturbing the soil. The issue is whether additional loading from a house floor will cause a settlement problem. Why do you think it will?

 

[n3jc]

Existing foundations are only 50 cm wide. According to my calculations the stress under existing foundation will be pretty big after additional load. i dont have many options here do I?
is there any other solution than leave it be just as it is or to add width to the foundation from the inner side of the basement wall?

 

[msquared48]

50 cm is almost 20" wide!

That width good for 2500 plf at 1500 psf soil bearing. That value should be good for most retail and residential strip footings.

Also, if the maximum stress is due to a point load, remember that you can spread out the load by at least twice the height of the stem wall and strip footing.

Mike McCann, PE, SE (WA)

 

[hokie66]

Yes, 500 (mm, not cm) is a decent width footing, and footings wider than this are rarely required in residential construction. Bearing stress is not normally the issue. Settlement normally controls, although admittedly bearing is the old/simple way of doing things.

 

[Canuck65]

A 20" wide footing is relatively standard for a 2 storey house with basement in reasonable soil conditions (Minimum 75 kPa). That being said, I usually adopt the following procedure when a 2nd storey addition is being contemplated on existing strip footings:
1. Calculate typical load on strip footing in existing condition. Compare against 75 kPa
2. Calculate typical load on strip footing after 2nd storey is added. Compare against 75 kPa
3. If the new load exceeds 75 kPa significantly then I have the client engage a geotechnical engineer to evaluate the soil bearing capacity at three test pit locations adjacent to the foundation walls (normally on the exterior side). Usually not a significant expense.
4. More times than not, in my geographical area, the geotech will come back with 100 or 150 kPa which then justifies the existing footings have sufficient capacity, without any modifications to the footings. It is a bit of a risk, but potential financial rewards are large.

If the footings really did need to be widened, I would underpin the entire footing ... lots of work and expense. I would not accept widening the footing as you have detailed.

 

[msquared48]

I agree.

Mike McCann, PE, SE (WA)

 

[n3jc]

75 kPa is very very low in my opinion. That means that at 50 cm width we can apply only 75 kn/m2 x 0,50 m = 37,50 kN/m. that is pretty low force! maybe its ok for wooden houses? I dont know...
But I cant imagine such a small load on the wall from 2 storey house with wooden roof, brick walls and concrete slabs. Force is more likely between 70 - 120 kN/m which means you need soil of 120 kN/m /0,5 m = 240 kN/m2 = 240 kPa!?

 

[hokie66]

As I said, settlement is the issue. What is the soil like, anyway? You haven't given us any idea as to that detail. 240 kPa could well be achievable. You just need to find out.

 

[n3jc]

Problem is i have no data on soil. There hasnt been any geotechnical research made to evaluate the soil bearing capacity.

I think its mandatory to suggest that to a client... what else can I do.